blkdevparts= Manual partition parsing of block device(s) for
embedded devices based on command line input.
- See Documentation/block/cmdline-partition.txt
+ See Documentation/block/cmdline-partition.rst
boot_delay= Milliseconds to delay each printk during boot.
Values larger than 10 seconds (10000) are changed to
elevator= [IOSCHED]
Format: { "mq-deadline" | "kyber" | "bfq" }
- See Documentation/block/deadline-iosched.txt,
- Documentation/block/kyber-iosched.txt and
- Documentation/block/bfq-iosched.txt for details.
+ See Documentation/block/deadline-iosched.rst,
+ Documentation/block/kyber-iosched.rst and
+ Documentation/block/bfq-iosched.rst for details.
elfcorehdr=[size[KMG]@]offset[KMG] [IA64,PPC,SH,X86,S390]
Specifies physical address of start of kernel core
--- /dev/null
+==========================
+BFQ (Budget Fair Queueing)
+==========================
+
+BFQ is a proportional-share I/O scheduler, with some extra
+low-latency capabilities. In addition to cgroups support (blkio or io
+controllers), BFQ's main features are:
+
+- BFQ guarantees a high system and application responsiveness, and a
+ low latency for time-sensitive applications, such as audio or video
+ players;
+- BFQ distributes bandwidth, and not just time, among processes or
+ groups (switching back to time distribution when needed to keep
+ throughput high).
+
+In its default configuration, BFQ privileges latency over
+throughput. So, when needed for achieving a lower latency, BFQ builds
+schedules that may lead to a lower throughput. If your main or only
+goal, for a given device, is to achieve the maximum-possible
+throughput at all times, then do switch off all low-latency heuristics
+for that device, by setting low_latency to 0. See Section 3 for
+details on how to configure BFQ for the desired tradeoff between
+latency and throughput, or on how to maximize throughput.
+
+As every I/O scheduler, BFQ adds some overhead to per-I/O-request
+processing. To give an idea of this overhead, the total,
+single-lock-protected, per-request processing time of BFQ---i.e., the
+sum of the execution times of the request insertion, dispatch and
+completion hooks---is, e.g., 1.9 us on an Intel Core i7-2760QM@2.40GHz
+(dated CPU for notebooks; time measured with simple code
+instrumentation, and using the throughput-sync.sh script of the S
+suite [1], in performance-profiling mode). To put this result into
+context, the total, single-lock-protected, per-request execution time
+of the lightest I/O scheduler available in blk-mq, mq-deadline, is 0.7
+us (mq-deadline is ~800 LOC, against ~10500 LOC for BFQ).
+
+Scheduling overhead further limits the maximum IOPS that a CPU can
+process (already limited by the execution of the rest of the I/O
+stack). To give an idea of the limits with BFQ, on slow or average
+CPUs, here are, first, the limits of BFQ for three different CPUs, on,
+respectively, an average laptop, an old desktop, and a cheap embedded
+system, in case full hierarchical support is enabled (i.e.,
+CONFIG_BFQ_GROUP_IOSCHED is set), but CONFIG_BFQ_CGROUP_DEBUG is not
+set (Section 4-2):
+- Intel i7-4850HQ: 400 KIOPS
+- AMD A8-3850: 250 KIOPS
+- ARM CortexTM-A53 Octa-core: 80 KIOPS
+
+If CONFIG_BFQ_CGROUP_DEBUG is set (and of course full hierarchical
+support is enabled), then the sustainable throughput with BFQ
+decreases, because all blkio.bfq* statistics are created and updated
+(Section 4-2). For BFQ, this leads to the following maximum
+sustainable throughputs, on the same systems as above:
+- Intel i7-4850HQ: 310 KIOPS
+- AMD A8-3850: 200 KIOPS
+- ARM CortexTM-A53 Octa-core: 56 KIOPS
+
+BFQ works for multi-queue devices too.
+
+.. The table of contents follow. Impatients can just jump to Section 3.
+
+.. CONTENTS
+
+ 1. When may BFQ be useful?
+ 1-1 Personal systems
+ 1-2 Server systems
+ 2. How does BFQ work?
+ 3. What are BFQ's tunables and how to properly configure BFQ?
+ 4. BFQ group scheduling
+ 4-1 Service guarantees provided
+ 4-2 Interface
+
+1. When may BFQ be useful?
+==========================
+
+BFQ provides the following benefits on personal and server systems.
+
+1-1 Personal systems
+--------------------
+
+Low latency for interactive applications
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Regardless of the actual background workload, BFQ guarantees that, for
+interactive tasks, the storage device is virtually as responsive as if
+it was idle. For example, even if one or more of the following
+background workloads are being executed:
+
+- one or more large files are being read, written or copied,
+- a tree of source files is being compiled,
+- one or more virtual machines are performing I/O,
+- a software update is in progress,
+- indexing daemons are scanning filesystems and updating their
+ databases,
+
+starting an application or loading a file from within an application
+takes about the same time as if the storage device was idle. As a
+comparison, with CFQ, NOOP or DEADLINE, and in the same conditions,
+applications experience high latencies, or even become unresponsive
+until the background workload terminates (also on SSDs).
+
+Low latency for soft real-time applications
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Also soft real-time applications, such as audio and video
+players/streamers, enjoy a low latency and a low drop rate, regardless
+of the background I/O workload. As a consequence, these applications
+do not suffer from almost any glitch due to the background workload.
+
+Higher speed for code-development tasks
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+If some additional workload happens to be executed in parallel, then
+BFQ executes the I/O-related components of typical code-development
+tasks (compilation, checkout, merge, ...) much more quickly than CFQ,
+NOOP or DEADLINE.
+
+High throughput
+^^^^^^^^^^^^^^^
+
+On hard disks, BFQ achieves up to 30% higher throughput than CFQ, and
+up to 150% higher throughput than DEADLINE and NOOP, with all the
+sequential workloads considered in our tests. With random workloads,
+and with all the workloads on flash-based devices, BFQ achieves,
+instead, about the same throughput as the other schedulers.
+
+Strong fairness, bandwidth and delay guarantees
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+BFQ distributes the device throughput, and not just the device time,
+among I/O-bound applications in proportion their weights, with any
+workload and regardless of the device parameters. From these bandwidth
+guarantees, it is possible to compute tight per-I/O-request delay
+guarantees by a simple formula. If not configured for strict service
+guarantees, BFQ switches to time-based resource sharing (only) for
+applications that would otherwise cause a throughput loss.
+
+1-2 Server systems
+------------------
+
+Most benefits for server systems follow from the same service
+properties as above. In particular, regardless of whether additional,
+possibly heavy workloads are being served, BFQ guarantees:
+
+* audio and video-streaming with zero or very low jitter and drop
+ rate;
+
+* fast retrieval of WEB pages and embedded objects;
+
+* real-time recording of data in live-dumping applications (e.g.,
+ packet logging);
+
+* responsiveness in local and remote access to a server.
+
+
+2. How does BFQ work?
+=====================
+
+BFQ is a proportional-share I/O scheduler, whose general structure,
+plus a lot of code, are borrowed from CFQ.
+
+- Each process doing I/O on a device is associated with a weight and a
+ `(bfq_)queue`.
+
+- BFQ grants exclusive access to the device, for a while, to one queue
+ (process) at a time, and implements this service model by
+ associating every queue with a budget, measured in number of
+ sectors.
+
+ - After a queue is granted access to the device, the budget of the
+ queue is decremented, on each request dispatch, by the size of the
+ request.
+
+ - The in-service queue is expired, i.e., its service is suspended,
+ only if one of the following events occurs: 1) the queue finishes
+ its budget, 2) the queue empties, 3) a "budget timeout" fires.
+
+ - The budget timeout prevents processes doing random I/O from
+ holding the device for too long and dramatically reducing
+ throughput.
+
+ - Actually, as in CFQ, a queue associated with a process issuing
+ sync requests may not be expired immediately when it empties. In
+ contrast, BFQ may idle the device for a short time interval,
+ giving the process the chance to go on being served if it issues
+ a new request in time. Device idling typically boosts the
+ throughput on rotational devices and on non-queueing flash-based
+ devices, if processes do synchronous and sequential I/O. In
+ addition, under BFQ, device idling is also instrumental in
+ guaranteeing the desired throughput fraction to processes
+ issuing sync requests (see the description of the slice_idle
+ tunable in this document, or [1, 2], for more details).
+
+ - With respect to idling for service guarantees, if several
+ processes are competing for the device at the same time, but
+ all processes and groups have the same weight, then BFQ
+ guarantees the expected throughput distribution without ever
+ idling the device. Throughput is thus as high as possible in
+ this common scenario.
+
+ - On flash-based storage with internal queueing of commands
+ (typically NCQ), device idling happens to be always detrimental
+ for throughput. So, with these devices, BFQ performs idling
+ only when strictly needed for service guarantees, i.e., for
+ guaranteeing low latency or fairness. In these cases, overall
+ throughput may be sub-optimal. No solution currently exists to
+ provide both strong service guarantees and optimal throughput
+ on devices with internal queueing.
+
+ - If low-latency mode is enabled (default configuration), BFQ
+ executes some special heuristics to detect interactive and soft
+ real-time applications (e.g., video or audio players/streamers),
+ and to reduce their latency. The most important action taken to
+ achieve this goal is to give to the queues associated with these
+ applications more than their fair share of the device
+ throughput. For brevity, we call just "weight-raising" the whole
+ sets of actions taken by BFQ to privilege these queues. In
+ particular, BFQ provides a milder form of weight-raising for
+ interactive applications, and a stronger form for soft real-time
+ applications.
+
+ - BFQ automatically deactivates idling for queues born in a burst of
+ queue creations. In fact, these queues are usually associated with
+ the processes of applications and services that benefit mostly
+ from a high throughput. Examples are systemd during boot, or git
+ grep.
+
+ - As CFQ, BFQ merges queues performing interleaved I/O, i.e.,
+ performing random I/O that becomes mostly sequential if
+ merged. Differently from CFQ, BFQ achieves this goal with a more
+ reactive mechanism, called Early Queue Merge (EQM). EQM is so
+ responsive in detecting interleaved I/O (cooperating processes),
+ that it enables BFQ to achieve a high throughput, by queue
+ merging, even for queues for which CFQ needs a different
+ mechanism, preemption, to get a high throughput. As such EQM is a
+ unified mechanism to achieve a high throughput with interleaved
+ I/O.
+
+ - Queues are scheduled according to a variant of WF2Q+, named
+ B-WF2Q+, and implemented using an augmented rb-tree to preserve an
+ O(log N) overall complexity. See [2] for more details. B-WF2Q+ is
+ also ready for hierarchical scheduling, details in Section 4.
+
+ - B-WF2Q+ guarantees a tight deviation with respect to an ideal,
+ perfectly fair, and smooth service. In particular, B-WF2Q+
+ guarantees that each queue receives a fraction of the device
+ throughput proportional to its weight, even if the throughput
+ fluctuates, and regardless of: the device parameters, the current
+ workload and the budgets assigned to the queue.
+
+ - The last, budget-independence, property (although probably
+ counterintuitive in the first place) is definitely beneficial, for
+ the following reasons:
+
+ - First, with any proportional-share scheduler, the maximum
+ deviation with respect to an ideal service is proportional to
+ the maximum budget (slice) assigned to queues. As a consequence,
+ BFQ can keep this deviation tight not only because of the
+ accurate service of B-WF2Q+, but also because BFQ *does not*
+ need to assign a larger budget to a queue to let the queue
+ receive a higher fraction of the device throughput.
+
+ - Second, BFQ is free to choose, for every process (queue), the
+ budget that best fits the needs of the process, or best
+ leverages the I/O pattern of the process. In particular, BFQ
+ updates queue budgets with a simple feedback-loop algorithm that
+ allows a high throughput to be achieved, while still providing
+ tight latency guarantees to time-sensitive applications. When
+ the in-service queue expires, this algorithm computes the next
+ budget of the queue so as to:
+
+ - Let large budgets be eventually assigned to the queues
+ associated with I/O-bound applications performing sequential
+ I/O: in fact, the longer these applications are served once
+ got access to the device, the higher the throughput is.
+
+ - Let small budgets be eventually assigned to the queues
+ associated with time-sensitive applications (which typically
+ perform sporadic and short I/O), because, the smaller the
+ budget assigned to a queue waiting for service is, the sooner
+ B-WF2Q+ will serve that queue (Subsec 3.3 in [2]).
+
+- If several processes are competing for the device at the same time,
+ but all processes and groups have the same weight, then BFQ
+ guarantees the expected throughput distribution without ever idling
+ the device. It uses preemption instead. Throughput is then much
+ higher in this common scenario.
+
+- ioprio classes are served in strict priority order, i.e.,
+ lower-priority queues are not served as long as there are
+ higher-priority queues. Among queues in the same class, the
+ bandwidth is distributed in proportion to the weight of each
+ queue. A very thin extra bandwidth is however guaranteed to
+ the Idle class, to prevent it from starving.
+
+
+3. What are BFQ's tunables and how to properly configure BFQ?
+=============================================================
+
+Most BFQ tunables affect service guarantees (basically latency and
+fairness) and throughput. For full details on how to choose the
+desired tradeoff between service guarantees and throughput, see the
+parameters slice_idle, strict_guarantees and low_latency. For details
+on how to maximise throughput, see slice_idle, timeout_sync and
+max_budget. The other performance-related parameters have been
+inherited from, and have been preserved mostly for compatibility with
+CFQ. So far, no performance improvement has been reported after
+changing the latter parameters in BFQ.
+
+In particular, the tunables back_seek-max, back_seek_penalty,
+fifo_expire_async and fifo_expire_sync below are the same as in
+CFQ. Their description is just copied from that for CFQ. Some
+considerations in the description of slice_idle are copied from CFQ
+too.
+
+per-process ioprio and weight
+-----------------------------
+
+Unless the cgroups interface is used (see "4. BFQ group scheduling"),
+weights can be assigned to processes only indirectly, through I/O
+priorities, and according to the relation:
+weight = (IOPRIO_BE_NR - ioprio) * 10.
+
+Beware that, if low-latency is set, then BFQ automatically raises the
+weight of the queues associated with interactive and soft real-time
+applications. Unset this tunable if you need/want to control weights.
+
+slice_idle
+----------
+
+This parameter specifies how long BFQ should idle for next I/O
+request, when certain sync BFQ queues become empty. By default
+slice_idle is a non-zero value. Idling has a double purpose: boosting
+throughput and making sure that the desired throughput distribution is
+respected (see the description of how BFQ works, and, if needed, the
+papers referred there).
+
+As for throughput, idling can be very helpful on highly seeky media
+like single spindle SATA/SAS disks where we can cut down on overall
+number of seeks and see improved throughput.
+
+Setting slice_idle to 0 will remove all the idling on queues and one
+should see an overall improved throughput on faster storage devices
+like multiple SATA/SAS disks in hardware RAID configuration, as well
+as flash-based storage with internal command queueing (and
+parallelism).
+
+So depending on storage and workload, it might be useful to set
+slice_idle=0. In general for SATA/SAS disks and software RAID of
+SATA/SAS disks keeping slice_idle enabled should be useful. For any
+configurations where there are multiple spindles behind single LUN
+(Host based hardware RAID controller or for storage arrays), or with
+flash-based fast storage, setting slice_idle=0 might end up in better
+throughput and acceptable latencies.
+
+Idling is however necessary to have service guarantees enforced in
+case of differentiated weights or differentiated I/O-request lengths.
+To see why, suppose that a given BFQ queue A must get several I/O
+requests served for each request served for another queue B. Idling
+ensures that, if A makes a new I/O request slightly after becoming
+empty, then no request of B is dispatched in the middle, and thus A
+does not lose the possibility to get more than one request dispatched
+before the next request of B is dispatched. Note that idling
+guarantees the desired differentiated treatment of queues only in
+terms of I/O-request dispatches. To guarantee that the actual service
+order then corresponds to the dispatch order, the strict_guarantees
+tunable must be set too.
+
+There is an important flipside for idling: apart from the above cases
+where it is beneficial also for throughput, idling can severely impact
+throughput. One important case is random workload. Because of this
+issue, BFQ tends to avoid idling as much as possible, when it is not
+beneficial also for throughput (as detailed in Section 2). As a
+consequence of this behavior, and of further issues described for the
+strict_guarantees tunable, short-term service guarantees may be
+occasionally violated. And, in some cases, these guarantees may be
+more important than guaranteeing maximum throughput. For example, in
+video playing/streaming, a very low drop rate may be more important
+than maximum throughput. In these cases, consider setting the
+strict_guarantees parameter.
+
+slice_idle_us
+-------------
+
+Controls the same tuning parameter as slice_idle, but in microseconds.
+Either tunable can be used to set idling behavior. Afterwards, the
+other tunable will reflect the newly set value in sysfs.
+
+strict_guarantees
+-----------------
+
+If this parameter is set (default: unset), then BFQ
+
+- always performs idling when the in-service queue becomes empty;
+
+- forces the device to serve one I/O request at a time, by dispatching a
+ new request only if there is no outstanding request.
+
+In the presence of differentiated weights or I/O-request sizes, both
+the above conditions are needed to guarantee that every BFQ queue
+receives its allotted share of the bandwidth. The first condition is
+needed for the reasons explained in the description of the slice_idle
+tunable. The second condition is needed because all modern storage
+devices reorder internally-queued requests, which may trivially break
+the service guarantees enforced by the I/O scheduler.
+
+Setting strict_guarantees may evidently affect throughput.
+
+back_seek_max
+-------------
+
+This specifies, given in Kbytes, the maximum "distance" for backward seeking.
+The distance is the amount of space from the current head location to the
+sectors that are backward in terms of distance.
+
+This parameter allows the scheduler to anticipate requests in the "backward"
+direction and consider them as being the "next" if they are within this
+distance from the current head location.
+
+back_seek_penalty
+-----------------
+
+This parameter is used to compute the cost of backward seeking. If the
+backward distance of request is just 1/back_seek_penalty from a "front"
+request, then the seeking cost of two requests is considered equivalent.
+
+So scheduler will not bias toward one or the other request (otherwise scheduler
+will bias toward front request). Default value of back_seek_penalty is 2.
+
+fifo_expire_async
+-----------------
+
+This parameter is used to set the timeout of asynchronous requests. Default
+value of this is 248ms.
+
+fifo_expire_sync
+----------------
+
+This parameter is used to set the timeout of synchronous requests. Default
+value of this is 124ms. In case to favor synchronous requests over asynchronous
+one, this value should be decreased relative to fifo_expire_async.
+
+low_latency
+-----------
+
+This parameter is used to enable/disable BFQ's low latency mode. By
+default, low latency mode is enabled. If enabled, interactive and soft
+real-time applications are privileged and experience a lower latency,
+as explained in more detail in the description of how BFQ works.
+
+DISABLE this mode if you need full control on bandwidth
+distribution. In fact, if it is enabled, then BFQ automatically
+increases the bandwidth share of privileged applications, as the main
+means to guarantee a lower latency to them.
+
+In addition, as already highlighted at the beginning of this document,
+DISABLE this mode if your only goal is to achieve a high throughput.
+In fact, privileging the I/O of some application over the rest may
+entail a lower throughput. To achieve the highest-possible throughput
+on a non-rotational device, setting slice_idle to 0 may be needed too
+(at the cost of giving up any strong guarantee on fairness and low
+latency).
+
+timeout_sync
+------------
+
+Maximum amount of device time that can be given to a task (queue) once
+it has been selected for service. On devices with costly seeks,
+increasing this time usually increases maximum throughput. On the
+opposite end, increasing this time coarsens the granularity of the
+short-term bandwidth and latency guarantees, especially if the
+following parameter is set to zero.
+
+max_budget
+----------
+
+Maximum amount of service, measured in sectors, that can be provided
+to a BFQ queue once it is set in service (of course within the limits
+of the above timeout). According to what said in the description of
+the algorithm, larger values increase the throughput in proportion to
+the percentage of sequential I/O requests issued. The price of larger
+values is that they coarsen the granularity of short-term bandwidth
+and latency guarantees.
+
+The default value is 0, which enables auto-tuning: BFQ sets max_budget
+to the maximum number of sectors that can be served during
+timeout_sync, according to the estimated peak rate.
+
+For specific devices, some users have occasionally reported to have
+reached a higher throughput by setting max_budget explicitly, i.e., by
+setting max_budget to a higher value than 0. In particular, they have
+set max_budget to higher values than those to which BFQ would have set
+it with auto-tuning. An alternative way to achieve this goal is to
+just increase the value of timeout_sync, leaving max_budget equal to 0.
+
+weights
+-------
+
+Read-only parameter, used to show the weights of the currently active
+BFQ queues.
+
+
+4. Group scheduling with BFQ
+============================
+
+BFQ supports both cgroups-v1 and cgroups-v2 io controllers, namely
+blkio and io. In particular, BFQ supports weight-based proportional
+share. To activate cgroups support, set BFQ_GROUP_IOSCHED.
+
+4-1 Service guarantees provided
+-------------------------------
+
+With BFQ, proportional share means true proportional share of the
+device bandwidth, according to group weights. For example, a group
+with weight 200 gets twice the bandwidth, and not just twice the time,
+of a group with weight 100.
+
+BFQ supports hierarchies (group trees) of any depth. Bandwidth is
+distributed among groups and processes in the expected way: for each
+group, the children of the group share the whole bandwidth of the
+group in proportion to their weights. In particular, this implies
+that, for each leaf group, every process of the group receives the
+same share of the whole group bandwidth, unless the ioprio of the
+process is modified.
+
+The resource-sharing guarantee for a group may partially or totally
+switch from bandwidth to time, if providing bandwidth guarantees to
+the group lowers the throughput too much. This switch occurs on a
+per-process basis: if a process of a leaf group causes throughput loss
+if served in such a way to receive its share of the bandwidth, then
+BFQ switches back to just time-based proportional share for that
+process.
+
+4-2 Interface
+-------------
+
+To get proportional sharing of bandwidth with BFQ for a given device,
+BFQ must of course be the active scheduler for that device.
+
+Within each group directory, the names of the files associated with
+BFQ-specific cgroup parameters and stats begin with the "bfq."
+prefix. So, with cgroups-v1 or cgroups-v2, the full prefix for
+BFQ-specific files is "blkio.bfq." or "io.bfq." For example, the group
+parameter to set the weight of a group with BFQ is blkio.bfq.weight
+or io.bfq.weight.
+
+As for cgroups-v1 (blkio controller), the exact set of stat files
+created, and kept up-to-date by bfq, depends on whether
+CONFIG_BFQ_CGROUP_DEBUG is set. If it is set, then bfq creates all
+the stat files documented in
+Documentation/cgroup-v1/blkio-controller.rst. If, instead,
+CONFIG_BFQ_CGROUP_DEBUG is not set, then bfq creates only the files::
+
+ blkio.bfq.io_service_bytes
+ blkio.bfq.io_service_bytes_recursive
+ blkio.bfq.io_serviced
+ blkio.bfq.io_serviced_recursive
+
+The value of CONFIG_BFQ_CGROUP_DEBUG greatly influences the maximum
+throughput sustainable with bfq, because updating the blkio.bfq.*
+stats is rather costly, especially for some of the stats enabled by
+CONFIG_BFQ_CGROUP_DEBUG.
+
+Parameters to set
+-----------------
+
+For each group, there is only the following parameter to set.
+
+weight (namely blkio.bfq.weight or io.bfq-weight): the weight of the
+group inside its parent. Available values: 1..10000 (default 100). The
+linear mapping between ioprio and weights, described at the beginning
+of the tunable section, is still valid, but all weights higher than
+IOPRIO_BE_NR*10 are mapped to ioprio 0.
+
+Recall that, if low-latency is set, then BFQ automatically raises the
+weight of the queues associated with interactive and soft real-time
+applications. Unset this tunable if you need/want to control weights.
+
+
+[1]
+ P. Valente, A. Avanzini, "Evolution of the BFQ Storage I/O
+ Scheduler", Proceedings of the First Workshop on Mobile System
+ Technologies (MST-2015), May 2015.
+
+ http://algogroup.unimore.it/people/paolo/disk_sched/mst-2015.pdf
+
+[2]
+ P. Valente and M. Andreolini, "Improving Application
+ Responsiveness with the BFQ Disk I/O Scheduler", Proceedings of
+ the 5th Annual International Systems and Storage Conference
+ (SYSTOR '12), June 2012.
+
+ Slightly extended version:
+
+ http://algogroup.unimore.it/people/paolo/disk_sched/bfq-v1-suite-results.pdf
+
+[3]
+ https://github.com/Algodev-github/S
+++ /dev/null
-BFQ (Budget Fair Queueing)
-==========================
-
-BFQ is a proportional-share I/O scheduler, with some extra
-low-latency capabilities. In addition to cgroups support (blkio or io
-controllers), BFQ's main features are:
-- BFQ guarantees a high system and application responsiveness, and a
- low latency for time-sensitive applications, such as audio or video
- players;
-- BFQ distributes bandwidth, and not just time, among processes or
- groups (switching back to time distribution when needed to keep
- throughput high).
-
-In its default configuration, BFQ privileges latency over
-throughput. So, when needed for achieving a lower latency, BFQ builds
-schedules that may lead to a lower throughput. If your main or only
-goal, for a given device, is to achieve the maximum-possible
-throughput at all times, then do switch off all low-latency heuristics
-for that device, by setting low_latency to 0. See Section 3 for
-details on how to configure BFQ for the desired tradeoff between
-latency and throughput, or on how to maximize throughput.
-
-As every I/O scheduler, BFQ adds some overhead to per-I/O-request
-processing. To give an idea of this overhead, the total,
-single-lock-protected, per-request processing time of BFQ---i.e., the
-sum of the execution times of the request insertion, dispatch and
-completion hooks---is, e.g., 1.9 us on an Intel Core i7-2760QM@2.40GHz
-(dated CPU for notebooks; time measured with simple code
-instrumentation, and using the throughput-sync.sh script of the S
-suite [1], in performance-profiling mode). To put this result into
-context, the total, single-lock-protected, per-request execution time
-of the lightest I/O scheduler available in blk-mq, mq-deadline, is 0.7
-us (mq-deadline is ~800 LOC, against ~10500 LOC for BFQ).
-
-Scheduling overhead further limits the maximum IOPS that a CPU can
-process (already limited by the execution of the rest of the I/O
-stack). To give an idea of the limits with BFQ, on slow or average
-CPUs, here are, first, the limits of BFQ for three different CPUs, on,
-respectively, an average laptop, an old desktop, and a cheap embedded
-system, in case full hierarchical support is enabled (i.e.,
-CONFIG_BFQ_GROUP_IOSCHED is set), but CONFIG_BFQ_CGROUP_DEBUG is not
-set (Section 4-2):
-- Intel i7-4850HQ: 400 KIOPS
-- AMD A8-3850: 250 KIOPS
-- ARM CortexTM-A53 Octa-core: 80 KIOPS
-
-If CONFIG_BFQ_CGROUP_DEBUG is set (and of course full hierarchical
-support is enabled), then the sustainable throughput with BFQ
-decreases, because all blkio.bfq* statistics are created and updated
-(Section 4-2). For BFQ, this leads to the following maximum
-sustainable throughputs, on the same systems as above:
-- Intel i7-4850HQ: 310 KIOPS
-- AMD A8-3850: 200 KIOPS
-- ARM CortexTM-A53 Octa-core: 56 KIOPS
-
-BFQ works for multi-queue devices too.
-
-The table of contents follow. Impatients can just jump to Section 3.
-
-CONTENTS
-
-1. When may BFQ be useful?
- 1-1 Personal systems
- 1-2 Server systems
-2. How does BFQ work?
-3. What are BFQ's tunables and how to properly configure BFQ?
-4. BFQ group scheduling
- 4-1 Service guarantees provided
- 4-2 Interface
-
-1. When may BFQ be useful?
-==========================
-
-BFQ provides the following benefits on personal and server systems.
-
-1-1 Personal systems
---------------------
-
-Low latency for interactive applications
-
-Regardless of the actual background workload, BFQ guarantees that, for
-interactive tasks, the storage device is virtually as responsive as if
-it was idle. For example, even if one or more of the following
-background workloads are being executed:
-- one or more large files are being read, written or copied,
-- a tree of source files is being compiled,
-- one or more virtual machines are performing I/O,
-- a software update is in progress,
-- indexing daemons are scanning filesystems and updating their
- databases,
-starting an application or loading a file from within an application
-takes about the same time as if the storage device was idle. As a
-comparison, with CFQ, NOOP or DEADLINE, and in the same conditions,
-applications experience high latencies, or even become unresponsive
-until the background workload terminates (also on SSDs).
-
-Low latency for soft real-time applications
-
-Also soft real-time applications, such as audio and video
-players/streamers, enjoy a low latency and a low drop rate, regardless
-of the background I/O workload. As a consequence, these applications
-do not suffer from almost any glitch due to the background workload.
-
-Higher speed for code-development tasks
-
-If some additional workload happens to be executed in parallel, then
-BFQ executes the I/O-related components of typical code-development
-tasks (compilation, checkout, merge, ...) much more quickly than CFQ,
-NOOP or DEADLINE.
-
-High throughput
-
-On hard disks, BFQ achieves up to 30% higher throughput than CFQ, and
-up to 150% higher throughput than DEADLINE and NOOP, with all the
-sequential workloads considered in our tests. With random workloads,
-and with all the workloads on flash-based devices, BFQ achieves,
-instead, about the same throughput as the other schedulers.
-
-Strong fairness, bandwidth and delay guarantees
-
-BFQ distributes the device throughput, and not just the device time,
-among I/O-bound applications in proportion their weights, with any
-workload and regardless of the device parameters. From these bandwidth
-guarantees, it is possible to compute tight per-I/O-request delay
-guarantees by a simple formula. If not configured for strict service
-guarantees, BFQ switches to time-based resource sharing (only) for
-applications that would otherwise cause a throughput loss.
-
-1-2 Server systems
-------------------
-
-Most benefits for server systems follow from the same service
-properties as above. In particular, regardless of whether additional,
-possibly heavy workloads are being served, BFQ guarantees:
-
-. audio and video-streaming with zero or very low jitter and drop
- rate;
-
-. fast retrieval of WEB pages and embedded objects;
-
-. real-time recording of data in live-dumping applications (e.g.,
- packet logging);
-
-. responsiveness in local and remote access to a server.
-
-
-2. How does BFQ work?
-=====================
-
-BFQ is a proportional-share I/O scheduler, whose general structure,
-plus a lot of code, are borrowed from CFQ.
-
-- Each process doing I/O on a device is associated with a weight and a
- (bfq_)queue.
-
-- BFQ grants exclusive access to the device, for a while, to one queue
- (process) at a time, and implements this service model by
- associating every queue with a budget, measured in number of
- sectors.
-
- - After a queue is granted access to the device, the budget of the
- queue is decremented, on each request dispatch, by the size of the
- request.
-
- - The in-service queue is expired, i.e., its service is suspended,
- only if one of the following events occurs: 1) the queue finishes
- its budget, 2) the queue empties, 3) a "budget timeout" fires.
-
- - The budget timeout prevents processes doing random I/O from
- holding the device for too long and dramatically reducing
- throughput.
-
- - Actually, as in CFQ, a queue associated with a process issuing
- sync requests may not be expired immediately when it empties. In
- contrast, BFQ may idle the device for a short time interval,
- giving the process the chance to go on being served if it issues
- a new request in time. Device idling typically boosts the
- throughput on rotational devices and on non-queueing flash-based
- devices, if processes do synchronous and sequential I/O. In
- addition, under BFQ, device idling is also instrumental in
- guaranteeing the desired throughput fraction to processes
- issuing sync requests (see the description of the slice_idle
- tunable in this document, or [1, 2], for more details).
-
- - With respect to idling for service guarantees, if several
- processes are competing for the device at the same time, but
- all processes and groups have the same weight, then BFQ
- guarantees the expected throughput distribution without ever
- idling the device. Throughput is thus as high as possible in
- this common scenario.
-
- - On flash-based storage with internal queueing of commands
- (typically NCQ), device idling happens to be always detrimental
- for throughput. So, with these devices, BFQ performs idling
- only when strictly needed for service guarantees, i.e., for
- guaranteeing low latency or fairness. In these cases, overall
- throughput may be sub-optimal. No solution currently exists to
- provide both strong service guarantees and optimal throughput
- on devices with internal queueing.
-
- - If low-latency mode is enabled (default configuration), BFQ
- executes some special heuristics to detect interactive and soft
- real-time applications (e.g., video or audio players/streamers),
- and to reduce their latency. The most important action taken to
- achieve this goal is to give to the queues associated with these
- applications more than their fair share of the device
- throughput. For brevity, we call just "weight-raising" the whole
- sets of actions taken by BFQ to privilege these queues. In
- particular, BFQ provides a milder form of weight-raising for
- interactive applications, and a stronger form for soft real-time
- applications.
-
- - BFQ automatically deactivates idling for queues born in a burst of
- queue creations. In fact, these queues are usually associated with
- the processes of applications and services that benefit mostly
- from a high throughput. Examples are systemd during boot, or git
- grep.
-
- - As CFQ, BFQ merges queues performing interleaved I/O, i.e.,
- performing random I/O that becomes mostly sequential if
- merged. Differently from CFQ, BFQ achieves this goal with a more
- reactive mechanism, called Early Queue Merge (EQM). EQM is so
- responsive in detecting interleaved I/O (cooperating processes),
- that it enables BFQ to achieve a high throughput, by queue
- merging, even for queues for which CFQ needs a different
- mechanism, preemption, to get a high throughput. As such EQM is a
- unified mechanism to achieve a high throughput with interleaved
- I/O.
-
- - Queues are scheduled according to a variant of WF2Q+, named
- B-WF2Q+, and implemented using an augmented rb-tree to preserve an
- O(log N) overall complexity. See [2] for more details. B-WF2Q+ is
- also ready for hierarchical scheduling, details in Section 4.
-
- - B-WF2Q+ guarantees a tight deviation with respect to an ideal,
- perfectly fair, and smooth service. In particular, B-WF2Q+
- guarantees that each queue receives a fraction of the device
- throughput proportional to its weight, even if the throughput
- fluctuates, and regardless of: the device parameters, the current
- workload and the budgets assigned to the queue.
-
- - The last, budget-independence, property (although probably
- counterintuitive in the first place) is definitely beneficial, for
- the following reasons:
-
- - First, with any proportional-share scheduler, the maximum
- deviation with respect to an ideal service is proportional to
- the maximum budget (slice) assigned to queues. As a consequence,
- BFQ can keep this deviation tight not only because of the
- accurate service of B-WF2Q+, but also because BFQ *does not*
- need to assign a larger budget to a queue to let the queue
- receive a higher fraction of the device throughput.
-
- - Second, BFQ is free to choose, for every process (queue), the
- budget that best fits the needs of the process, or best
- leverages the I/O pattern of the process. In particular, BFQ
- updates queue budgets with a simple feedback-loop algorithm that
- allows a high throughput to be achieved, while still providing
- tight latency guarantees to time-sensitive applications. When
- the in-service queue expires, this algorithm computes the next
- budget of the queue so as to:
-
- - Let large budgets be eventually assigned to the queues
- associated with I/O-bound applications performing sequential
- I/O: in fact, the longer these applications are served once
- got access to the device, the higher the throughput is.
-
- - Let small budgets be eventually assigned to the queues
- associated with time-sensitive applications (which typically
- perform sporadic and short I/O), because, the smaller the
- budget assigned to a queue waiting for service is, the sooner
- B-WF2Q+ will serve that queue (Subsec 3.3 in [2]).
-
-- If several processes are competing for the device at the same time,
- but all processes and groups have the same weight, then BFQ
- guarantees the expected throughput distribution without ever idling
- the device. It uses preemption instead. Throughput is then much
- higher in this common scenario.
-
-- ioprio classes are served in strict priority order, i.e.,
- lower-priority queues are not served as long as there are
- higher-priority queues. Among queues in the same class, the
- bandwidth is distributed in proportion to the weight of each
- queue. A very thin extra bandwidth is however guaranteed to
- the Idle class, to prevent it from starving.
-
-
-3. What are BFQ's tunables and how to properly configure BFQ?
-=============================================================
-
-Most BFQ tunables affect service guarantees (basically latency and
-fairness) and throughput. For full details on how to choose the
-desired tradeoff between service guarantees and throughput, see the
-parameters slice_idle, strict_guarantees and low_latency. For details
-on how to maximise throughput, see slice_idle, timeout_sync and
-max_budget. The other performance-related parameters have been
-inherited from, and have been preserved mostly for compatibility with
-CFQ. So far, no performance improvement has been reported after
-changing the latter parameters in BFQ.
-
-In particular, the tunables back_seek-max, back_seek_penalty,
-fifo_expire_async and fifo_expire_sync below are the same as in
-CFQ. Their description is just copied from that for CFQ. Some
-considerations in the description of slice_idle are copied from CFQ
-too.
-
-per-process ioprio and weight
------------------------------
-
-Unless the cgroups interface is used (see "4. BFQ group scheduling"),
-weights can be assigned to processes only indirectly, through I/O
-priorities, and according to the relation:
-weight = (IOPRIO_BE_NR - ioprio) * 10.
-
-Beware that, if low-latency is set, then BFQ automatically raises the
-weight of the queues associated with interactive and soft real-time
-applications. Unset this tunable if you need/want to control weights.
-
-slice_idle
-----------
-
-This parameter specifies how long BFQ should idle for next I/O
-request, when certain sync BFQ queues become empty. By default
-slice_idle is a non-zero value. Idling has a double purpose: boosting
-throughput and making sure that the desired throughput distribution is
-respected (see the description of how BFQ works, and, if needed, the
-papers referred there).
-
-As for throughput, idling can be very helpful on highly seeky media
-like single spindle SATA/SAS disks where we can cut down on overall
-number of seeks and see improved throughput.
-
-Setting slice_idle to 0 will remove all the idling on queues and one
-should see an overall improved throughput on faster storage devices
-like multiple SATA/SAS disks in hardware RAID configuration, as well
-as flash-based storage with internal command queueing (and
-parallelism).
-
-So depending on storage and workload, it might be useful to set
-slice_idle=0. In general for SATA/SAS disks and software RAID of
-SATA/SAS disks keeping slice_idle enabled should be useful. For any
-configurations where there are multiple spindles behind single LUN
-(Host based hardware RAID controller or for storage arrays), or with
-flash-based fast storage, setting slice_idle=0 might end up in better
-throughput and acceptable latencies.
-
-Idling is however necessary to have service guarantees enforced in
-case of differentiated weights or differentiated I/O-request lengths.
-To see why, suppose that a given BFQ queue A must get several I/O
-requests served for each request served for another queue B. Idling
-ensures that, if A makes a new I/O request slightly after becoming
-empty, then no request of B is dispatched in the middle, and thus A
-does not lose the possibility to get more than one request dispatched
-before the next request of B is dispatched. Note that idling
-guarantees the desired differentiated treatment of queues only in
-terms of I/O-request dispatches. To guarantee that the actual service
-order then corresponds to the dispatch order, the strict_guarantees
-tunable must be set too.
-
-There is an important flipside for idling: apart from the above cases
-where it is beneficial also for throughput, idling can severely impact
-throughput. One important case is random workload. Because of this
-issue, BFQ tends to avoid idling as much as possible, when it is not
-beneficial also for throughput (as detailed in Section 2). As a
-consequence of this behavior, and of further issues described for the
-strict_guarantees tunable, short-term service guarantees may be
-occasionally violated. And, in some cases, these guarantees may be
-more important than guaranteeing maximum throughput. For example, in
-video playing/streaming, a very low drop rate may be more important
-than maximum throughput. In these cases, consider setting the
-strict_guarantees parameter.
-
-slice_idle_us
--------------
-
-Controls the same tuning parameter as slice_idle, but in microseconds.
-Either tunable can be used to set idling behavior. Afterwards, the
-other tunable will reflect the newly set value in sysfs.
-
-strict_guarantees
------------------
-
-If this parameter is set (default: unset), then BFQ
-
-- always performs idling when the in-service queue becomes empty;
-
-- forces the device to serve one I/O request at a time, by dispatching a
- new request only if there is no outstanding request.
-
-In the presence of differentiated weights or I/O-request sizes, both
-the above conditions are needed to guarantee that every BFQ queue
-receives its allotted share of the bandwidth. The first condition is
-needed for the reasons explained in the description of the slice_idle
-tunable. The second condition is needed because all modern storage
-devices reorder internally-queued requests, which may trivially break
-the service guarantees enforced by the I/O scheduler.
-
-Setting strict_guarantees may evidently affect throughput.
-
-back_seek_max
--------------
-
-This specifies, given in Kbytes, the maximum "distance" for backward seeking.
-The distance is the amount of space from the current head location to the
-sectors that are backward in terms of distance.
-
-This parameter allows the scheduler to anticipate requests in the "backward"
-direction and consider them as being the "next" if they are within this
-distance from the current head location.
-
-back_seek_penalty
------------------
-
-This parameter is used to compute the cost of backward seeking. If the
-backward distance of request is just 1/back_seek_penalty from a "front"
-request, then the seeking cost of two requests is considered equivalent.
-
-So scheduler will not bias toward one or the other request (otherwise scheduler
-will bias toward front request). Default value of back_seek_penalty is 2.
-
-fifo_expire_async
------------------
-
-This parameter is used to set the timeout of asynchronous requests. Default
-value of this is 248ms.
-
-fifo_expire_sync
-----------------
-
-This parameter is used to set the timeout of synchronous requests. Default
-value of this is 124ms. In case to favor synchronous requests over asynchronous
-one, this value should be decreased relative to fifo_expire_async.
-
-low_latency
------------
-
-This parameter is used to enable/disable BFQ's low latency mode. By
-default, low latency mode is enabled. If enabled, interactive and soft
-real-time applications are privileged and experience a lower latency,
-as explained in more detail in the description of how BFQ works.
-
-DISABLE this mode if you need full control on bandwidth
-distribution. In fact, if it is enabled, then BFQ automatically
-increases the bandwidth share of privileged applications, as the main
-means to guarantee a lower latency to them.
-
-In addition, as already highlighted at the beginning of this document,
-DISABLE this mode if your only goal is to achieve a high throughput.
-In fact, privileging the I/O of some application over the rest may
-entail a lower throughput. To achieve the highest-possible throughput
-on a non-rotational device, setting slice_idle to 0 may be needed too
-(at the cost of giving up any strong guarantee on fairness and low
-latency).
-
-timeout_sync
-------------
-
-Maximum amount of device time that can be given to a task (queue) once
-it has been selected for service. On devices with costly seeks,
-increasing this time usually increases maximum throughput. On the
-opposite end, increasing this time coarsens the granularity of the
-short-term bandwidth and latency guarantees, especially if the
-following parameter is set to zero.
-
-max_budget
-----------
-
-Maximum amount of service, measured in sectors, that can be provided
-to a BFQ queue once it is set in service (of course within the limits
-of the above timeout). According to what said in the description of
-the algorithm, larger values increase the throughput in proportion to
-the percentage of sequential I/O requests issued. The price of larger
-values is that they coarsen the granularity of short-term bandwidth
-and latency guarantees.
-
-The default value is 0, which enables auto-tuning: BFQ sets max_budget
-to the maximum number of sectors that can be served during
-timeout_sync, according to the estimated peak rate.
-
-For specific devices, some users have occasionally reported to have
-reached a higher throughput by setting max_budget explicitly, i.e., by
-setting max_budget to a higher value than 0. In particular, they have
-set max_budget to higher values than those to which BFQ would have set
-it with auto-tuning. An alternative way to achieve this goal is to
-just increase the value of timeout_sync, leaving max_budget equal to 0.
-
-weights
--------
-
-Read-only parameter, used to show the weights of the currently active
-BFQ queues.
-
-
-4. Group scheduling with BFQ
-============================
-
-BFQ supports both cgroups-v1 and cgroups-v2 io controllers, namely
-blkio and io. In particular, BFQ supports weight-based proportional
-share. To activate cgroups support, set BFQ_GROUP_IOSCHED.
-
-4-1 Service guarantees provided
--------------------------------
-
-With BFQ, proportional share means true proportional share of the
-device bandwidth, according to group weights. For example, a group
-with weight 200 gets twice the bandwidth, and not just twice the time,
-of a group with weight 100.
-
-BFQ supports hierarchies (group trees) of any depth. Bandwidth is
-distributed among groups and processes in the expected way: for each
-group, the children of the group share the whole bandwidth of the
-group in proportion to their weights. In particular, this implies
-that, for each leaf group, every process of the group receives the
-same share of the whole group bandwidth, unless the ioprio of the
-process is modified.
-
-The resource-sharing guarantee for a group may partially or totally
-switch from bandwidth to time, if providing bandwidth guarantees to
-the group lowers the throughput too much. This switch occurs on a
-per-process basis: if a process of a leaf group causes throughput loss
-if served in such a way to receive its share of the bandwidth, then
-BFQ switches back to just time-based proportional share for that
-process.
-
-4-2 Interface
--------------
-
-To get proportional sharing of bandwidth with BFQ for a given device,
-BFQ must of course be the active scheduler for that device.
-
-Within each group directory, the names of the files associated with
-BFQ-specific cgroup parameters and stats begin with the "bfq."
-prefix. So, with cgroups-v1 or cgroups-v2, the full prefix for
-BFQ-specific files is "blkio.bfq." or "io.bfq." For example, the group
-parameter to set the weight of a group with BFQ is blkio.bfq.weight
-or io.bfq.weight.
-
-As for cgroups-v1 (blkio controller), the exact set of stat files
-created, and kept up-to-date by bfq, depends on whether
-CONFIG_BFQ_CGROUP_DEBUG is set. If it is set, then bfq creates all
-the stat files documented in
-Documentation/cgroup-v1/blkio-controller.rst. If, instead,
-CONFIG_BFQ_CGROUP_DEBUG is not set, then bfq creates only the files
-blkio.bfq.io_service_bytes
-blkio.bfq.io_service_bytes_recursive
-blkio.bfq.io_serviced
-blkio.bfq.io_serviced_recursive
-
-The value of CONFIG_BFQ_CGROUP_DEBUG greatly influences the maximum
-throughput sustainable with bfq, because updating the blkio.bfq.*
-stats is rather costly, especially for some of the stats enabled by
-CONFIG_BFQ_CGROUP_DEBUG.
-
-Parameters to set
------------------
-
-For each group, there is only the following parameter to set.
-
-weight (namely blkio.bfq.weight or io.bfq-weight): the weight of the
-group inside its parent. Available values: 1..10000 (default 100). The
-linear mapping between ioprio and weights, described at the beginning
-of the tunable section, is still valid, but all weights higher than
-IOPRIO_BE_NR*10 are mapped to ioprio 0.
-
-Recall that, if low-latency is set, then BFQ automatically raises the
-weight of the queues associated with interactive and soft real-time
-applications. Unset this tunable if you need/want to control weights.
-
-
-[1] P. Valente, A. Avanzini, "Evolution of the BFQ Storage I/O
- Scheduler", Proceedings of the First Workshop on Mobile System
- Technologies (MST-2015), May 2015.
- http://algogroup.unimore.it/people/paolo/disk_sched/mst-2015.pdf
-
-[2] P. Valente and M. Andreolini, "Improving Application
- Responsiveness with the BFQ Disk I/O Scheduler", Proceedings of
- the 5th Annual International Systems and Storage Conference
- (SYSTOR '12), June 2012.
- Slightly extended version:
- http://algogroup.unimore.it/people/paolo/disk_sched/bfq-v1-suite-
- results.pdf
-
-[3] https://github.com/Algodev-github/S
--- /dev/null
+=====================================================
+Notes on the Generic Block Layer Rewrite in Linux 2.5
+=====================================================
+
+.. note::
+
+ It seems that there are lot of outdated stuff here. This seems
+ to be written somewhat as a task list. Yet, eventually, something
+ here might still be useful.
+
+Notes Written on Jan 15, 2002:
+ - Jens Axboe <jens.axboe@oracle.com>
+ - Suparna Bhattacharya <suparna@in.ibm.com>
+
+Last Updated May 2, 2002
+
+September 2003: Updated I/O Scheduler portions
+ - Nick Piggin <npiggin@kernel.dk>
+
+Introduction
+============
+
+These are some notes describing some aspects of the 2.5 block layer in the
+context of the bio rewrite. The idea is to bring out some of the key
+changes and a glimpse of the rationale behind those changes.
+
+Please mail corrections & suggestions to suparna@in.ibm.com.
+
+Credits
+=======
+
+2.5 bio rewrite:
+ - Jens Axboe <jens.axboe@oracle.com>
+
+Many aspects of the generic block layer redesign were driven by and evolved
+over discussions, prior patches and the collective experience of several
+people. See sections 8 and 9 for a list of some related references.
+
+The following people helped with review comments and inputs for this
+document:
+
+ - Christoph Hellwig <hch@infradead.org>
+ - Arjan van de Ven <arjanv@redhat.com>
+ - Randy Dunlap <rdunlap@xenotime.net>
+ - Andre Hedrick <andre@linux-ide.org>
+
+The following people helped with fixes/contributions to the bio patches
+while it was still work-in-progress:
+
+ - David S. Miller <davem@redhat.com>
+
+
+.. Description of Contents:
+
+ 1. Scope for tuning of logic to various needs
+ 1.1 Tuning based on device or low level driver capabilities
+ - Per-queue parameters
+ - Highmem I/O support
+ - I/O scheduler modularization
+ 1.2 Tuning based on high level requirements/capabilities
+ 1.2.1 Request Priority/Latency
+ 1.3 Direct access/bypass to lower layers for diagnostics and special
+ device operations
+ 1.3.1 Pre-built commands
+ 2. New flexible and generic but minimalist i/o structure or descriptor
+ (instead of using buffer heads at the i/o layer)
+ 2.1 Requirements/Goals addressed
+ 2.2 The bio struct in detail (multi-page io unit)
+ 2.3 Changes in the request structure
+ 3. Using bios
+ 3.1 Setup/teardown (allocation, splitting)
+ 3.2 Generic bio helper routines
+ 3.2.1 Traversing segments and completion units in a request
+ 3.2.2 Setting up DMA scatterlists
+ 3.2.3 I/O completion
+ 3.2.4 Implications for drivers that do not interpret bios (don't handle
+ multiple segments)
+ 3.3 I/O submission
+ 4. The I/O scheduler
+ 5. Scalability related changes
+ 5.1 Granular locking: Removal of io_request_lock
+ 5.2 Prepare for transition to 64 bit sector_t
+ 6. Other Changes/Implications
+ 6.1 Partition re-mapping handled by the generic block layer
+ 7. A few tips on migration of older drivers
+ 8. A list of prior/related/impacted patches/ideas
+ 9. Other References/Discussion Threads
+
+
+Bio Notes
+=========
+
+Let us discuss the changes in the context of how some overall goals for the
+block layer are addressed.
+
+1. Scope for tuning the generic logic to satisfy various requirements
+=====================================================================
+
+The block layer design supports adaptable abstractions to handle common
+processing with the ability to tune the logic to an appropriate extent
+depending on the nature of the device and the requirements of the caller.
+One of the objectives of the rewrite was to increase the degree of tunability
+and to enable higher level code to utilize underlying device/driver
+capabilities to the maximum extent for better i/o performance. This is
+important especially in the light of ever improving hardware capabilities
+and application/middleware software designed to take advantage of these
+capabilities.
+
+1.1 Tuning based on low level device / driver capabilities
+----------------------------------------------------------
+
+Sophisticated devices with large built-in caches, intelligent i/o scheduling
+optimizations, high memory DMA support, etc may find some of the
+generic processing an overhead, while for less capable devices the
+generic functionality is essential for performance or correctness reasons.
+Knowledge of some of the capabilities or parameters of the device should be
+used at the generic block layer to take the right decisions on
+behalf of the driver.
+
+How is this achieved ?
+
+Tuning at a per-queue level:
+
+i. Per-queue limits/values exported to the generic layer by the driver
+
+Various parameters that the generic i/o scheduler logic uses are set at
+a per-queue level (e.g maximum request size, maximum number of segments in
+a scatter-gather list, logical block size)
+
+Some parameters that were earlier available as global arrays indexed by
+major/minor are now directly associated with the queue. Some of these may
+move into the block device structure in the future. Some characteristics
+have been incorporated into a queue flags field rather than separate fields
+in themselves. There are blk_queue_xxx functions to set the parameters,
+rather than update the fields directly
+
+Some new queue property settings:
+
+ blk_queue_bounce_limit(q, u64 dma_address)
+ Enable I/O to highmem pages, dma_address being the
+ limit. No highmem default.
+
+ blk_queue_max_sectors(q, max_sectors)
+ Sets two variables that limit the size of the request.
+
+ - The request queue's max_sectors, which is a soft size in
+ units of 512 byte sectors, and could be dynamically varied
+ by the core kernel.
+
+ - The request queue's max_hw_sectors, which is a hard limit
+ and reflects the maximum size request a driver can handle
+ in units of 512 byte sectors.
+
+ The default for both max_sectors and max_hw_sectors is
+ 255. The upper limit of max_sectors is 1024.
+
+ blk_queue_max_phys_segments(q, max_segments)
+ Maximum physical segments you can handle in a request. 128
+ default (driver limit). (See 3.2.2)
+
+ blk_queue_max_hw_segments(q, max_segments)
+ Maximum dma segments the hardware can handle in a request. 128
+ default (host adapter limit, after dma remapping).
+ (See 3.2.2)
+
+ blk_queue_max_segment_size(q, max_seg_size)
+ Maximum size of a clustered segment, 64kB default.
+
+ blk_queue_logical_block_size(q, logical_block_size)
+ Lowest possible sector size that the hardware can operate
+ on, 512 bytes default.
+
+New queue flags:
+
+ QUEUE_FLAG_CLUSTER (see 3.2.2)
+ QUEUE_FLAG_QUEUED (see 3.2.4)
+
+
+ii. High-mem i/o capabilities are now considered the default
+
+The generic bounce buffer logic, present in 2.4, where the block layer would
+by default copyin/out i/o requests on high-memory buffers to low-memory buffers
+assuming that the driver wouldn't be able to handle it directly, has been
+changed in 2.5. The bounce logic is now applied only for memory ranges
+for which the device cannot handle i/o. A driver can specify this by
+setting the queue bounce limit for the request queue for the device
+(blk_queue_bounce_limit()). This avoids the inefficiencies of the copyin/out
+where a device is capable of handling high memory i/o.
+
+In order to enable high-memory i/o where the device is capable of supporting
+it, the pci dma mapping routines and associated data structures have now been
+modified to accomplish a direct page -> bus translation, without requiring
+a virtual address mapping (unlike the earlier scheme of virtual address
+-> bus translation). So this works uniformly for high-memory pages (which
+do not have a corresponding kernel virtual address space mapping) and
+low-memory pages.
+
+Note: Please refer to Documentation/DMA-API-HOWTO.txt for a discussion
+on PCI high mem DMA aspects and mapping of scatter gather lists, and support
+for 64 bit PCI.
+
+Special handling is required only for cases where i/o needs to happen on
+pages at physical memory addresses beyond what the device can support. In these
+cases, a bounce bio representing a buffer from the supported memory range
+is used for performing the i/o with copyin/copyout as needed depending on
+the type of the operation. For example, in case of a read operation, the
+data read has to be copied to the original buffer on i/o completion, so a
+callback routine is set up to do this, while for write, the data is copied
+from the original buffer to the bounce buffer prior to issuing the
+operation. Since an original buffer may be in a high memory area that's not
+mapped in kernel virtual addr, a kmap operation may be required for
+performing the copy, and special care may be needed in the completion path
+as it may not be in irq context. Special care is also required (by way of
+GFP flags) when allocating bounce buffers, to avoid certain highmem
+deadlock possibilities.
+
+It is also possible that a bounce buffer may be allocated from high-memory
+area that's not mapped in kernel virtual addr, but within the range that the
+device can use directly; so the bounce page may need to be kmapped during
+copy operations. [Note: This does not hold in the current implementation,
+though]
+
+There are some situations when pages from high memory may need to
+be kmapped, even if bounce buffers are not necessary. For example a device
+may need to abort DMA operations and revert to PIO for the transfer, in
+which case a virtual mapping of the page is required. For SCSI it is also
+done in some scenarios where the low level driver cannot be trusted to
+handle a single sg entry correctly. The driver is expected to perform the
+kmaps as needed on such occasions as appropriate. A driver could also use
+the blk_queue_bounce() routine on its own to bounce highmem i/o to low
+memory for specific requests if so desired.
+
+iii. The i/o scheduler algorithm itself can be replaced/set as appropriate
+
+As in 2.4, it is possible to plugin a brand new i/o scheduler for a particular
+queue or pick from (copy) existing generic schedulers and replace/override
+certain portions of it. The 2.5 rewrite provides improved modularization
+of the i/o scheduler. There are more pluggable callbacks, e.g for init,
+add request, extract request, which makes it possible to abstract specific
+i/o scheduling algorithm aspects and details outside of the generic loop.
+It also makes it possible to completely hide the implementation details of
+the i/o scheduler from block drivers.
+
+I/O scheduler wrappers are to be used instead of accessing the queue directly.
+See section 4. The I/O scheduler for details.
+
+1.2 Tuning Based on High level code capabilities
+------------------------------------------------
+
+i. Application capabilities for raw i/o
+
+This comes from some of the high-performance database/middleware
+requirements where an application prefers to make its own i/o scheduling
+decisions based on an understanding of the access patterns and i/o
+characteristics
+
+ii. High performance filesystems or other higher level kernel code's
+capabilities
+
+Kernel components like filesystems could also take their own i/o scheduling
+decisions for optimizing performance. Journalling filesystems may need
+some control over i/o ordering.
+
+What kind of support exists at the generic block layer for this ?
+
+The flags and rw fields in the bio structure can be used for some tuning
+from above e.g indicating that an i/o is just a readahead request, or priority
+settings (currently unused). As far as user applications are concerned they
+would need an additional mechanism either via open flags or ioctls, or some
+other upper level mechanism to communicate such settings to block.
+
+1.2.1 Request Priority/Latency
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Todo/Under discussion::
+
+ Arjan's proposed request priority scheme allows higher levels some broad
+ control (high/med/low) over the priority of an i/o request vs other pending
+ requests in the queue. For example it allows reads for bringing in an
+ executable page on demand to be given a higher priority over pending write
+ requests which haven't aged too much on the queue. Potentially this priority
+ could even be exposed to applications in some manner, providing higher level
+ tunability. Time based aging avoids starvation of lower priority
+ requests. Some bits in the bi_opf flags field in the bio structure are
+ intended to be used for this priority information.
+
+
+1.3 Direct Access to Low level Device/Driver Capabilities (Bypass mode)
+-----------------------------------------------------------------------
+
+(e.g Diagnostics, Systems Management)
+
+There are situations where high-level code needs to have direct access to
+the low level device capabilities or requires the ability to issue commands
+to the device bypassing some of the intermediate i/o layers.
+These could, for example, be special control commands issued through ioctl
+interfaces, or could be raw read/write commands that stress the drive's
+capabilities for certain kinds of fitness tests. Having direct interfaces at
+multiple levels without having to pass through upper layers makes
+it possible to perform bottom up validation of the i/o path, layer by
+layer, starting from the media.
+
+The normal i/o submission interfaces, e.g submit_bio, could be bypassed
+for specially crafted requests which such ioctl or diagnostics
+interfaces would typically use, and the elevator add_request routine
+can instead be used to directly insert such requests in the queue or preferably
+the blk_do_rq routine can be used to place the request on the queue and
+wait for completion. Alternatively, sometimes the caller might just
+invoke a lower level driver specific interface with the request as a
+parameter.
+
+If the request is a means for passing on special information associated with
+the command, then such information is associated with the request->special
+field (rather than misuse the request->buffer field which is meant for the
+request data buffer's virtual mapping).
+
+For passing request data, the caller must build up a bio descriptor
+representing the concerned memory buffer if the underlying driver interprets
+bio segments or uses the block layer end*request* functions for i/o
+completion. Alternatively one could directly use the request->buffer field to
+specify the virtual address of the buffer, if the driver expects buffer
+addresses passed in this way and ignores bio entries for the request type
+involved. In the latter case, the driver would modify and manage the
+request->buffer, request->sector and request->nr_sectors or
+request->current_nr_sectors fields itself rather than using the block layer
+end_request or end_that_request_first completion interfaces.
+(See 2.3 or Documentation/block/request.rst for a brief explanation of
+the request structure fields)
+
+::
+
+ [TBD: end_that_request_last should be usable even in this case;
+ Perhaps an end_that_direct_request_first routine could be implemented to make
+ handling direct requests easier for such drivers; Also for drivers that
+ expect bios, a helper function could be provided for setting up a bio
+ corresponding to a data buffer]
+
+ <JENS: I dont understand the above, why is end_that_request_first() not
+ usable? Or _last for that matter. I must be missing something>
+
+ <SUP: What I meant here was that if the request doesn't have a bio, then
+ end_that_request_first doesn't modify nr_sectors or current_nr_sectors,
+ and hence can't be used for advancing request state settings on the
+ completion of partial transfers. The driver has to modify these fields
+ directly by hand.
+ This is because end_that_request_first only iterates over the bio list,
+ and always returns 0 if there are none associated with the request.
+ _last works OK in this case, and is not a problem, as I mentioned earlier
+ >
+
+1.3.1 Pre-built Commands
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+A request can be created with a pre-built custom command to be sent directly
+to the device. The cmd block in the request structure has room for filling
+in the command bytes. (i.e rq->cmd is now 16 bytes in size, and meant for
+command pre-building, and the type of the request is now indicated
+through rq->flags instead of via rq->cmd)
+
+The request structure flags can be set up to indicate the type of request
+in such cases (REQ_PC: direct packet command passed to driver, REQ_BLOCK_PC:
+packet command issued via blk_do_rq, REQ_SPECIAL: special request).
+
+It can help to pre-build device commands for requests in advance.
+Drivers can now specify a request prepare function (q->prep_rq_fn) that the
+block layer would invoke to pre-build device commands for a given request,
+or perform other preparatory processing for the request. This is routine is
+called by elv_next_request(), i.e. typically just before servicing a request.
+(The prepare function would not be called for requests that have RQF_DONTPREP
+enabled)
+
+Aside:
+ Pre-building could possibly even be done early, i.e before placing the
+ request on the queue, rather than construct the command on the fly in the
+ driver while servicing the request queue when it may affect latencies in
+ interrupt context or responsiveness in general. One way to add early
+ pre-building would be to do it whenever we fail to merge on a request.
+ Now REQ_NOMERGE is set in the request flags to skip this one in the future,
+ which means that it will not change before we feed it to the device. So
+ the pre-builder hook can be invoked there.
+
+
+2. Flexible and generic but minimalist i/o structure/descriptor
+===============================================================
+
+2.1 Reason for a new structure and requirements addressed
+---------------------------------------------------------
+
+Prior to 2.5, buffer heads were used as the unit of i/o at the generic block
+layer, and the low level request structure was associated with a chain of
+buffer heads for a contiguous i/o request. This led to certain inefficiencies
+when it came to large i/o requests and readv/writev style operations, as it
+forced such requests to be broken up into small chunks before being passed
+on to the generic block layer, only to be merged by the i/o scheduler
+when the underlying device was capable of handling the i/o in one shot.
+Also, using the buffer head as an i/o structure for i/os that didn't originate
+from the buffer cache unnecessarily added to the weight of the descriptors
+which were generated for each such chunk.
+
+The following were some of the goals and expectations considered in the
+redesign of the block i/o data structure in 2.5.
+
+1. Should be appropriate as a descriptor for both raw and buffered i/o -
+ avoid cache related fields which are irrelevant in the direct/page i/o path,
+ or filesystem block size alignment restrictions which may not be relevant
+ for raw i/o.
+2. Ability to represent high-memory buffers (which do not have a virtual
+ address mapping in kernel address space).
+3. Ability to represent large i/os w/o unnecessarily breaking them up (i.e
+ greater than PAGE_SIZE chunks in one shot)
+4. At the same time, ability to retain independent identity of i/os from
+ different sources or i/o units requiring individual completion (e.g. for
+ latency reasons)
+5. Ability to represent an i/o involving multiple physical memory segments
+ (including non-page aligned page fragments, as specified via readv/writev)
+ without unnecessarily breaking it up, if the underlying device is capable of
+ handling it.
+6. Preferably should be based on a memory descriptor structure that can be
+ passed around different types of subsystems or layers, maybe even
+ networking, without duplication or extra copies of data/descriptor fields
+ themselves in the process
+7. Ability to handle the possibility of splits/merges as the structure passes
+ through layered drivers (lvm, md, evms), with minimal overhead.
+
+The solution was to define a new structure (bio) for the block layer,
+instead of using the buffer head structure (bh) directly, the idea being
+avoidance of some associated baggage and limitations. The bio structure
+is uniformly used for all i/o at the block layer ; it forms a part of the
+bh structure for buffered i/o, and in the case of raw/direct i/o kiobufs are
+mapped to bio structures.
+
+2.2 The bio struct
+------------------
+
+The bio structure uses a vector representation pointing to an array of tuples
+of <page, offset, len> to describe the i/o buffer, and has various other
+fields describing i/o parameters and state that needs to be maintained for
+performing the i/o.
+
+Notice that this representation means that a bio has no virtual address
+mapping at all (unlike buffer heads).
+
+::
+
+ struct bio_vec {
+ struct page *bv_page;
+ unsigned short bv_len;
+ unsigned short bv_offset;
+ };
+
+ /*
+ * main unit of I/O for the block layer and lower layers (ie drivers)
+ */
+ struct bio {
+ struct bio *bi_next; /* request queue link */
+ struct block_device *bi_bdev; /* target device */
+ unsigned long bi_flags; /* status, command, etc */
+ unsigned long bi_opf; /* low bits: r/w, high: priority */
+
+ unsigned int bi_vcnt; /* how may bio_vec's */
+ struct bvec_iter bi_iter; /* current index into bio_vec array */
+
+ unsigned int bi_size; /* total size in bytes */
+ unsigned short bi_hw_segments; /* segments after DMA remapping */
+ unsigned int bi_max; /* max bio_vecs we can hold
+ used as index into pool */
+ struct bio_vec *bi_io_vec; /* the actual vec list */
+ bio_end_io_t *bi_end_io; /* bi_end_io (bio) */
+ atomic_t bi_cnt; /* pin count: free when it hits zero */
+ void *bi_private;
+ };
+
+With this multipage bio design:
+
+- Large i/os can be sent down in one go using a bio_vec list consisting
+ of an array of <page, offset, len> fragments (similar to the way fragments
+ are represented in the zero-copy network code)
+- Splitting of an i/o request across multiple devices (as in the case of
+ lvm or raid) is achieved by cloning the bio (where the clone points to
+ the same bi_io_vec array, but with the index and size accordingly modified)
+- A linked list of bios is used as before for unrelated merges [*]_ - this
+ avoids reallocs and makes independent completions easier to handle.
+- Code that traverses the req list can find all the segments of a bio
+ by using rq_for_each_segment. This handles the fact that a request
+ has multiple bios, each of which can have multiple segments.
+- Drivers which can't process a large bio in one shot can use the bi_iter
+ field to keep track of the next bio_vec entry to process.
+ (e.g a 1MB bio_vec needs to be handled in max 128kB chunks for IDE)
+ [TBD: Should preferably also have a bi_voffset and bi_vlen to avoid modifying
+ bi_offset an len fields]
+
+.. [*]
+
+ unrelated merges -- a request ends up containing two or more bios that
+ didn't originate from the same place.
+
+bi_end_io() i/o callback gets called on i/o completion of the entire bio.
+
+At a lower level, drivers build a scatter gather list from the merged bios.
+The scatter gather list is in the form of an array of <page, offset, len>
+entries with their corresponding dma address mappings filled in at the
+appropriate time. As an optimization, contiguous physical pages can be
+covered by a single entry where <page> refers to the first page and <len>
+covers the range of pages (up to 16 contiguous pages could be covered this
+way). There is a helper routine (blk_rq_map_sg) which drivers can use to build
+the sg list.
+
+Note: Right now the only user of bios with more than one page is ll_rw_kio,
+which in turn means that only raw I/O uses it (direct i/o may not work
+right now). The intent however is to enable clustering of pages etc to
+become possible. The pagebuf abstraction layer from SGI also uses multi-page
+bios, but that is currently not included in the stock development kernels.
+The same is true of Andrew Morton's work-in-progress multipage bio writeout
+and readahead patches.
+
+2.3 Changes in the Request Structure
+------------------------------------
+
+The request structure is the structure that gets passed down to low level
+drivers. The block layer make_request function builds up a request structure,
+places it on the queue and invokes the drivers request_fn. The driver makes
+use of block layer helper routine elv_next_request to pull the next request
+off the queue. Control or diagnostic functions might bypass block and directly
+invoke underlying driver entry points passing in a specially constructed
+request structure.
+
+Only some relevant fields (mainly those which changed or may be referred
+to in some of the discussion here) are listed below, not necessarily in
+the order in which they occur in the structure (see include/linux/blkdev.h)
+Refer to Documentation/block/request.rst for details about all the request
+structure fields and a quick reference about the layers which are
+supposed to use or modify those fields::
+
+ struct request {
+ struct list_head queuelist; /* Not meant to be directly accessed by
+ the driver.
+ Used by q->elv_next_request_fn
+ rq->queue is gone
+ */
+ .
+ .
+ unsigned char cmd[16]; /* prebuilt command data block */
+ unsigned long flags; /* also includes earlier rq->cmd settings */
+ .
+ .
+ sector_t sector; /* this field is now of type sector_t instead of int
+ preparation for 64 bit sectors */
+ .
+ .
+
+ /* Number of scatter-gather DMA addr+len pairs after
+ * physical address coalescing is performed.
+ */
+ unsigned short nr_phys_segments;
+
+ /* Number of scatter-gather addr+len pairs after
+ * physical and DMA remapping hardware coalescing is performed.
+ * This is the number of scatter-gather entries the driver
+ * will actually have to deal with after DMA mapping is done.
+ */
+ unsigned short nr_hw_segments;
+
+ /* Various sector counts */
+ unsigned long nr_sectors; /* no. of sectors left: driver modifiable */
+ unsigned long hard_nr_sectors; /* block internal copy of above */
+ unsigned int current_nr_sectors; /* no. of sectors left in the
+ current segment:driver modifiable */
+ unsigned long hard_cur_sectors; /* block internal copy of the above */
+ .
+ .
+ int tag; /* command tag associated with request */
+ void *special; /* same as before */
+ char *buffer; /* valid only for low memory buffers up to
+ current_nr_sectors */
+ .
+ .
+ struct bio *bio, *biotail; /* bio list instead of bh */
+ struct request_list *rl;
+ }
+
+See the req_ops and req_flag_bits definitions for an explanation of the various
+flags available. Some bits are used by the block layer or i/o scheduler.
+
+The behaviour of the various sector counts are almost the same as before,
+except that since we have multi-segment bios, current_nr_sectors refers
+to the numbers of sectors in the current segment being processed which could
+be one of the many segments in the current bio (i.e i/o completion unit).
+The nr_sectors value refers to the total number of sectors in the whole
+request that remain to be transferred (no change). The purpose of the
+hard_xxx values is for block to remember these counts every time it hands
+over the request to the driver. These values are updated by block on
+end_that_request_first, i.e. every time the driver completes a part of the
+transfer and invokes block end*request helpers to mark this. The
+driver should not modify these values. The block layer sets up the
+nr_sectors and current_nr_sectors fields (based on the corresponding
+hard_xxx values and the number of bytes transferred) and updates it on
+every transfer that invokes end_that_request_first. It does the same for the
+buffer, bio, bio->bi_iter fields too.
+
+The buffer field is just a virtual address mapping of the current segment
+of the i/o buffer in cases where the buffer resides in low-memory. For high
+memory i/o, this field is not valid and must not be used by drivers.
+
+Code that sets up its own request structures and passes them down to
+a driver needs to be careful about interoperation with the block layer helper
+functions which the driver uses. (Section 1.3)
+
+3. Using bios
+=============
+
+3.1 Setup/Teardown
+------------------
+
+There are routines for managing the allocation, and reference counting, and
+freeing of bios (bio_alloc, bio_get, bio_put).
+
+This makes use of Ingo Molnar's mempool implementation, which enables
+subsystems like bio to maintain their own reserve memory pools for guaranteed
+deadlock-free allocations during extreme VM load. For example, the VM
+subsystem makes use of the block layer to writeout dirty pages in order to be
+able to free up memory space, a case which needs careful handling. The
+allocation logic draws from the preallocated emergency reserve in situations
+where it cannot allocate through normal means. If the pool is empty and it
+can wait, then it would trigger action that would help free up memory or
+replenish the pool (without deadlocking) and wait for availability in the pool.
+If it is in IRQ context, and hence not in a position to do this, allocation
+could fail if the pool is empty. In general mempool always first tries to
+perform allocation without having to wait, even if it means digging into the
+pool as long it is not less that 50% full.
+
+On a free, memory is released to the pool or directly freed depending on
+the current availability in the pool. The mempool interface lets the
+subsystem specify the routines to be used for normal alloc and free. In the
+case of bio, these routines make use of the standard slab allocator.
+
+The caller of bio_alloc is expected to taken certain steps to avoid
+deadlocks, e.g. avoid trying to allocate more memory from the pool while
+already holding memory obtained from the pool.
+
+::
+
+ [TBD: This is a potential issue, though a rare possibility
+ in the bounce bio allocation that happens in the current code, since
+ it ends up allocating a second bio from the same pool while
+ holding the original bio ]
+
+Memory allocated from the pool should be released back within a limited
+amount of time (in the case of bio, that would be after the i/o is completed).
+This ensures that if part of the pool has been used up, some work (in this
+case i/o) must already be in progress and memory would be available when it
+is over. If allocating from multiple pools in the same code path, the order
+or hierarchy of allocation needs to be consistent, just the way one deals
+with multiple locks.
+
+The bio_alloc routine also needs to allocate the bio_vec_list (bvec_alloc())
+for a non-clone bio. There are the 6 pools setup for different size biovecs,
+so bio_alloc(gfp_mask, nr_iovecs) will allocate a vec_list of the
+given size from these slabs.
+
+The bio_get() routine may be used to hold an extra reference on a bio prior
+to i/o submission, if the bio fields are likely to be accessed after the
+i/o is issued (since the bio may otherwise get freed in case i/o completion
+happens in the meantime).
+
+The bio_clone_fast() routine may be used to duplicate a bio, where the clone
+shares the bio_vec_list with the original bio (i.e. both point to the
+same bio_vec_list). This would typically be used for splitting i/o requests
+in lvm or md.
+
+3.2 Generic bio helper Routines
+-------------------------------
+
+3.2.1 Traversing segments and completion units in a request
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The macro rq_for_each_segment() should be used for traversing the bios
+in the request list (drivers should avoid directly trying to do it
+themselves). Using these helpers should also make it easier to cope
+with block changes in the future.
+
+::
+
+ struct req_iterator iter;
+ rq_for_each_segment(bio_vec, rq, iter)
+ /* bio_vec is now current segment */
+
+I/O completion callbacks are per-bio rather than per-segment, so drivers
+that traverse bio chains on completion need to keep that in mind. Drivers
+which don't make a distinction between segments and completion units would
+need to be reorganized to support multi-segment bios.
+
+3.2.2 Setting up DMA scatterlists
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The blk_rq_map_sg() helper routine would be used for setting up scatter
+gather lists from a request, so a driver need not do it on its own.
+
+ nr_segments = blk_rq_map_sg(q, rq, scatterlist);
+
+The helper routine provides a level of abstraction which makes it easier
+to modify the internals of request to scatterlist conversion down the line
+without breaking drivers. The blk_rq_map_sg routine takes care of several
+things like collapsing physically contiguous segments (if QUEUE_FLAG_CLUSTER
+is set) and correct segment accounting to avoid exceeding the limits which
+the i/o hardware can handle, based on various queue properties.
+
+- Prevents a clustered segment from crossing a 4GB mem boundary
+- Avoids building segments that would exceed the number of physical
+ memory segments that the driver can handle (phys_segments) and the
+ number that the underlying hardware can handle at once, accounting for
+ DMA remapping (hw_segments) (i.e. IOMMU aware limits).
+
+Routines which the low level driver can use to set up the segment limits:
+
+blk_queue_max_hw_segments() : Sets an upper limit of the maximum number of
+hw data segments in a request (i.e. the maximum number of address/length
+pairs the host adapter can actually hand to the device at once)
+
+blk_queue_max_phys_segments() : Sets an upper limit on the maximum number
+of physical data segments in a request (i.e. the largest sized scatter list
+a driver could handle)
+
+3.2.3 I/O completion
+^^^^^^^^^^^^^^^^^^^^
+
+The existing generic block layer helper routines end_request,
+end_that_request_first and end_that_request_last can be used for i/o
+completion (and setting things up so the rest of the i/o or the next
+request can be kicked of) as before. With the introduction of multi-page
+bio support, end_that_request_first requires an additional argument indicating
+the number of sectors completed.
+
+3.2.4 Implications for drivers that do not interpret bios
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+(don't handle multiple segments)
+
+Drivers that do not interpret bios e.g those which do not handle multiple
+segments and do not support i/o into high memory addresses (require bounce
+buffers) and expect only virtually mapped buffers, can access the rq->buffer
+field. As before the driver should use current_nr_sectors to determine the
+size of remaining data in the current segment (that is the maximum it can
+transfer in one go unless it interprets segments), and rely on the block layer
+end_request, or end_that_request_first/last to take care of all accounting
+and transparent mapping of the next bio segment when a segment boundary
+is crossed on completion of a transfer. (The end*request* functions should
+be used if only if the request has come down from block/bio path, not for
+direct access requests which only specify rq->buffer without a valid rq->bio)
+
+3.3 I/O Submission
+------------------
+
+The routine submit_bio() is used to submit a single io. Higher level i/o
+routines make use of this:
+
+(a) Buffered i/o:
+
+The routine submit_bh() invokes submit_bio() on a bio corresponding to the
+bh, allocating the bio if required. ll_rw_block() uses submit_bh() as before.
+
+(b) Kiobuf i/o (for raw/direct i/o):
+
+The ll_rw_kio() routine breaks up the kiobuf into page sized chunks and
+maps the array to one or more multi-page bios, issuing submit_bio() to
+perform the i/o on each of these.
+
+The embedded bh array in the kiobuf structure has been removed and no
+preallocation of bios is done for kiobufs. [The intent is to remove the
+blocks array as well, but it's currently in there to kludge around direct i/o.]
+Thus kiobuf allocation has switched back to using kmalloc rather than vmalloc.
+
+Todo/Observation:
+
+ A single kiobuf structure is assumed to correspond to a contiguous range
+ of data, so brw_kiovec() invokes ll_rw_kio for each kiobuf in a kiovec.
+ So right now it wouldn't work for direct i/o on non-contiguous blocks.
+ This is to be resolved. The eventual direction is to replace kiobuf
+ by kvec's.
+
+ Badari Pulavarty has a patch to implement direct i/o correctly using
+ bio and kvec.
+
+
+(c) Page i/o:
+
+Todo/Under discussion:
+
+ Andrew Morton's multi-page bio patches attempt to issue multi-page
+ writeouts (and reads) from the page cache, by directly building up
+ large bios for submission completely bypassing the usage of buffer
+ heads. This work is still in progress.
+
+ Christoph Hellwig had some code that uses bios for page-io (rather than
+ bh). This isn't included in bio as yet. Christoph was also working on a
+ design for representing virtual/real extents as an entity and modifying
+ some of the address space ops interfaces to utilize this abstraction rather
+ than buffer_heads. (This is somewhat along the lines of the SGI XFS pagebuf
+ abstraction, but intended to be as lightweight as possible).
+
+(d) Direct access i/o:
+
+Direct access requests that do not contain bios would be submitted differently
+as discussed earlier in section 1.3.
+
+Aside:
+
+ Kvec i/o:
+
+ Ben LaHaise's aio code uses a slightly different structure instead
+ of kiobufs, called a kvec_cb. This contains an array of <page, offset, len>
+ tuples (very much like the networking code), together with a callback function
+ and data pointer. This is embedded into a brw_cb structure when passed
+ to brw_kvec_async().
+
+ Now it should be possible to directly map these kvecs to a bio. Just as while
+ cloning, in this case rather than PRE_BUILT bio_vecs, we set the bi_io_vec
+ array pointer to point to the veclet array in kvecs.
+
+ TBD: In order for this to work, some changes are needed in the way multi-page
+ bios are handled today. The values of the tuples in such a vector passed in
+ from higher level code should not be modified by the block layer in the course
+ of its request processing, since that would make it hard for the higher layer
+ to continue to use the vector descriptor (kvec) after i/o completes. Instead,
+ all such transient state should either be maintained in the request structure,
+ and passed on in some way to the endio completion routine.
+
+
+4. The I/O scheduler
+====================
+
+I/O scheduler, a.k.a. elevator, is implemented in two layers. Generic dispatch
+queue and specific I/O schedulers. Unless stated otherwise, elevator is used
+to refer to both parts and I/O scheduler to specific I/O schedulers.
+
+Block layer implements generic dispatch queue in `block/*.c`.
+The generic dispatch queue is responsible for requeueing, handling non-fs
+requests and all other subtleties.
+
+Specific I/O schedulers are responsible for ordering normal filesystem
+requests. They can also choose to delay certain requests to improve
+throughput or whatever purpose. As the plural form indicates, there are
+multiple I/O schedulers. They can be built as modules but at least one should
+be built inside the kernel. Each queue can choose different one and can also
+change to another one dynamically.
+
+A block layer call to the i/o scheduler follows the convention elv_xxx(). This
+calls elevator_xxx_fn in the elevator switch (block/elevator.c). Oh, xxx
+and xxx might not match exactly, but use your imagination. If an elevator
+doesn't implement a function, the switch does nothing or some minimal house
+keeping work.
+
+4.1. I/O scheduler API
+----------------------
+
+The functions an elevator may implement are: (* are mandatory)
+
+=============================== ================================================
+elevator_merge_fn called to query requests for merge with a bio
+
+elevator_merge_req_fn called when two requests get merged. the one
+ which gets merged into the other one will be
+ never seen by I/O scheduler again. IOW, after
+ being merged, the request is gone.
+
+elevator_merged_fn called when a request in the scheduler has been
+ involved in a merge. It is used in the deadline
+ scheduler for example, to reposition the request
+ if its sorting order has changed.
+
+elevator_allow_merge_fn called whenever the block layer determines
+ that a bio can be merged into an existing
+ request safely. The io scheduler may still
+ want to stop a merge at this point if it
+ results in some sort of conflict internally,
+ this hook allows it to do that. Note however
+ that two *requests* can still be merged at later
+ time. Currently the io scheduler has no way to
+ prevent that. It can only learn about the fact
+ from elevator_merge_req_fn callback.
+
+elevator_dispatch_fn* fills the dispatch queue with ready requests.
+ I/O schedulers are free to postpone requests by
+ not filling the dispatch queue unless @force
+ is non-zero. Once dispatched, I/O schedulers
+ are not allowed to manipulate the requests -
+ they belong to generic dispatch queue.
+
+elevator_add_req_fn* called to add a new request into the scheduler
+
+elevator_former_req_fn
+elevator_latter_req_fn These return the request before or after the
+ one specified in disk sort order. Used by the
+ block layer to find merge possibilities.
+
+elevator_completed_req_fn called when a request is completed.
+
+elevator_may_queue_fn returns true if the scheduler wants to allow the
+ current context to queue a new request even if
+ it is over the queue limit. This must be used
+ very carefully!!
+
+elevator_set_req_fn
+elevator_put_req_fn Must be used to allocate and free any elevator
+ specific storage for a request.
+
+elevator_activate_req_fn Called when device driver first sees a request.
+ I/O schedulers can use this callback to
+ determine when actual execution of a request
+ starts.
+elevator_deactivate_req_fn Called when device driver decides to delay
+ a request by requeueing it.
+
+elevator_init_fn*
+elevator_exit_fn Allocate and free any elevator specific storage
+ for a queue.
+=============================== ================================================
+
+4.2 Request flows seen by I/O schedulers
+----------------------------------------
+
+All requests seen by I/O schedulers strictly follow one of the following three
+flows.
+
+ set_req_fn ->
+
+ i. add_req_fn -> (merged_fn ->)* -> dispatch_fn -> activate_req_fn ->
+ (deactivate_req_fn -> activate_req_fn ->)* -> completed_req_fn
+ ii. add_req_fn -> (merged_fn ->)* -> merge_req_fn
+ iii. [none]
+
+ -> put_req_fn
+
+4.3 I/O scheduler implementation
+--------------------------------
+
+The generic i/o scheduler algorithm attempts to sort/merge/batch requests for
+optimal disk scan and request servicing performance (based on generic
+principles and device capabilities), optimized for:
+
+i. improved throughput
+ii. improved latency
+iii. better utilization of h/w & CPU time
+
+Characteristics:
+
+i. Binary tree
+AS and deadline i/o schedulers use red black binary trees for disk position
+sorting and searching, and a fifo linked list for time-based searching. This
+gives good scalability and good availability of information. Requests are
+almost always dispatched in disk sort order, so a cache is kept of the next
+request in sort order to prevent binary tree lookups.
+
+This arrangement is not a generic block layer characteristic however, so
+elevators may implement queues as they please.
+
+ii. Merge hash
+AS and deadline use a hash table indexed by the last sector of a request. This
+enables merging code to quickly look up "back merge" candidates, even when
+multiple I/O streams are being performed at once on one disk.
+
+"Front merges", a new request being merged at the front of an existing request,
+are far less common than "back merges" due to the nature of most I/O patterns.
+Front merges are handled by the binary trees in AS and deadline schedulers.
+
+iii. Plugging the queue to batch requests in anticipation of opportunities for
+ merge/sort optimizations
+
+Plugging is an approach that the current i/o scheduling algorithm resorts to so
+that it collects up enough requests in the queue to be able to take
+advantage of the sorting/merging logic in the elevator. If the
+queue is empty when a request comes in, then it plugs the request queue
+(sort of like plugging the bath tub of a vessel to get fluid to build up)
+till it fills up with a few more requests, before starting to service
+the requests. This provides an opportunity to merge/sort the requests before
+passing them down to the device. There are various conditions when the queue is
+unplugged (to open up the flow again), either through a scheduled task or
+could be on demand. For example wait_on_buffer sets the unplugging going
+through sync_buffer() running blk_run_address_space(mapping). Or the caller
+can do it explicity through blk_unplug(bdev). So in the read case,
+the queue gets explicitly unplugged as part of waiting for completion on that
+buffer.
+
+Aside:
+ This is kind of controversial territory, as it's not clear if plugging is
+ always the right thing to do. Devices typically have their own queues,
+ and allowing a big queue to build up in software, while letting the device be
+ idle for a while may not always make sense. The trick is to handle the fine
+ balance between when to plug and when to open up. Also now that we have
+ multi-page bios being queued in one shot, we may not need to wait to merge
+ a big request from the broken up pieces coming by.
+
+4.4 I/O contexts
+----------------
+
+I/O contexts provide a dynamically allocated per process data area. They may
+be used in I/O schedulers, and in the block layer (could be used for IO statis,
+priorities for example). See `*io_context` in block/ll_rw_blk.c, and as-iosched.c
+for an example of usage in an i/o scheduler.
+
+
+5. Scalability related changes
+==============================
+
+5.1 Granular Locking: io_request_lock replaced by a per-queue lock
+------------------------------------------------------------------
+
+The global io_request_lock has been removed as of 2.5, to avoid
+the scalability bottleneck it was causing, and has been replaced by more
+granular locking. The request queue structure has a pointer to the
+lock to be used for that queue. As a result, locking can now be
+per-queue, with a provision for sharing a lock across queues if
+necessary (e.g the scsi layer sets the queue lock pointers to the
+corresponding adapter lock, which results in a per host locking
+granularity). The locking semantics are the same, i.e. locking is
+still imposed by the block layer, grabbing the lock before
+request_fn execution which it means that lots of older drivers
+should still be SMP safe. Drivers are free to drop the queue
+lock themselves, if required. Drivers that explicitly used the
+io_request_lock for serialization need to be modified accordingly.
+Usually it's as easy as adding a global lock::
+
+ static DEFINE_SPINLOCK(my_driver_lock);
+
+and passing the address to that lock to blk_init_queue().
+
+5.2 64 bit sector numbers (sector_t prepares for 64 bit support)
+----------------------------------------------------------------
+
+The sector number used in the bio structure has been changed to sector_t,
+which could be defined as 64 bit in preparation for 64 bit sector support.
+
+6. Other Changes/Implications
+=============================
+
+6.1 Partition re-mapping handled by the generic block layer
+-----------------------------------------------------------
+
+In 2.5 some of the gendisk/partition related code has been reorganized.
+Now the generic block layer performs partition-remapping early and thus
+provides drivers with a sector number relative to whole device, rather than
+having to take partition number into account in order to arrive at the true
+sector number. The routine blk_partition_remap() is invoked by
+generic_make_request even before invoking the queue specific make_request_fn,
+so the i/o scheduler also gets to operate on whole disk sector numbers. This
+should typically not require changes to block drivers, it just never gets
+to invoke its own partition sector offset calculations since all bios
+sent are offset from the beginning of the device.
+
+
+7. A Few Tips on Migration of older drivers
+===========================================
+
+Old-style drivers that just use CURRENT and ignores clustered requests,
+may not need much change. The generic layer will automatically handle
+clustered requests, multi-page bios, etc for the driver.
+
+For a low performance driver or hardware that is PIO driven or just doesn't
+support scatter-gather changes should be minimal too.
+
+The following are some points to keep in mind when converting old drivers
+to bio.
+
+Drivers should use elv_next_request to pick up requests and are no longer
+supposed to handle looping directly over the request list.
+(struct request->queue has been removed)
+
+Now end_that_request_first takes an additional number_of_sectors argument.
+It used to handle always just the first buffer_head in a request, now
+it will loop and handle as many sectors (on a bio-segment granularity)
+as specified.
+
+Now bh->b_end_io is replaced by bio->bi_end_io, but most of the time the
+right thing to use is bio_endio(bio) instead.
+
+If the driver is dropping the io_request_lock from its request_fn strategy,
+then it just needs to replace that with q->queue_lock instead.
+
+As described in Sec 1.1, drivers can set max sector size, max segment size
+etc per queue now. Drivers that used to define their own merge functions i
+to handle things like this can now just use the blk_queue_* functions at
+blk_init_queue time.
+
+Drivers no longer have to map a {partition, sector offset} into the
+correct absolute location anymore, this is done by the block layer, so
+where a driver received a request ala this before::
+
+ rq->rq_dev = mk_kdev(3, 5); /* /dev/hda5 */
+ rq->sector = 0; /* first sector on hda5 */
+
+it will now see::
+
+ rq->rq_dev = mk_kdev(3, 0); /* /dev/hda */
+ rq->sector = 123128; /* offset from start of disk */
+
+As mentioned, there is no virtual mapping of a bio. For DMA, this is
+not a problem as the driver probably never will need a virtual mapping.
+Instead it needs a bus mapping (dma_map_page for a single segment or
+use dma_map_sg for scatter gather) to be able to ship it to the driver. For
+PIO drivers (or drivers that need to revert to PIO transfer once in a
+while (IDE for example)), where the CPU is doing the actual data
+transfer a virtual mapping is needed. If the driver supports highmem I/O,
+(Sec 1.1, (ii) ) it needs to use kmap_atomic or similar to temporarily map
+a bio into the virtual address space.
+
+
+8. Prior/Related/Impacted patches
+=================================
+
+8.1. Earlier kiobuf patches (sct/axboe/chait/hch/mkp)
+-----------------------------------------------------
+
+- orig kiobuf & raw i/o patches (now in 2.4 tree)
+- direct kiobuf based i/o to devices (no intermediate bh's)
+- page i/o using kiobuf
+- kiobuf splitting for lvm (mkp)
+- elevator support for kiobuf request merging (axboe)
+
+8.2. Zero-copy networking (Dave Miller)
+---------------------------------------
+
+8.3. SGI XFS - pagebuf patches - use of kiobufs
+-----------------------------------------------
+8.4. Multi-page pioent patch for bio (Christoph Hellwig)
+--------------------------------------------------------
+8.5. Direct i/o implementation (Andrea Arcangeli) since 2.4.10-pre11
+--------------------------------------------------------------------
+8.6. Async i/o implementation patch (Ben LaHaise)
+-------------------------------------------------
+8.7. EVMS layering design (IBM EVMS team)
+-----------------------------------------
+8.8. Larger page cache size patch (Ben LaHaise) and Large page size (Daniel Phillips)
+-------------------------------------------------------------------------------------
+
+ => larger contiguous physical memory buffers
+
+8.9. VM reservations patch (Ben LaHaise)
+----------------------------------------
+8.10. Write clustering patches ? (Marcelo/Quintela/Riel ?)
+----------------------------------------------------------
+8.11. Block device in page cache patch (Andrea Archangeli) - now in 2.4.10+
+---------------------------------------------------------------------------
+8.12. Multiple block-size transfers for faster raw i/o (Shailabh Nagar, Badari)
+-------------------------------------------------------------------------------
+8.13 Priority based i/o scheduler - prepatches (Arjan van de Ven)
+------------------------------------------------------------------
+8.14 IDE Taskfile i/o patch (Andre Hedrick)
+--------------------------------------------
+8.15 Multi-page writeout and readahead patches (Andrew Morton)
+---------------------------------------------------------------
+8.16 Direct i/o patches for 2.5 using kvec and bio (Badari Pulavarthy)
+-----------------------------------------------------------------------
+
+9. Other References
+===================
+
+9.1 The Splice I/O Model
+------------------------
+
+Larry McVoy (and subsequent discussions on lkml, and Linus' comments - Jan 2001
+
+9.2 Discussions about kiobuf and bh design
+------------------------------------------
+
+On lkml between sct, linus, alan et al - Feb-March 2001 (many of the
+initial thoughts that led to bio were brought up in this discussion thread)
+
+9.3 Discussions on mempool on lkml - Dec 2001.
+----------------------------------------------
+++ /dev/null
- Notes on the Generic Block Layer Rewrite in Linux 2.5
- =====================================================
-
-Notes Written on Jan 15, 2002:
- Jens Axboe <jens.axboe@oracle.com>
- Suparna Bhattacharya <suparna@in.ibm.com>
-
-Last Updated May 2, 2002
-September 2003: Updated I/O Scheduler portions
- Nick Piggin <npiggin@kernel.dk>
-
-Introduction:
-
-These are some notes describing some aspects of the 2.5 block layer in the
-context of the bio rewrite. The idea is to bring out some of the key
-changes and a glimpse of the rationale behind those changes.
-
-Please mail corrections & suggestions to suparna@in.ibm.com.
-
-Credits:
----------
-
-2.5 bio rewrite:
- Jens Axboe <jens.axboe@oracle.com>
-
-Many aspects of the generic block layer redesign were driven by and evolved
-over discussions, prior patches and the collective experience of several
-people. See sections 8 and 9 for a list of some related references.
-
-The following people helped with review comments and inputs for this
-document:
- Christoph Hellwig <hch@infradead.org>
- Arjan van de Ven <arjanv@redhat.com>
- Randy Dunlap <rdunlap@xenotime.net>
- Andre Hedrick <andre@linux-ide.org>
-
-The following people helped with fixes/contributions to the bio patches
-while it was still work-in-progress:
- David S. Miller <davem@redhat.com>
-
-
-Description of Contents:
-------------------------
-
-1. Scope for tuning of logic to various needs
- 1.1 Tuning based on device or low level driver capabilities
- - Per-queue parameters
- - Highmem I/O support
- - I/O scheduler modularization
- 1.2 Tuning based on high level requirements/capabilities
- 1.2.1 Request Priority/Latency
- 1.3 Direct access/bypass to lower layers for diagnostics and special
- device operations
- 1.3.1 Pre-built commands
-2. New flexible and generic but minimalist i/o structure or descriptor
- (instead of using buffer heads at the i/o layer)
- 2.1 Requirements/Goals addressed
- 2.2 The bio struct in detail (multi-page io unit)
- 2.3 Changes in the request structure
-3. Using bios
- 3.1 Setup/teardown (allocation, splitting)
- 3.2 Generic bio helper routines
- 3.2.1 Traversing segments and completion units in a request
- 3.2.2 Setting up DMA scatterlists
- 3.2.3 I/O completion
- 3.2.4 Implications for drivers that do not interpret bios (don't handle
- multiple segments)
- 3.3 I/O submission
-4. The I/O scheduler
-5. Scalability related changes
- 5.1 Granular locking: Removal of io_request_lock
- 5.2 Prepare for transition to 64 bit sector_t
-6. Other Changes/Implications
- 6.1 Partition re-mapping handled by the generic block layer
-7. A few tips on migration of older drivers
-8. A list of prior/related/impacted patches/ideas
-9. Other References/Discussion Threads
-
----------------------------------------------------------------------------
-
-Bio Notes
---------
-
-Let us discuss the changes in the context of how some overall goals for the
-block layer are addressed.
-
-1. Scope for tuning the generic logic to satisfy various requirements
-
-The block layer design supports adaptable abstractions to handle common
-processing with the ability to tune the logic to an appropriate extent
-depending on the nature of the device and the requirements of the caller.
-One of the objectives of the rewrite was to increase the degree of tunability
-and to enable higher level code to utilize underlying device/driver
-capabilities to the maximum extent for better i/o performance. This is
-important especially in the light of ever improving hardware capabilities
-and application/middleware software designed to take advantage of these
-capabilities.
-
-1.1 Tuning based on low level device / driver capabilities
-
-Sophisticated devices with large built-in caches, intelligent i/o scheduling
-optimizations, high memory DMA support, etc may find some of the
-generic processing an overhead, while for less capable devices the
-generic functionality is essential for performance or correctness reasons.
-Knowledge of some of the capabilities or parameters of the device should be
-used at the generic block layer to take the right decisions on
-behalf of the driver.
-
-How is this achieved ?
-
-Tuning at a per-queue level:
-
-i. Per-queue limits/values exported to the generic layer by the driver
-
-Various parameters that the generic i/o scheduler logic uses are set at
-a per-queue level (e.g maximum request size, maximum number of segments in
-a scatter-gather list, logical block size)
-
-Some parameters that were earlier available as global arrays indexed by
-major/minor are now directly associated with the queue. Some of these may
-move into the block device structure in the future. Some characteristics
-have been incorporated into a queue flags field rather than separate fields
-in themselves. There are blk_queue_xxx functions to set the parameters,
-rather than update the fields directly
-
-Some new queue property settings:
-
- blk_queue_bounce_limit(q, u64 dma_address)
- Enable I/O to highmem pages, dma_address being the
- limit. No highmem default.
-
- blk_queue_max_sectors(q, max_sectors)
- Sets two variables that limit the size of the request.
-
- - The request queue's max_sectors, which is a soft size in
- units of 512 byte sectors, and could be dynamically varied
- by the core kernel.
-
- - The request queue's max_hw_sectors, which is a hard limit
- and reflects the maximum size request a driver can handle
- in units of 512 byte sectors.
-
- The default for both max_sectors and max_hw_sectors is
- 255. The upper limit of max_sectors is 1024.
-
- blk_queue_max_phys_segments(q, max_segments)
- Maximum physical segments you can handle in a request. 128
- default (driver limit). (See 3.2.2)
-
- blk_queue_max_hw_segments(q, max_segments)
- Maximum dma segments the hardware can handle in a request. 128
- default (host adapter limit, after dma remapping).
- (See 3.2.2)
-
- blk_queue_max_segment_size(q, max_seg_size)
- Maximum size of a clustered segment, 64kB default.
-
- blk_queue_logical_block_size(q, logical_block_size)
- Lowest possible sector size that the hardware can operate
- on, 512 bytes default.
-
-New queue flags:
-
- QUEUE_FLAG_CLUSTER (see 3.2.2)
- QUEUE_FLAG_QUEUED (see 3.2.4)
-
-
-ii. High-mem i/o capabilities are now considered the default
-
-The generic bounce buffer logic, present in 2.4, where the block layer would
-by default copyin/out i/o requests on high-memory buffers to low-memory buffers
-assuming that the driver wouldn't be able to handle it directly, has been
-changed in 2.5. The bounce logic is now applied only for memory ranges
-for which the device cannot handle i/o. A driver can specify this by
-setting the queue bounce limit for the request queue for the device
-(blk_queue_bounce_limit()). This avoids the inefficiencies of the copyin/out
-where a device is capable of handling high memory i/o.
-
-In order to enable high-memory i/o where the device is capable of supporting
-it, the pci dma mapping routines and associated data structures have now been
-modified to accomplish a direct page -> bus translation, without requiring
-a virtual address mapping (unlike the earlier scheme of virtual address
--> bus translation). So this works uniformly for high-memory pages (which
-do not have a corresponding kernel virtual address space mapping) and
-low-memory pages.
-
-Note: Please refer to Documentation/DMA-API-HOWTO.txt for a discussion
-on PCI high mem DMA aspects and mapping of scatter gather lists, and support
-for 64 bit PCI.
-
-Special handling is required only for cases where i/o needs to happen on
-pages at physical memory addresses beyond what the device can support. In these
-cases, a bounce bio representing a buffer from the supported memory range
-is used for performing the i/o with copyin/copyout as needed depending on
-the type of the operation. For example, in case of a read operation, the
-data read has to be copied to the original buffer on i/o completion, so a
-callback routine is set up to do this, while for write, the data is copied
-from the original buffer to the bounce buffer prior to issuing the
-operation. Since an original buffer may be in a high memory area that's not
-mapped in kernel virtual addr, a kmap operation may be required for
-performing the copy, and special care may be needed in the completion path
-as it may not be in irq context. Special care is also required (by way of
-GFP flags) when allocating bounce buffers, to avoid certain highmem
-deadlock possibilities.
-
-It is also possible that a bounce buffer may be allocated from high-memory
-area that's not mapped in kernel virtual addr, but within the range that the
-device can use directly; so the bounce page may need to be kmapped during
-copy operations. [Note: This does not hold in the current implementation,
-though]
-
-There are some situations when pages from high memory may need to
-be kmapped, even if bounce buffers are not necessary. For example a device
-may need to abort DMA operations and revert to PIO for the transfer, in
-which case a virtual mapping of the page is required. For SCSI it is also
-done in some scenarios where the low level driver cannot be trusted to
-handle a single sg entry correctly. The driver is expected to perform the
-kmaps as needed on such occasions as appropriate. A driver could also use
-the blk_queue_bounce() routine on its own to bounce highmem i/o to low
-memory for specific requests if so desired.
-
-iii. The i/o scheduler algorithm itself can be replaced/set as appropriate
-
-As in 2.4, it is possible to plugin a brand new i/o scheduler for a particular
-queue or pick from (copy) existing generic schedulers and replace/override
-certain portions of it. The 2.5 rewrite provides improved modularization
-of the i/o scheduler. There are more pluggable callbacks, e.g for init,
-add request, extract request, which makes it possible to abstract specific
-i/o scheduling algorithm aspects and details outside of the generic loop.
-It also makes it possible to completely hide the implementation details of
-the i/o scheduler from block drivers.
-
-I/O scheduler wrappers are to be used instead of accessing the queue directly.
-See section 4. The I/O scheduler for details.
-
-1.2 Tuning Based on High level code capabilities
-
-i. Application capabilities for raw i/o
-
-This comes from some of the high-performance database/middleware
-requirements where an application prefers to make its own i/o scheduling
-decisions based on an understanding of the access patterns and i/o
-characteristics
-
-ii. High performance filesystems or other higher level kernel code's
-capabilities
-
-Kernel components like filesystems could also take their own i/o scheduling
-decisions for optimizing performance. Journalling filesystems may need
-some control over i/o ordering.
-
-What kind of support exists at the generic block layer for this ?
-
-The flags and rw fields in the bio structure can be used for some tuning
-from above e.g indicating that an i/o is just a readahead request, or priority
-settings (currently unused). As far as user applications are concerned they
-would need an additional mechanism either via open flags or ioctls, or some
-other upper level mechanism to communicate such settings to block.
-
-1.2.1 Request Priority/Latency
-
-Todo/Under discussion:
-Arjan's proposed request priority scheme allows higher levels some broad
- control (high/med/low) over the priority of an i/o request vs other pending
- requests in the queue. For example it allows reads for bringing in an
- executable page on demand to be given a higher priority over pending write
- requests which haven't aged too much on the queue. Potentially this priority
- could even be exposed to applications in some manner, providing higher level
- tunability. Time based aging avoids starvation of lower priority
- requests. Some bits in the bi_opf flags field in the bio structure are
- intended to be used for this priority information.
-
-
-1.3 Direct Access to Low level Device/Driver Capabilities (Bypass mode)
- (e.g Diagnostics, Systems Management)
-
-There are situations where high-level code needs to have direct access to
-the low level device capabilities or requires the ability to issue commands
-to the device bypassing some of the intermediate i/o layers.
-These could, for example, be special control commands issued through ioctl
-interfaces, or could be raw read/write commands that stress the drive's
-capabilities for certain kinds of fitness tests. Having direct interfaces at
-multiple levels without having to pass through upper layers makes
-it possible to perform bottom up validation of the i/o path, layer by
-layer, starting from the media.
-
-The normal i/o submission interfaces, e.g submit_bio, could be bypassed
-for specially crafted requests which such ioctl or diagnostics
-interfaces would typically use, and the elevator add_request routine
-can instead be used to directly insert such requests in the queue or preferably
-the blk_do_rq routine can be used to place the request on the queue and
-wait for completion. Alternatively, sometimes the caller might just
-invoke a lower level driver specific interface with the request as a
-parameter.
-
-If the request is a means for passing on special information associated with
-the command, then such information is associated with the request->special
-field (rather than misuse the request->buffer field which is meant for the
-request data buffer's virtual mapping).
-
-For passing request data, the caller must build up a bio descriptor
-representing the concerned memory buffer if the underlying driver interprets
-bio segments or uses the block layer end*request* functions for i/o
-completion. Alternatively one could directly use the request->buffer field to
-specify the virtual address of the buffer, if the driver expects buffer
-addresses passed in this way and ignores bio entries for the request type
-involved. In the latter case, the driver would modify and manage the
-request->buffer, request->sector and request->nr_sectors or
-request->current_nr_sectors fields itself rather than using the block layer
-end_request or end_that_request_first completion interfaces.
-(See 2.3 or Documentation/block/request.txt for a brief explanation of
-the request structure fields)
-
-[TBD: end_that_request_last should be usable even in this case;
-Perhaps an end_that_direct_request_first routine could be implemented to make
-handling direct requests easier for such drivers; Also for drivers that
-expect bios, a helper function could be provided for setting up a bio
-corresponding to a data buffer]
-
-<JENS: I dont understand the above, why is end_that_request_first() not
-usable? Or _last for that matter. I must be missing something>
-<SUP: What I meant here was that if the request doesn't have a bio, then
- end_that_request_first doesn't modify nr_sectors or current_nr_sectors,
- and hence can't be used for advancing request state settings on the
- completion of partial transfers. The driver has to modify these fields
- directly by hand.
- This is because end_that_request_first only iterates over the bio list,
- and always returns 0 if there are none associated with the request.
- _last works OK in this case, and is not a problem, as I mentioned earlier
->
-
-1.3.1 Pre-built Commands
-
-A request can be created with a pre-built custom command to be sent directly
-to the device. The cmd block in the request structure has room for filling
-in the command bytes. (i.e rq->cmd is now 16 bytes in size, and meant for
-command pre-building, and the type of the request is now indicated
-through rq->flags instead of via rq->cmd)
-
-The request structure flags can be set up to indicate the type of request
-in such cases (REQ_PC: direct packet command passed to driver, REQ_BLOCK_PC:
-packet command issued via blk_do_rq, REQ_SPECIAL: special request).
-
-It can help to pre-build device commands for requests in advance.
-Drivers can now specify a request prepare function (q->prep_rq_fn) that the
-block layer would invoke to pre-build device commands for a given request,
-or perform other preparatory processing for the request. This is routine is
-called by elv_next_request(), i.e. typically just before servicing a request.
-(The prepare function would not be called for requests that have RQF_DONTPREP
-enabled)
-
-Aside:
- Pre-building could possibly even be done early, i.e before placing the
- request on the queue, rather than construct the command on the fly in the
- driver while servicing the request queue when it may affect latencies in
- interrupt context or responsiveness in general. One way to add early
- pre-building would be to do it whenever we fail to merge on a request.
- Now REQ_NOMERGE is set in the request flags to skip this one in the future,
- which means that it will not change before we feed it to the device. So
- the pre-builder hook can be invoked there.
-
-
-2. Flexible and generic but minimalist i/o structure/descriptor.
-
-2.1 Reason for a new structure and requirements addressed
-
-Prior to 2.5, buffer heads were used as the unit of i/o at the generic block
-layer, and the low level request structure was associated with a chain of
-buffer heads for a contiguous i/o request. This led to certain inefficiencies
-when it came to large i/o requests and readv/writev style operations, as it
-forced such requests to be broken up into small chunks before being passed
-on to the generic block layer, only to be merged by the i/o scheduler
-when the underlying device was capable of handling the i/o in one shot.
-Also, using the buffer head as an i/o structure for i/os that didn't originate
-from the buffer cache unnecessarily added to the weight of the descriptors
-which were generated for each such chunk.
-
-The following were some of the goals and expectations considered in the
-redesign of the block i/o data structure in 2.5.
-
-i. Should be appropriate as a descriptor for both raw and buffered i/o -
- avoid cache related fields which are irrelevant in the direct/page i/o path,
- or filesystem block size alignment restrictions which may not be relevant
- for raw i/o.
-ii. Ability to represent high-memory buffers (which do not have a virtual
- address mapping in kernel address space).
-iii.Ability to represent large i/os w/o unnecessarily breaking them up (i.e
- greater than PAGE_SIZE chunks in one shot)
-iv. At the same time, ability to retain independent identity of i/os from
- different sources or i/o units requiring individual completion (e.g. for
- latency reasons)
-v. Ability to represent an i/o involving multiple physical memory segments
- (including non-page aligned page fragments, as specified via readv/writev)
- without unnecessarily breaking it up, if the underlying device is capable of
- handling it.
-vi. Preferably should be based on a memory descriptor structure that can be
- passed around different types of subsystems or layers, maybe even
- networking, without duplication or extra copies of data/descriptor fields
- themselves in the process
-vii.Ability to handle the possibility of splits/merges as the structure passes
- through layered drivers (lvm, md, evms), with minimal overhead.
-
-The solution was to define a new structure (bio) for the block layer,
-instead of using the buffer head structure (bh) directly, the idea being
-avoidance of some associated baggage and limitations. The bio structure
-is uniformly used for all i/o at the block layer ; it forms a part of the
-bh structure for buffered i/o, and in the case of raw/direct i/o kiobufs are
-mapped to bio structures.
-
-2.2 The bio struct
-
-The bio structure uses a vector representation pointing to an array of tuples
-of <page, offset, len> to describe the i/o buffer, and has various other
-fields describing i/o parameters and state that needs to be maintained for
-performing the i/o.
-
-Notice that this representation means that a bio has no virtual address
-mapping at all (unlike buffer heads).
-
-struct bio_vec {
- struct page *bv_page;
- unsigned short bv_len;
- unsigned short bv_offset;
-};
-
-/*
- * main unit of I/O for the block layer and lower layers (ie drivers)
- */
-struct bio {
- struct bio *bi_next; /* request queue link */
- struct block_device *bi_bdev; /* target device */
- unsigned long bi_flags; /* status, command, etc */
- unsigned long bi_opf; /* low bits: r/w, high: priority */
-
- unsigned int bi_vcnt; /* how may bio_vec's */
- struct bvec_iter bi_iter; /* current index into bio_vec array */
-
- unsigned int bi_size; /* total size in bytes */
- unsigned short bi_hw_segments; /* segments after DMA remapping */
- unsigned int bi_max; /* max bio_vecs we can hold
- used as index into pool */
- struct bio_vec *bi_io_vec; /* the actual vec list */
- bio_end_io_t *bi_end_io; /* bi_end_io (bio) */
- atomic_t bi_cnt; /* pin count: free when it hits zero */
- void *bi_private;
-};
-
-With this multipage bio design:
-
-- Large i/os can be sent down in one go using a bio_vec list consisting
- of an array of <page, offset, len> fragments (similar to the way fragments
- are represented in the zero-copy network code)
-- Splitting of an i/o request across multiple devices (as in the case of
- lvm or raid) is achieved by cloning the bio (where the clone points to
- the same bi_io_vec array, but with the index and size accordingly modified)
-- A linked list of bios is used as before for unrelated merges (*) - this
- avoids reallocs and makes independent completions easier to handle.
-- Code that traverses the req list can find all the segments of a bio
- by using rq_for_each_segment. This handles the fact that a request
- has multiple bios, each of which can have multiple segments.
-- Drivers which can't process a large bio in one shot can use the bi_iter
- field to keep track of the next bio_vec entry to process.
- (e.g a 1MB bio_vec needs to be handled in max 128kB chunks for IDE)
- [TBD: Should preferably also have a bi_voffset and bi_vlen to avoid modifying
- bi_offset an len fields]
-
-(*) unrelated merges -- a request ends up containing two or more bios that
- didn't originate from the same place.
-
-bi_end_io() i/o callback gets called on i/o completion of the entire bio.
-
-At a lower level, drivers build a scatter gather list from the merged bios.
-The scatter gather list is in the form of an array of <page, offset, len>
-entries with their corresponding dma address mappings filled in at the
-appropriate time. As an optimization, contiguous physical pages can be
-covered by a single entry where <page> refers to the first page and <len>
-covers the range of pages (up to 16 contiguous pages could be covered this
-way). There is a helper routine (blk_rq_map_sg) which drivers can use to build
-the sg list.
-
-Note: Right now the only user of bios with more than one page is ll_rw_kio,
-which in turn means that only raw I/O uses it (direct i/o may not work
-right now). The intent however is to enable clustering of pages etc to
-become possible. The pagebuf abstraction layer from SGI also uses multi-page
-bios, but that is currently not included in the stock development kernels.
-The same is true of Andrew Morton's work-in-progress multipage bio writeout
-and readahead patches.
-
-2.3 Changes in the Request Structure
-
-The request structure is the structure that gets passed down to low level
-drivers. The block layer make_request function builds up a request structure,
-places it on the queue and invokes the drivers request_fn. The driver makes
-use of block layer helper routine elv_next_request to pull the next request
-off the queue. Control or diagnostic functions might bypass block and directly
-invoke underlying driver entry points passing in a specially constructed
-request structure.
-
-Only some relevant fields (mainly those which changed or may be referred
-to in some of the discussion here) are listed below, not necessarily in
-the order in which they occur in the structure (see include/linux/blkdev.h)
-Refer to Documentation/block/request.txt for details about all the request
-structure fields and a quick reference about the layers which are
-supposed to use or modify those fields.
-
-struct request {
- struct list_head queuelist; /* Not meant to be directly accessed by
- the driver.
- Used by q->elv_next_request_fn
- rq->queue is gone
- */
- .
- .
- unsigned char cmd[16]; /* prebuilt command data block */
- unsigned long flags; /* also includes earlier rq->cmd settings */
- .
- .
- sector_t sector; /* this field is now of type sector_t instead of int
- preparation for 64 bit sectors */
- .
- .
-
- /* Number of scatter-gather DMA addr+len pairs after
- * physical address coalescing is performed.
- */
- unsigned short nr_phys_segments;
-
- /* Number of scatter-gather addr+len pairs after
- * physical and DMA remapping hardware coalescing is performed.
- * This is the number of scatter-gather entries the driver
- * will actually have to deal with after DMA mapping is done.
- */
- unsigned short nr_hw_segments;
-
- /* Various sector counts */
- unsigned long nr_sectors; /* no. of sectors left: driver modifiable */
- unsigned long hard_nr_sectors; /* block internal copy of above */
- unsigned int current_nr_sectors; /* no. of sectors left in the
- current segment:driver modifiable */
- unsigned long hard_cur_sectors; /* block internal copy of the above */
- .
- .
- int tag; /* command tag associated with request */
- void *special; /* same as before */
- char *buffer; /* valid only for low memory buffers up to
- current_nr_sectors */
- .
- .
- struct bio *bio, *biotail; /* bio list instead of bh */
- struct request_list *rl;
-}
-
-See the req_ops and req_flag_bits definitions for an explanation of the various
-flags available. Some bits are used by the block layer or i/o scheduler.
-
-The behaviour of the various sector counts are almost the same as before,
-except that since we have multi-segment bios, current_nr_sectors refers
-to the numbers of sectors in the current segment being processed which could
-be one of the many segments in the current bio (i.e i/o completion unit).
-The nr_sectors value refers to the total number of sectors in the whole
-request that remain to be transferred (no change). The purpose of the
-hard_xxx values is for block to remember these counts every time it hands
-over the request to the driver. These values are updated by block on
-end_that_request_first, i.e. every time the driver completes a part of the
-transfer and invokes block end*request helpers to mark this. The
-driver should not modify these values. The block layer sets up the
-nr_sectors and current_nr_sectors fields (based on the corresponding
-hard_xxx values and the number of bytes transferred) and updates it on
-every transfer that invokes end_that_request_first. It does the same for the
-buffer, bio, bio->bi_iter fields too.
-
-The buffer field is just a virtual address mapping of the current segment
-of the i/o buffer in cases where the buffer resides in low-memory. For high
-memory i/o, this field is not valid and must not be used by drivers.
-
-Code that sets up its own request structures and passes them down to
-a driver needs to be careful about interoperation with the block layer helper
-functions which the driver uses. (Section 1.3)
-
-3. Using bios
-
-3.1 Setup/Teardown
-
-There are routines for managing the allocation, and reference counting, and
-freeing of bios (bio_alloc, bio_get, bio_put).
-
-This makes use of Ingo Molnar's mempool implementation, which enables
-subsystems like bio to maintain their own reserve memory pools for guaranteed
-deadlock-free allocations during extreme VM load. For example, the VM
-subsystem makes use of the block layer to writeout dirty pages in order to be
-able to free up memory space, a case which needs careful handling. The
-allocation logic draws from the preallocated emergency reserve in situations
-where it cannot allocate through normal means. If the pool is empty and it
-can wait, then it would trigger action that would help free up memory or
-replenish the pool (without deadlocking) and wait for availability in the pool.
-If it is in IRQ context, and hence not in a position to do this, allocation
-could fail if the pool is empty. In general mempool always first tries to
-perform allocation without having to wait, even if it means digging into the
-pool as long it is not less that 50% full.
-
-On a free, memory is released to the pool or directly freed depending on
-the current availability in the pool. The mempool interface lets the
-subsystem specify the routines to be used for normal alloc and free. In the
-case of bio, these routines make use of the standard slab allocator.
-
-The caller of bio_alloc is expected to taken certain steps to avoid
-deadlocks, e.g. avoid trying to allocate more memory from the pool while
-already holding memory obtained from the pool.
-[TBD: This is a potential issue, though a rare possibility
- in the bounce bio allocation that happens in the current code, since
- it ends up allocating a second bio from the same pool while
- holding the original bio ]
-
-Memory allocated from the pool should be released back within a limited
-amount of time (in the case of bio, that would be after the i/o is completed).
-This ensures that if part of the pool has been used up, some work (in this
-case i/o) must already be in progress and memory would be available when it
-is over. If allocating from multiple pools in the same code path, the order
-or hierarchy of allocation needs to be consistent, just the way one deals
-with multiple locks.
-
-The bio_alloc routine also needs to allocate the bio_vec_list (bvec_alloc())
-for a non-clone bio. There are the 6 pools setup for different size biovecs,
-so bio_alloc(gfp_mask, nr_iovecs) will allocate a vec_list of the
-given size from these slabs.
-
-The bio_get() routine may be used to hold an extra reference on a bio prior
-to i/o submission, if the bio fields are likely to be accessed after the
-i/o is issued (since the bio may otherwise get freed in case i/o completion
-happens in the meantime).
-
-The bio_clone_fast() routine may be used to duplicate a bio, where the clone
-shares the bio_vec_list with the original bio (i.e. both point to the
-same bio_vec_list). This would typically be used for splitting i/o requests
-in lvm or md.
-
-3.2 Generic bio helper Routines
-
-3.2.1 Traversing segments and completion units in a request
-
-The macro rq_for_each_segment() should be used for traversing the bios
-in the request list (drivers should avoid directly trying to do it
-themselves). Using these helpers should also make it easier to cope
-with block changes in the future.
-
- struct req_iterator iter;
- rq_for_each_segment(bio_vec, rq, iter)
- /* bio_vec is now current segment */
-
-I/O completion callbacks are per-bio rather than per-segment, so drivers
-that traverse bio chains on completion need to keep that in mind. Drivers
-which don't make a distinction between segments and completion units would
-need to be reorganized to support multi-segment bios.
-
-3.2.2 Setting up DMA scatterlists
-
-The blk_rq_map_sg() helper routine would be used for setting up scatter
-gather lists from a request, so a driver need not do it on its own.
-
- nr_segments = blk_rq_map_sg(q, rq, scatterlist);
-
-The helper routine provides a level of abstraction which makes it easier
-to modify the internals of request to scatterlist conversion down the line
-without breaking drivers. The blk_rq_map_sg routine takes care of several
-things like collapsing physically contiguous segments (if QUEUE_FLAG_CLUSTER
-is set) and correct segment accounting to avoid exceeding the limits which
-the i/o hardware can handle, based on various queue properties.
-
-- Prevents a clustered segment from crossing a 4GB mem boundary
-- Avoids building segments that would exceed the number of physical
- memory segments that the driver can handle (phys_segments) and the
- number that the underlying hardware can handle at once, accounting for
- DMA remapping (hw_segments) (i.e. IOMMU aware limits).
-
-Routines which the low level driver can use to set up the segment limits:
-
-blk_queue_max_hw_segments() : Sets an upper limit of the maximum number of
-hw data segments in a request (i.e. the maximum number of address/length
-pairs the host adapter can actually hand to the device at once)
-
-blk_queue_max_phys_segments() : Sets an upper limit on the maximum number
-of physical data segments in a request (i.e. the largest sized scatter list
-a driver could handle)
-
-3.2.3 I/O completion
-
-The existing generic block layer helper routines end_request,
-end_that_request_first and end_that_request_last can be used for i/o
-completion (and setting things up so the rest of the i/o or the next
-request can be kicked of) as before. With the introduction of multi-page
-bio support, end_that_request_first requires an additional argument indicating
-the number of sectors completed.
-
-3.2.4 Implications for drivers that do not interpret bios (don't handle
- multiple segments)
-
-Drivers that do not interpret bios e.g those which do not handle multiple
-segments and do not support i/o into high memory addresses (require bounce
-buffers) and expect only virtually mapped buffers, can access the rq->buffer
-field. As before the driver should use current_nr_sectors to determine the
-size of remaining data in the current segment (that is the maximum it can
-transfer in one go unless it interprets segments), and rely on the block layer
-end_request, or end_that_request_first/last to take care of all accounting
-and transparent mapping of the next bio segment when a segment boundary
-is crossed on completion of a transfer. (The end*request* functions should
-be used if only if the request has come down from block/bio path, not for
-direct access requests which only specify rq->buffer without a valid rq->bio)
-
-3.3 I/O Submission
-
-The routine submit_bio() is used to submit a single io. Higher level i/o
-routines make use of this:
-
-(a) Buffered i/o:
-The routine submit_bh() invokes submit_bio() on a bio corresponding to the
-bh, allocating the bio if required. ll_rw_block() uses submit_bh() as before.
-
-(b) Kiobuf i/o (for raw/direct i/o):
-The ll_rw_kio() routine breaks up the kiobuf into page sized chunks and
-maps the array to one or more multi-page bios, issuing submit_bio() to
-perform the i/o on each of these.
-
-The embedded bh array in the kiobuf structure has been removed and no
-preallocation of bios is done for kiobufs. [The intent is to remove the
-blocks array as well, but it's currently in there to kludge around direct i/o.]
-Thus kiobuf allocation has switched back to using kmalloc rather than vmalloc.
-
-Todo/Observation:
-
- A single kiobuf structure is assumed to correspond to a contiguous range
- of data, so brw_kiovec() invokes ll_rw_kio for each kiobuf in a kiovec.
- So right now it wouldn't work for direct i/o on non-contiguous blocks.
- This is to be resolved. The eventual direction is to replace kiobuf
- by kvec's.
-
- Badari Pulavarty has a patch to implement direct i/o correctly using
- bio and kvec.
-
-
-(c) Page i/o:
-Todo/Under discussion:
-
- Andrew Morton's multi-page bio patches attempt to issue multi-page
- writeouts (and reads) from the page cache, by directly building up
- large bios for submission completely bypassing the usage of buffer
- heads. This work is still in progress.
-
- Christoph Hellwig had some code that uses bios for page-io (rather than
- bh). This isn't included in bio as yet. Christoph was also working on a
- design for representing virtual/real extents as an entity and modifying
- some of the address space ops interfaces to utilize this abstraction rather
- than buffer_heads. (This is somewhat along the lines of the SGI XFS pagebuf
- abstraction, but intended to be as lightweight as possible).
-
-(d) Direct access i/o:
-Direct access requests that do not contain bios would be submitted differently
-as discussed earlier in section 1.3.
-
-Aside:
-
- Kvec i/o:
-
- Ben LaHaise's aio code uses a slightly different structure instead
- of kiobufs, called a kvec_cb. This contains an array of <page, offset, len>
- tuples (very much like the networking code), together with a callback function
- and data pointer. This is embedded into a brw_cb structure when passed
- to brw_kvec_async().
-
- Now it should be possible to directly map these kvecs to a bio. Just as while
- cloning, in this case rather than PRE_BUILT bio_vecs, we set the bi_io_vec
- array pointer to point to the veclet array in kvecs.
-
- TBD: In order for this to work, some changes are needed in the way multi-page
- bios are handled today. The values of the tuples in such a vector passed in
- from higher level code should not be modified by the block layer in the course
- of its request processing, since that would make it hard for the higher layer
- to continue to use the vector descriptor (kvec) after i/o completes. Instead,
- all such transient state should either be maintained in the request structure,
- and passed on in some way to the endio completion routine.
-
-
-4. The I/O scheduler
-I/O scheduler, a.k.a. elevator, is implemented in two layers. Generic dispatch
-queue and specific I/O schedulers. Unless stated otherwise, elevator is used
-to refer to both parts and I/O scheduler to specific I/O schedulers.
-
-Block layer implements generic dispatch queue in block/*.c.
-The generic dispatch queue is responsible for requeueing, handling non-fs
-requests and all other subtleties.
-
-Specific I/O schedulers are responsible for ordering normal filesystem
-requests. They can also choose to delay certain requests to improve
-throughput or whatever purpose. As the plural form indicates, there are
-multiple I/O schedulers. They can be built as modules but at least one should
-be built inside the kernel. Each queue can choose different one and can also
-change to another one dynamically.
-
-A block layer call to the i/o scheduler follows the convention elv_xxx(). This
-calls elevator_xxx_fn in the elevator switch (block/elevator.c). Oh, xxx
-and xxx might not match exactly, but use your imagination. If an elevator
-doesn't implement a function, the switch does nothing or some minimal house
-keeping work.
-
-4.1. I/O scheduler API
-
-The functions an elevator may implement are: (* are mandatory)
-elevator_merge_fn called to query requests for merge with a bio
-
-elevator_merge_req_fn called when two requests get merged. the one
- which gets merged into the other one will be
- never seen by I/O scheduler again. IOW, after
- being merged, the request is gone.
-
-elevator_merged_fn called when a request in the scheduler has been
- involved in a merge. It is used in the deadline
- scheduler for example, to reposition the request
- if its sorting order has changed.
-
-elevator_allow_merge_fn called whenever the block layer determines
- that a bio can be merged into an existing
- request safely. The io scheduler may still
- want to stop a merge at this point if it
- results in some sort of conflict internally,
- this hook allows it to do that. Note however
- that two *requests* can still be merged at later
- time. Currently the io scheduler has no way to
- prevent that. It can only learn about the fact
- from elevator_merge_req_fn callback.
-
-elevator_dispatch_fn* fills the dispatch queue with ready requests.
- I/O schedulers are free to postpone requests by
- not filling the dispatch queue unless @force
- is non-zero. Once dispatched, I/O schedulers
- are not allowed to manipulate the requests -
- they belong to generic dispatch queue.
-
-elevator_add_req_fn* called to add a new request into the scheduler
-
-elevator_former_req_fn
-elevator_latter_req_fn These return the request before or after the
- one specified in disk sort order. Used by the
- block layer to find merge possibilities.
-
-elevator_completed_req_fn called when a request is completed.
-
-elevator_may_queue_fn returns true if the scheduler wants to allow the
- current context to queue a new request even if
- it is over the queue limit. This must be used
- very carefully!!
-
-elevator_set_req_fn
-elevator_put_req_fn Must be used to allocate and free any elevator
- specific storage for a request.
-
-elevator_activate_req_fn Called when device driver first sees a request.
- I/O schedulers can use this callback to
- determine when actual execution of a request
- starts.
-elevator_deactivate_req_fn Called when device driver decides to delay
- a request by requeueing it.
-
-elevator_init_fn*
-elevator_exit_fn Allocate and free any elevator specific storage
- for a queue.
-
-4.2 Request flows seen by I/O schedulers
-All requests seen by I/O schedulers strictly follow one of the following three
-flows.
-
- set_req_fn ->
-
- i. add_req_fn -> (merged_fn ->)* -> dispatch_fn -> activate_req_fn ->
- (deactivate_req_fn -> activate_req_fn ->)* -> completed_req_fn
- ii. add_req_fn -> (merged_fn ->)* -> merge_req_fn
- iii. [none]
-
- -> put_req_fn
-
-4.3 I/O scheduler implementation
-The generic i/o scheduler algorithm attempts to sort/merge/batch requests for
-optimal disk scan and request servicing performance (based on generic
-principles and device capabilities), optimized for:
-i. improved throughput
-ii. improved latency
-iii. better utilization of h/w & CPU time
-
-Characteristics:
-
-i. Binary tree
-AS and deadline i/o schedulers use red black binary trees for disk position
-sorting and searching, and a fifo linked list for time-based searching. This
-gives good scalability and good availability of information. Requests are
-almost always dispatched in disk sort order, so a cache is kept of the next
-request in sort order to prevent binary tree lookups.
-
-This arrangement is not a generic block layer characteristic however, so
-elevators may implement queues as they please.
-
-ii. Merge hash
-AS and deadline use a hash table indexed by the last sector of a request. This
-enables merging code to quickly look up "back merge" candidates, even when
-multiple I/O streams are being performed at once on one disk.
-
-"Front merges", a new request being merged at the front of an existing request,
-are far less common than "back merges" due to the nature of most I/O patterns.
-Front merges are handled by the binary trees in AS and deadline schedulers.
-
-iii. Plugging the queue to batch requests in anticipation of opportunities for
- merge/sort optimizations
-
-Plugging is an approach that the current i/o scheduling algorithm resorts to so
-that it collects up enough requests in the queue to be able to take
-advantage of the sorting/merging logic in the elevator. If the
-queue is empty when a request comes in, then it plugs the request queue
-(sort of like plugging the bath tub of a vessel to get fluid to build up)
-till it fills up with a few more requests, before starting to service
-the requests. This provides an opportunity to merge/sort the requests before
-passing them down to the device. There are various conditions when the queue is
-unplugged (to open up the flow again), either through a scheduled task or
-could be on demand. For example wait_on_buffer sets the unplugging going
-through sync_buffer() running blk_run_address_space(mapping). Or the caller
-can do it explicity through blk_unplug(bdev). So in the read case,
-the queue gets explicitly unplugged as part of waiting for completion on that
-buffer.
-
-Aside:
- This is kind of controversial territory, as it's not clear if plugging is
- always the right thing to do. Devices typically have their own queues,
- and allowing a big queue to build up in software, while letting the device be
- idle for a while may not always make sense. The trick is to handle the fine
- balance between when to plug and when to open up. Also now that we have
- multi-page bios being queued in one shot, we may not need to wait to merge
- a big request from the broken up pieces coming by.
-
-4.4 I/O contexts
-I/O contexts provide a dynamically allocated per process data area. They may
-be used in I/O schedulers, and in the block layer (could be used for IO statis,
-priorities for example). See *io_context in block/ll_rw_blk.c, and as-iosched.c
-for an example of usage in an i/o scheduler.
-
-
-5. Scalability related changes
-
-5.1 Granular Locking: io_request_lock replaced by a per-queue lock
-
-The global io_request_lock has been removed as of 2.5, to avoid
-the scalability bottleneck it was causing, and has been replaced by more
-granular locking. The request queue structure has a pointer to the
-lock to be used for that queue. As a result, locking can now be
-per-queue, with a provision for sharing a lock across queues if
-necessary (e.g the scsi layer sets the queue lock pointers to the
-corresponding adapter lock, which results in a per host locking
-granularity). The locking semantics are the same, i.e. locking is
-still imposed by the block layer, grabbing the lock before
-request_fn execution which it means that lots of older drivers
-should still be SMP safe. Drivers are free to drop the queue
-lock themselves, if required. Drivers that explicitly used the
-io_request_lock for serialization need to be modified accordingly.
-Usually it's as easy as adding a global lock:
-
- static DEFINE_SPINLOCK(my_driver_lock);
-
-and passing the address to that lock to blk_init_queue().
-
-5.2 64 bit sector numbers (sector_t prepares for 64 bit support)
-
-The sector number used in the bio structure has been changed to sector_t,
-which could be defined as 64 bit in preparation for 64 bit sector support.
-
-6. Other Changes/Implications
-
-6.1 Partition re-mapping handled by the generic block layer
-
-In 2.5 some of the gendisk/partition related code has been reorganized.
-Now the generic block layer performs partition-remapping early and thus
-provides drivers with a sector number relative to whole device, rather than
-having to take partition number into account in order to arrive at the true
-sector number. The routine blk_partition_remap() is invoked by
-generic_make_request even before invoking the queue specific make_request_fn,
-so the i/o scheduler also gets to operate on whole disk sector numbers. This
-should typically not require changes to block drivers, it just never gets
-to invoke its own partition sector offset calculations since all bios
-sent are offset from the beginning of the device.
-
-
-7. A Few Tips on Migration of older drivers
-
-Old-style drivers that just use CURRENT and ignores clustered requests,
-may not need much change. The generic layer will automatically handle
-clustered requests, multi-page bios, etc for the driver.
-
-For a low performance driver or hardware that is PIO driven or just doesn't
-support scatter-gather changes should be minimal too.
-
-The following are some points to keep in mind when converting old drivers
-to bio.
-
-Drivers should use elv_next_request to pick up requests and are no longer
-supposed to handle looping directly over the request list.
-(struct request->queue has been removed)
-
-Now end_that_request_first takes an additional number_of_sectors argument.
-It used to handle always just the first buffer_head in a request, now
-it will loop and handle as many sectors (on a bio-segment granularity)
-as specified.
-
-Now bh->b_end_io is replaced by bio->bi_end_io, but most of the time the
-right thing to use is bio_endio(bio) instead.
-
-If the driver is dropping the io_request_lock from its request_fn strategy,
-then it just needs to replace that with q->queue_lock instead.
-
-As described in Sec 1.1, drivers can set max sector size, max segment size
-etc per queue now. Drivers that used to define their own merge functions i
-to handle things like this can now just use the blk_queue_* functions at
-blk_init_queue time.
-
-Drivers no longer have to map a {partition, sector offset} into the
-correct absolute location anymore, this is done by the block layer, so
-where a driver received a request ala this before:
-
- rq->rq_dev = mk_kdev(3, 5); /* /dev/hda5 */
- rq->sector = 0; /* first sector on hda5 */
-
- it will now see
-
- rq->rq_dev = mk_kdev(3, 0); /* /dev/hda */
- rq->sector = 123128; /* offset from start of disk */
-
-As mentioned, there is no virtual mapping of a bio. For DMA, this is
-not a problem as the driver probably never will need a virtual mapping.
-Instead it needs a bus mapping (dma_map_page for a single segment or
-use dma_map_sg for scatter gather) to be able to ship it to the driver. For
-PIO drivers (or drivers that need to revert to PIO transfer once in a
-while (IDE for example)), where the CPU is doing the actual data
-transfer a virtual mapping is needed. If the driver supports highmem I/O,
-(Sec 1.1, (ii) ) it needs to use kmap_atomic or similar to temporarily map
-a bio into the virtual address space.
-
-
-8. Prior/Related/Impacted patches
-
-8.1. Earlier kiobuf patches (sct/axboe/chait/hch/mkp)
-- orig kiobuf & raw i/o patches (now in 2.4 tree)
-- direct kiobuf based i/o to devices (no intermediate bh's)
-- page i/o using kiobuf
-- kiobuf splitting for lvm (mkp)
-- elevator support for kiobuf request merging (axboe)
-8.2. Zero-copy networking (Dave Miller)
-8.3. SGI XFS - pagebuf patches - use of kiobufs
-8.4. Multi-page pioent patch for bio (Christoph Hellwig)
-8.5. Direct i/o implementation (Andrea Arcangeli) since 2.4.10-pre11
-8.6. Async i/o implementation patch (Ben LaHaise)
-8.7. EVMS layering design (IBM EVMS team)
-8.8. Larger page cache size patch (Ben LaHaise) and
- Large page size (Daniel Phillips)
- => larger contiguous physical memory buffers
-8.9. VM reservations patch (Ben LaHaise)
-8.10. Write clustering patches ? (Marcelo/Quintela/Riel ?)
-8.11. Block device in page cache patch (Andrea Archangeli) - now in 2.4.10+
-8.12. Multiple block-size transfers for faster raw i/o (Shailabh Nagar,
- Badari)
-8.13 Priority based i/o scheduler - prepatches (Arjan van de Ven)
-8.14 IDE Taskfile i/o patch (Andre Hedrick)
-8.15 Multi-page writeout and readahead patches (Andrew Morton)
-8.16 Direct i/o patches for 2.5 using kvec and bio (Badari Pulavarthy)
-
-9. Other References:
-
-9.1 The Splice I/O Model - Larry McVoy (and subsequent discussions on lkml,
-and Linus' comments - Jan 2001)
-9.2 Discussions about kiobuf and bh design on lkml between sct, linus, alan
-et al - Feb-March 2001 (many of the initial thoughts that led to bio were
-brought up in this discussion thread)
-9.3 Discussions on mempool on lkml - Dec 2001.
-
--- /dev/null
+======================================
+Immutable biovecs and biovec iterators
+======================================
+
+Kent Overstreet <kmo@daterainc.com>
+
+As of 3.13, biovecs should never be modified after a bio has been submitted.
+Instead, we have a new struct bvec_iter which represents a range of a biovec -
+the iterator will be modified as the bio is completed, not the biovec.
+
+More specifically, old code that needed to partially complete a bio would
+update bi_sector and bi_size, and advance bi_idx to the next biovec. If it
+ended up partway through a biovec, it would increment bv_offset and decrement
+bv_len by the number of bytes completed in that biovec.
+
+In the new scheme of things, everything that must be mutated in order to
+partially complete a bio is segregated into struct bvec_iter: bi_sector,
+bi_size and bi_idx have been moved there; and instead of modifying bv_offset
+and bv_len, struct bvec_iter has bi_bvec_done, which represents the number of
+bytes completed in the current bvec.
+
+There are a bunch of new helper macros for hiding the gory details - in
+particular, presenting the illusion of partially completed biovecs so that
+normal code doesn't have to deal with bi_bvec_done.
+
+ * Driver code should no longer refer to biovecs directly; we now have
+ bio_iovec() and bio_iter_iovec() macros that return literal struct biovecs,
+ constructed from the raw biovecs but taking into account bi_bvec_done and
+ bi_size.
+
+ bio_for_each_segment() has been updated to take a bvec_iter argument
+ instead of an integer (that corresponded to bi_idx); for a lot of code the
+ conversion just required changing the types of the arguments to
+ bio_for_each_segment().
+
+ * Advancing a bvec_iter is done with bio_advance_iter(); bio_advance() is a
+ wrapper around bio_advance_iter() that operates on bio->bi_iter, and also
+ advances the bio integrity's iter if present.
+
+ There is a lower level advance function - bvec_iter_advance() - which takes
+ a pointer to a biovec, not a bio; this is used by the bio integrity code.
+
+What's all this get us?
+=======================
+
+Having a real iterator, and making biovecs immutable, has a number of
+advantages:
+
+ * Before, iterating over bios was very awkward when you weren't processing
+ exactly one bvec at a time - for example, bio_copy_data() in fs/bio.c,
+ which copies the contents of one bio into another. Because the biovecs
+ wouldn't necessarily be the same size, the old code was tricky convoluted -
+ it had to walk two different bios at the same time, keeping both bi_idx and
+ and offset into the current biovec for each.
+
+ The new code is much more straightforward - have a look. This sort of
+ pattern comes up in a lot of places; a lot of drivers were essentially open
+ coding bvec iterators before, and having common implementation considerably
+ simplifies a lot of code.
+
+ * Before, any code that might need to use the biovec after the bio had been
+ completed (perhaps to copy the data somewhere else, or perhaps to resubmit
+ it somewhere else if there was an error) had to save the entire bvec array
+ - again, this was being done in a fair number of places.
+
+ * Biovecs can be shared between multiple bios - a bvec iter can represent an
+ arbitrary range of an existing biovec, both starting and ending midway
+ through biovecs. This is what enables efficient splitting of arbitrary
+ bios. Note that this means we _only_ use bi_size to determine when we've
+ reached the end of a bio, not bi_vcnt - and the bio_iovec() macro takes
+ bi_size into account when constructing biovecs.
+
+ * Splitting bios is now much simpler. The old bio_split() didn't even work on
+ bios with more than a single bvec! Now, we can efficiently split arbitrary
+ size bios - because the new bio can share the old bio's biovec.
+
+ Care must be taken to ensure the biovec isn't freed while the split bio is
+ still using it, in case the original bio completes first, though. Using
+ bio_chain() when splitting bios helps with this.
+
+ * Submitting partially completed bios is now perfectly fine - this comes up
+ occasionally in stacking block drivers and various code (e.g. md and
+ bcache) had some ugly workarounds for this.
+
+ It used to be the case that submitting a partially completed bio would work
+ fine to _most_ devices, but since accessing the raw bvec array was the
+ norm, not all drivers would respect bi_idx and those would break. Now,
+ since all drivers _must_ go through the bvec iterator - and have been
+ audited to make sure they are - submitting partially completed bios is
+ perfectly fine.
+
+Other implications:
+===================
+
+ * Almost all usage of bi_idx is now incorrect and has been removed; instead,
+ where previously you would have used bi_idx you'd now use a bvec_iter,
+ probably passing it to one of the helper macros.
+
+ I.e. instead of using bio_iovec_idx() (or bio->bi_iovec[bio->bi_idx]), you
+ now use bio_iter_iovec(), which takes a bvec_iter and returns a
+ literal struct bio_vec - constructed on the fly from the raw biovec but
+ taking into account bi_bvec_done (and bi_size).
+
+ * bi_vcnt can't be trusted or relied upon by driver code - i.e. anything that
+ doesn't actually own the bio. The reason is twofold: firstly, it's not
+ actually needed for iterating over the bio anymore - we only use bi_size.
+ Secondly, when cloning a bio and reusing (a portion of) the original bio's
+ biovec, in order to calculate bi_vcnt for the new bio we'd have to iterate
+ over all the biovecs in the new bio - which is silly as it's not needed.
+
+ So, don't use bi_vcnt anymore.
+
+ * The current interface allows the block layer to split bios as needed, so we
+ could eliminate a lot of complexity particularly in stacked drivers. Code
+ that creates bios can then create whatever size bios are convenient, and
+ more importantly stacked drivers don't have to deal with both their own bio
+ size limitations and the limitations of the underlying devices. Thus
+ there's no need to define ->merge_bvec_fn() callbacks for individual block
+ drivers.
+
+Usage of helpers:
+=================
+
+* The following helpers whose names have the suffix of `_all` can only be used
+ on non-BIO_CLONED bio. They are usually used by filesystem code. Drivers
+ shouldn't use them because the bio may have been split before it reached the
+ driver.
+
+::
+
+ bio_for_each_segment_all()
+ bio_first_bvec_all()
+ bio_first_page_all()
+ bio_last_bvec_all()
+
+* The following helpers iterate over single-page segment. The passed 'struct
+ bio_vec' will contain a single-page IO vector during the iteration::
+
+ bio_for_each_segment()
+ bio_for_each_segment_all()
+
+* The following helpers iterate over multi-page bvec. The passed 'struct
+ bio_vec' will contain a multi-page IO vector during the iteration::
+
+ bio_for_each_bvec()
+ rq_for_each_bvec()
+++ /dev/null
-
-Immutable biovecs and biovec iterators:
-=======================================
-
-Kent Overstreet <kmo@daterainc.com>
-
-As of 3.13, biovecs should never be modified after a bio has been submitted.
-Instead, we have a new struct bvec_iter which represents a range of a biovec -
-the iterator will be modified as the bio is completed, not the biovec.
-
-More specifically, old code that needed to partially complete a bio would
-update bi_sector and bi_size, and advance bi_idx to the next biovec. If it
-ended up partway through a biovec, it would increment bv_offset and decrement
-bv_len by the number of bytes completed in that biovec.
-
-In the new scheme of things, everything that must be mutated in order to
-partially complete a bio is segregated into struct bvec_iter: bi_sector,
-bi_size and bi_idx have been moved there; and instead of modifying bv_offset
-and bv_len, struct bvec_iter has bi_bvec_done, which represents the number of
-bytes completed in the current bvec.
-
-There are a bunch of new helper macros for hiding the gory details - in
-particular, presenting the illusion of partially completed biovecs so that
-normal code doesn't have to deal with bi_bvec_done.
-
- * Driver code should no longer refer to biovecs directly; we now have
- bio_iovec() and bio_iter_iovec() macros that return literal struct biovecs,
- constructed from the raw biovecs but taking into account bi_bvec_done and
- bi_size.
-
- bio_for_each_segment() has been updated to take a bvec_iter argument
- instead of an integer (that corresponded to bi_idx); for a lot of code the
- conversion just required changing the types of the arguments to
- bio_for_each_segment().
-
- * Advancing a bvec_iter is done with bio_advance_iter(); bio_advance() is a
- wrapper around bio_advance_iter() that operates on bio->bi_iter, and also
- advances the bio integrity's iter if present.
-
- There is a lower level advance function - bvec_iter_advance() - which takes
- a pointer to a biovec, not a bio; this is used by the bio integrity code.
-
-What's all this get us?
-=======================
-
-Having a real iterator, and making biovecs immutable, has a number of
-advantages:
-
- * Before, iterating over bios was very awkward when you weren't processing
- exactly one bvec at a time - for example, bio_copy_data() in fs/bio.c,
- which copies the contents of one bio into another. Because the biovecs
- wouldn't necessarily be the same size, the old code was tricky convoluted -
- it had to walk two different bios at the same time, keeping both bi_idx and
- and offset into the current biovec for each.
-
- The new code is much more straightforward - have a look. This sort of
- pattern comes up in a lot of places; a lot of drivers were essentially open
- coding bvec iterators before, and having common implementation considerably
- simplifies a lot of code.
-
- * Before, any code that might need to use the biovec after the bio had been
- completed (perhaps to copy the data somewhere else, or perhaps to resubmit
- it somewhere else if there was an error) had to save the entire bvec array
- - again, this was being done in a fair number of places.
-
- * Biovecs can be shared between multiple bios - a bvec iter can represent an
- arbitrary range of an existing biovec, both starting and ending midway
- through biovecs. This is what enables efficient splitting of arbitrary
- bios. Note that this means we _only_ use bi_size to determine when we've
- reached the end of a bio, not bi_vcnt - and the bio_iovec() macro takes
- bi_size into account when constructing biovecs.
-
- * Splitting bios is now much simpler. The old bio_split() didn't even work on
- bios with more than a single bvec! Now, we can efficiently split arbitrary
- size bios - because the new bio can share the old bio's biovec.
-
- Care must be taken to ensure the biovec isn't freed while the split bio is
- still using it, in case the original bio completes first, though. Using
- bio_chain() when splitting bios helps with this.
-
- * Submitting partially completed bios is now perfectly fine - this comes up
- occasionally in stacking block drivers and various code (e.g. md and
- bcache) had some ugly workarounds for this.
-
- It used to be the case that submitting a partially completed bio would work
- fine to _most_ devices, but since accessing the raw bvec array was the
- norm, not all drivers would respect bi_idx and those would break. Now,
- since all drivers _must_ go through the bvec iterator - and have been
- audited to make sure they are - submitting partially completed bios is
- perfectly fine.
-
-Other implications:
-===================
-
- * Almost all usage of bi_idx is now incorrect and has been removed; instead,
- where previously you would have used bi_idx you'd now use a bvec_iter,
- probably passing it to one of the helper macros.
-
- I.e. instead of using bio_iovec_idx() (or bio->bi_iovec[bio->bi_idx]), you
- now use bio_iter_iovec(), which takes a bvec_iter and returns a
- literal struct bio_vec - constructed on the fly from the raw biovec but
- taking into account bi_bvec_done (and bi_size).
-
- * bi_vcnt can't be trusted or relied upon by driver code - i.e. anything that
- doesn't actually own the bio. The reason is twofold: firstly, it's not
- actually needed for iterating over the bio anymore - we only use bi_size.
- Secondly, when cloning a bio and reusing (a portion of) the original bio's
- biovec, in order to calculate bi_vcnt for the new bio we'd have to iterate
- over all the biovecs in the new bio - which is silly as it's not needed.
-
- So, don't use bi_vcnt anymore.
-
- * The current interface allows the block layer to split bios as needed, so we
- could eliminate a lot of complexity particularly in stacked drivers. Code
- that creates bios can then create whatever size bios are convenient, and
- more importantly stacked drivers don't have to deal with both their own bio
- size limitations and the limitations of the underlying devices. Thus
- there's no need to define ->merge_bvec_fn() callbacks for individual block
- drivers.
-
-Usage of helpers:
-=================
-
-* The following helpers whose names have the suffix of "_all" can only be used
-on non-BIO_CLONED bio. They are usually used by filesystem code. Drivers
-shouldn't use them because the bio may have been split before it reached the
-driver.
-
- bio_for_each_segment_all()
- bio_first_bvec_all()
- bio_first_page_all()
- bio_last_bvec_all()
-
-* The following helpers iterate over single-page segment. The passed 'struct
-bio_vec' will contain a single-page IO vector during the iteration
-
- bio_for_each_segment()
- bio_for_each_segment_all()
-
-* The following helpers iterate over multi-page bvec. The passed 'struct
-bio_vec' will contain a multi-page IO vector during the iteration
-
- bio_for_each_bvec()
- rq_for_each_bvec()
--- /dev/null
+===============================
+Generic Block Device Capability
+===============================
+
+This file documents the sysfs file block/<disk>/capability
+
+capability is a hex word indicating which capabilities a specific disk
+supports. For more information on bits not listed here, see
+include/linux/genhd.h
+
+GENHD_FL_MEDIA_CHANGE_NOTIFY
+----------------------------
+
+Value: 4
+
+When this bit is set, the disk supports Asynchronous Notification
+of media change events. These events will be broadcast to user
+space via kernel uevent.
+++ /dev/null
-Generic Block Device Capability
-===============================================================================
-This file documents the sysfs file block/<disk>/capability
-
-capability is a hex word indicating which capabilities a specific disk
-supports. For more information on bits not listed here, see
-include/linux/genhd.h
-
-Capability Value
--------------------------------------------------------------------------------
-GENHD_FL_MEDIA_CHANGE_NOTIFY 4
- When this bit is set, the disk supports Asynchronous Notification
- of media change events. These events will be broadcast to user
- space via kernel uevent.
-
--- /dev/null
+==============================================
+Embedded device command line partition parsing
+==============================================
+
+The "blkdevparts" command line option adds support for reading the
+block device partition table from the kernel command line.
+
+It is typically used for fixed block (eMMC) embedded devices.
+It has no MBR, so saves storage space. Bootloader can be easily accessed
+by absolute address of data on the block device.
+Users can easily change the partition.
+
+The format for the command line is just like mtdparts:
+
+blkdevparts=<blkdev-def>[;<blkdev-def>]
+ <blkdev-def> := <blkdev-id>:<partdef>[,<partdef>]
+ <partdef> := <size>[@<offset>](part-name)
+
+<blkdev-id>
+ block device disk name. Embedded device uses fixed block device.
+ Its disk name is also fixed, such as: mmcblk0, mmcblk1, mmcblk0boot0.
+
+<size>
+ partition size, in bytes, such as: 512, 1m, 1G.
+ size may contain an optional suffix of (upper or lower case):
+
+ K, M, G, T, P, E.
+
+ "-" is used to denote all remaining space.
+
+<offset>
+ partition start address, in bytes.
+ offset may contain an optional suffix of (upper or lower case):
+
+ K, M, G, T, P, E.
+
+(part-name)
+ partition name. Kernel sends uevent with "PARTNAME". Application can
+ create a link to block device partition with the name "PARTNAME".
+ User space application can access partition by partition name.
+
+Example:
+
+ eMMC disk names are "mmcblk0" and "mmcblk0boot0".
+
+ bootargs::
+
+ 'blkdevparts=mmcblk0:1G(data0),1G(data1),-;mmcblk0boot0:1m(boot),-(kernel)'
+
+ dmesg::
+
+ mmcblk0: p1(data0) p2(data1) p3()
+ mmcblk0boot0: p1(boot) p2(kernel)
+++ /dev/null
-Embedded device command line partition parsing
-=====================================================================
-
-The "blkdevparts" command line option adds support for reading the
-block device partition table from the kernel command line.
-
-It is typically used for fixed block (eMMC) embedded devices.
-It has no MBR, so saves storage space. Bootloader can be easily accessed
-by absolute address of data on the block device.
-Users can easily change the partition.
-
-The format for the command line is just like mtdparts:
-
-blkdevparts=<blkdev-def>[;<blkdev-def>]
- <blkdev-def> := <blkdev-id>:<partdef>[,<partdef>]
- <partdef> := <size>[@<offset>](part-name)
-
-<blkdev-id>
- block device disk name. Embedded device uses fixed block device.
- Its disk name is also fixed, such as: mmcblk0, mmcblk1, mmcblk0boot0.
-
-<size>
- partition size, in bytes, such as: 512, 1m, 1G.
- size may contain an optional suffix of (upper or lower case):
- K, M, G, T, P, E.
- "-" is used to denote all remaining space.
-
-<offset>
- partition start address, in bytes.
- offset may contain an optional suffix of (upper or lower case):
- K, M, G, T, P, E.
-
-(part-name)
- partition name. Kernel sends uevent with "PARTNAME". Application can
- create a link to block device partition with the name "PARTNAME".
- User space application can access partition by partition name.
-
-Example:
- eMMC disk names are "mmcblk0" and "mmcblk0boot0".
-
- bootargs:
- 'blkdevparts=mmcblk0:1G(data0),1G(data1),-;mmcblk0boot0:1m(boot),-(kernel)'
-
- dmesg:
- mmcblk0: p1(data0) p2(data1) p3()
- mmcblk0boot0: p1(boot) p2(kernel)
--- /dev/null
+==============
+Data Integrity
+==============
+
+1. Introduction
+===============
+
+Modern filesystems feature checksumming of data and metadata to
+protect against data corruption. However, the detection of the
+corruption is done at read time which could potentially be months
+after the data was written. At that point the original data that the
+application tried to write is most likely lost.
+
+The solution is to ensure that the disk is actually storing what the
+application meant it to. Recent additions to both the SCSI family
+protocols (SBC Data Integrity Field, SCC protection proposal) as well
+as SATA/T13 (External Path Protection) try to remedy this by adding
+support for appending integrity metadata to an I/O. The integrity
+metadata (or protection information in SCSI terminology) includes a
+checksum for each sector as well as an incrementing counter that
+ensures the individual sectors are written in the right order. And
+for some protection schemes also that the I/O is written to the right
+place on disk.
+
+Current storage controllers and devices implement various protective
+measures, for instance checksumming and scrubbing. But these
+technologies are working in their own isolated domains or at best
+between adjacent nodes in the I/O path. The interesting thing about
+DIF and the other integrity extensions is that the protection format
+is well defined and every node in the I/O path can verify the
+integrity of the I/O and reject it if corruption is detected. This
+allows not only corruption prevention but also isolation of the point
+of failure.
+
+2. The Data Integrity Extensions
+================================
+
+As written, the protocol extensions only protect the path between
+controller and storage device. However, many controllers actually
+allow the operating system to interact with the integrity metadata
+(IMD). We have been working with several FC/SAS HBA vendors to enable
+the protection information to be transferred to and from their
+controllers.
+
+The SCSI Data Integrity Field works by appending 8 bytes of protection
+information to each sector. The data + integrity metadata is stored
+in 520 byte sectors on disk. Data + IMD are interleaved when
+transferred between the controller and target. The T13 proposal is
+similar.
+
+Because it is highly inconvenient for operating systems to deal with
+520 (and 4104) byte sectors, we approached several HBA vendors and
+encouraged them to allow separation of the data and integrity metadata
+scatter-gather lists.
+
+The controller will interleave the buffers on write and split them on
+read. This means that Linux can DMA the data buffers to and from
+host memory without changes to the page cache.
+
+Also, the 16-bit CRC checksum mandated by both the SCSI and SATA specs
+is somewhat heavy to compute in software. Benchmarks found that
+calculating this checksum had a significant impact on system
+performance for a number of workloads. Some controllers allow a
+lighter-weight checksum to be used when interfacing with the operating
+system. Emulex, for instance, supports the TCP/IP checksum instead.
+The IP checksum received from the OS is converted to the 16-bit CRC
+when writing and vice versa. This allows the integrity metadata to be
+generated by Linux or the application at very low cost (comparable to
+software RAID5).
+
+The IP checksum is weaker than the CRC in terms of detecting bit
+errors. However, the strength is really in the separation of the data
+buffers and the integrity metadata. These two distinct buffers must
+match up for an I/O to complete.
+
+The separation of the data and integrity metadata buffers as well as
+the choice in checksums is referred to as the Data Integrity
+Extensions. As these extensions are outside the scope of the protocol
+bodies (T10, T13), Oracle and its partners are trying to standardize
+them within the Storage Networking Industry Association.
+
+3. Kernel Changes
+=================
+
+The data integrity framework in Linux enables protection information
+to be pinned to I/Os and sent to/received from controllers that
+support it.
+
+The advantage to the integrity extensions in SCSI and SATA is that
+they enable us to protect the entire path from application to storage
+device. However, at the same time this is also the biggest
+disadvantage. It means that the protection information must be in a
+format that can be understood by the disk.
+
+Generally Linux/POSIX applications are agnostic to the intricacies of
+the storage devices they are accessing. The virtual filesystem switch
+and the block layer make things like hardware sector size and
+transport protocols completely transparent to the application.
+
+However, this level of detail is required when preparing the
+protection information to send to a disk. Consequently, the very
+concept of an end-to-end protection scheme is a layering violation.
+It is completely unreasonable for an application to be aware whether
+it is accessing a SCSI or SATA disk.
+
+The data integrity support implemented in Linux attempts to hide this
+from the application. As far as the application (and to some extent
+the kernel) is concerned, the integrity metadata is opaque information
+that's attached to the I/O.
+
+The current implementation allows the block layer to automatically
+generate the protection information for any I/O. Eventually the
+intent is to move the integrity metadata calculation to userspace for
+user data. Metadata and other I/O that originates within the kernel
+will still use the automatic generation interface.
+
+Some storage devices allow each hardware sector to be tagged with a
+16-bit value. The owner of this tag space is the owner of the block
+device. I.e. the filesystem in most cases. The filesystem can use
+this extra space to tag sectors as they see fit. Because the tag
+space is limited, the block interface allows tagging bigger chunks by
+way of interleaving. This way, 8*16 bits of information can be
+attached to a typical 4KB filesystem block.
+
+This also means that applications such as fsck and mkfs will need
+access to manipulate the tags from user space. A passthrough
+interface for this is being worked on.
+
+
+4. Block Layer Implementation Details
+=====================================
+
+4.1 Bio
+-------
+
+The data integrity patches add a new field to struct bio when
+CONFIG_BLK_DEV_INTEGRITY is enabled. bio_integrity(bio) returns a
+pointer to a struct bip which contains the bio integrity payload.
+Essentially a bip is a trimmed down struct bio which holds a bio_vec
+containing the integrity metadata and the required housekeeping
+information (bvec pool, vector count, etc.)
+
+A kernel subsystem can enable data integrity protection on a bio by
+calling bio_integrity_alloc(bio). This will allocate and attach the
+bip to the bio.
+
+Individual pages containing integrity metadata can subsequently be
+attached using bio_integrity_add_page().
+
+bio_free() will automatically free the bip.
+
+
+4.2 Block Device
+----------------
+
+Because the format of the protection data is tied to the physical
+disk, each block device has been extended with a block integrity
+profile (struct blk_integrity). This optional profile is registered
+with the block layer using blk_integrity_register().
+
+The profile contains callback functions for generating and verifying
+the protection data, as well as getting and setting application tags.
+The profile also contains a few constants to aid in completing,
+merging and splitting the integrity metadata.
+
+Layered block devices will need to pick a profile that's appropriate
+for all subdevices. blk_integrity_compare() can help with that. DM
+and MD linear, RAID0 and RAID1 are currently supported. RAID4/5/6
+will require extra work due to the application tag.
+
+
+5.0 Block Layer Integrity API
+=============================
+
+5.1 Normal Filesystem
+---------------------
+
+ The normal filesystem is unaware that the underlying block device
+ is capable of sending/receiving integrity metadata. The IMD will
+ be automatically generated by the block layer at submit_bio() time
+ in case of a WRITE. A READ request will cause the I/O integrity
+ to be verified upon completion.
+
+ IMD generation and verification can be toggled using the::
+
+ /sys/block/<bdev>/integrity/write_generate
+
+ and::
+
+ /sys/block/<bdev>/integrity/read_verify
+
+ flags.
+
+
+5.2 Integrity-Aware Filesystem
+------------------------------
+
+ A filesystem that is integrity-aware can prepare I/Os with IMD
+ attached. It can also use the application tag space if this is
+ supported by the block device.
+
+
+ `bool bio_integrity_prep(bio);`
+
+ To generate IMD for WRITE and to set up buffers for READ, the
+ filesystem must call bio_integrity_prep(bio).
+
+ Prior to calling this function, the bio data direction and start
+ sector must be set, and the bio should have all data pages
+ added. It is up to the caller to ensure that the bio does not
+ change while I/O is in progress.
+ Complete bio with error if prepare failed for some reson.
+
+
+5.3 Passing Existing Integrity Metadata
+---------------------------------------
+
+ Filesystems that either generate their own integrity metadata or
+ are capable of transferring IMD from user space can use the
+ following calls:
+
+
+ `struct bip * bio_integrity_alloc(bio, gfp_mask, nr_pages);`
+
+ Allocates the bio integrity payload and hangs it off of the bio.
+ nr_pages indicate how many pages of protection data need to be
+ stored in the integrity bio_vec list (similar to bio_alloc()).
+
+ The integrity payload will be freed at bio_free() time.
+
+
+ `int bio_integrity_add_page(bio, page, len, offset);`
+
+ Attaches a page containing integrity metadata to an existing
+ bio. The bio must have an existing bip,
+ i.e. bio_integrity_alloc() must have been called. For a WRITE,
+ the integrity metadata in the pages must be in a format
+ understood by the target device with the notable exception that
+ the sector numbers will be remapped as the request traverses the
+ I/O stack. This implies that the pages added using this call
+ will be modified during I/O! The first reference tag in the
+ integrity metadata must have a value of bip->bip_sector.
+
+ Pages can be added using bio_integrity_add_page() as long as
+ there is room in the bip bio_vec array (nr_pages).
+
+ Upon completion of a READ operation, the attached pages will
+ contain the integrity metadata received from the storage device.
+ It is up to the receiver to process them and verify data
+ integrity upon completion.
+
+
+5.4 Registering A Block Device As Capable Of Exchanging Integrity Metadata
+--------------------------------------------------------------------------
+
+ To enable integrity exchange on a block device the gendisk must be
+ registered as capable:
+
+ `int blk_integrity_register(gendisk, blk_integrity);`
+
+ The blk_integrity struct is a template and should contain the
+ following::
+
+ static struct blk_integrity my_profile = {
+ .name = "STANDARDSBODY-TYPE-VARIANT-CSUM",
+ .generate_fn = my_generate_fn,
+ .verify_fn = my_verify_fn,
+ .tuple_size = sizeof(struct my_tuple_size),
+ .tag_size = <tag bytes per hw sector>,
+ };
+
+ 'name' is a text string which will be visible in sysfs. This is
+ part of the userland API so chose it carefully and never change
+ it. The format is standards body-type-variant.
+ E.g. T10-DIF-TYPE1-IP or T13-EPP-0-CRC.
+
+ 'generate_fn' generates appropriate integrity metadata (for WRITE).
+
+ 'verify_fn' verifies that the data buffer matches the integrity
+ metadata.
+
+ 'tuple_size' must be set to match the size of the integrity
+ metadata per sector. I.e. 8 for DIF and EPP.
+
+ 'tag_size' must be set to identify how many bytes of tag space
+ are available per hardware sector. For DIF this is either 2 or
+ 0 depending on the value of the Control Mode Page ATO bit.
+
+----------------------------------------------------------------------
+
+2007-12-24 Martin K. Petersen <martin.petersen@oracle.com>
+++ /dev/null
-----------------------------------------------------------------------
-1. INTRODUCTION
-
-Modern filesystems feature checksumming of data and metadata to
-protect against data corruption. However, the detection of the
-corruption is done at read time which could potentially be months
-after the data was written. At that point the original data that the
-application tried to write is most likely lost.
-
-The solution is to ensure that the disk is actually storing what the
-application meant it to. Recent additions to both the SCSI family
-protocols (SBC Data Integrity Field, SCC protection proposal) as well
-as SATA/T13 (External Path Protection) try to remedy this by adding
-support for appending integrity metadata to an I/O. The integrity
-metadata (or protection information in SCSI terminology) includes a
-checksum for each sector as well as an incrementing counter that
-ensures the individual sectors are written in the right order. And
-for some protection schemes also that the I/O is written to the right
-place on disk.
-
-Current storage controllers and devices implement various protective
-measures, for instance checksumming and scrubbing. But these
-technologies are working in their own isolated domains or at best
-between adjacent nodes in the I/O path. The interesting thing about
-DIF and the other integrity extensions is that the protection format
-is well defined and every node in the I/O path can verify the
-integrity of the I/O and reject it if corruption is detected. This
-allows not only corruption prevention but also isolation of the point
-of failure.
-
-----------------------------------------------------------------------
-2. THE DATA INTEGRITY EXTENSIONS
-
-As written, the protocol extensions only protect the path between
-controller and storage device. However, many controllers actually
-allow the operating system to interact with the integrity metadata
-(IMD). We have been working with several FC/SAS HBA vendors to enable
-the protection information to be transferred to and from their
-controllers.
-
-The SCSI Data Integrity Field works by appending 8 bytes of protection
-information to each sector. The data + integrity metadata is stored
-in 520 byte sectors on disk. Data + IMD are interleaved when
-transferred between the controller and target. The T13 proposal is
-similar.
-
-Because it is highly inconvenient for operating systems to deal with
-520 (and 4104) byte sectors, we approached several HBA vendors and
-encouraged them to allow separation of the data and integrity metadata
-scatter-gather lists.
-
-The controller will interleave the buffers on write and split them on
-read. This means that Linux can DMA the data buffers to and from
-host memory without changes to the page cache.
-
-Also, the 16-bit CRC checksum mandated by both the SCSI and SATA specs
-is somewhat heavy to compute in software. Benchmarks found that
-calculating this checksum had a significant impact on system
-performance for a number of workloads. Some controllers allow a
-lighter-weight checksum to be used when interfacing with the operating
-system. Emulex, for instance, supports the TCP/IP checksum instead.
-The IP checksum received from the OS is converted to the 16-bit CRC
-when writing and vice versa. This allows the integrity metadata to be
-generated by Linux or the application at very low cost (comparable to
-software RAID5).
-
-The IP checksum is weaker than the CRC in terms of detecting bit
-errors. However, the strength is really in the separation of the data
-buffers and the integrity metadata. These two distinct buffers must
-match up for an I/O to complete.
-
-The separation of the data and integrity metadata buffers as well as
-the choice in checksums is referred to as the Data Integrity
-Extensions. As these extensions are outside the scope of the protocol
-bodies (T10, T13), Oracle and its partners are trying to standardize
-them within the Storage Networking Industry Association.
-
-----------------------------------------------------------------------
-3. KERNEL CHANGES
-
-The data integrity framework in Linux enables protection information
-to be pinned to I/Os and sent to/received from controllers that
-support it.
-
-The advantage to the integrity extensions in SCSI and SATA is that
-they enable us to protect the entire path from application to storage
-device. However, at the same time this is also the biggest
-disadvantage. It means that the protection information must be in a
-format that can be understood by the disk.
-
-Generally Linux/POSIX applications are agnostic to the intricacies of
-the storage devices they are accessing. The virtual filesystem switch
-and the block layer make things like hardware sector size and
-transport protocols completely transparent to the application.
-
-However, this level of detail is required when preparing the
-protection information to send to a disk. Consequently, the very
-concept of an end-to-end protection scheme is a layering violation.
-It is completely unreasonable for an application to be aware whether
-it is accessing a SCSI or SATA disk.
-
-The data integrity support implemented in Linux attempts to hide this
-from the application. As far as the application (and to some extent
-the kernel) is concerned, the integrity metadata is opaque information
-that's attached to the I/O.
-
-The current implementation allows the block layer to automatically
-generate the protection information for any I/O. Eventually the
-intent is to move the integrity metadata calculation to userspace for
-user data. Metadata and other I/O that originates within the kernel
-will still use the automatic generation interface.
-
-Some storage devices allow each hardware sector to be tagged with a
-16-bit value. The owner of this tag space is the owner of the block
-device. I.e. the filesystem in most cases. The filesystem can use
-this extra space to tag sectors as they see fit. Because the tag
-space is limited, the block interface allows tagging bigger chunks by
-way of interleaving. This way, 8*16 bits of information can be
-attached to a typical 4KB filesystem block.
-
-This also means that applications such as fsck and mkfs will need
-access to manipulate the tags from user space. A passthrough
-interface for this is being worked on.
-
-
-----------------------------------------------------------------------
-4. BLOCK LAYER IMPLEMENTATION DETAILS
-
-4.1 BIO
-
-The data integrity patches add a new field to struct bio when
-CONFIG_BLK_DEV_INTEGRITY is enabled. bio_integrity(bio) returns a
-pointer to a struct bip which contains the bio integrity payload.
-Essentially a bip is a trimmed down struct bio which holds a bio_vec
-containing the integrity metadata and the required housekeeping
-information (bvec pool, vector count, etc.)
-
-A kernel subsystem can enable data integrity protection on a bio by
-calling bio_integrity_alloc(bio). This will allocate and attach the
-bip to the bio.
-
-Individual pages containing integrity metadata can subsequently be
-attached using bio_integrity_add_page().
-
-bio_free() will automatically free the bip.
-
-
-4.2 BLOCK DEVICE
-
-Because the format of the protection data is tied to the physical
-disk, each block device has been extended with a block integrity
-profile (struct blk_integrity). This optional profile is registered
-with the block layer using blk_integrity_register().
-
-The profile contains callback functions for generating and verifying
-the protection data, as well as getting and setting application tags.
-The profile also contains a few constants to aid in completing,
-merging and splitting the integrity metadata.
-
-Layered block devices will need to pick a profile that's appropriate
-for all subdevices. blk_integrity_compare() can help with that. DM
-and MD linear, RAID0 and RAID1 are currently supported. RAID4/5/6
-will require extra work due to the application tag.
-
-
-----------------------------------------------------------------------
-5.0 BLOCK LAYER INTEGRITY API
-
-5.1 NORMAL FILESYSTEM
-
- The normal filesystem is unaware that the underlying block device
- is capable of sending/receiving integrity metadata. The IMD will
- be automatically generated by the block layer at submit_bio() time
- in case of a WRITE. A READ request will cause the I/O integrity
- to be verified upon completion.
-
- IMD generation and verification can be toggled using the
-
- /sys/block/<bdev>/integrity/write_generate
-
- and
-
- /sys/block/<bdev>/integrity/read_verify
-
- flags.
-
-
-5.2 INTEGRITY-AWARE FILESYSTEM
-
- A filesystem that is integrity-aware can prepare I/Os with IMD
- attached. It can also use the application tag space if this is
- supported by the block device.
-
-
- bool bio_integrity_prep(bio);
-
- To generate IMD for WRITE and to set up buffers for READ, the
- filesystem must call bio_integrity_prep(bio).
-
- Prior to calling this function, the bio data direction and start
- sector must be set, and the bio should have all data pages
- added. It is up to the caller to ensure that the bio does not
- change while I/O is in progress.
- Complete bio with error if prepare failed for some reson.
-
-
-5.3 PASSING EXISTING INTEGRITY METADATA
-
- Filesystems that either generate their own integrity metadata or
- are capable of transferring IMD from user space can use the
- following calls:
-
-
- struct bip * bio_integrity_alloc(bio, gfp_mask, nr_pages);
-
- Allocates the bio integrity payload and hangs it off of the bio.
- nr_pages indicate how many pages of protection data need to be
- stored in the integrity bio_vec list (similar to bio_alloc()).
-
- The integrity payload will be freed at bio_free() time.
-
-
- int bio_integrity_add_page(bio, page, len, offset);
-
- Attaches a page containing integrity metadata to an existing
- bio. The bio must have an existing bip,
- i.e. bio_integrity_alloc() must have been called. For a WRITE,
- the integrity metadata in the pages must be in a format
- understood by the target device with the notable exception that
- the sector numbers will be remapped as the request traverses the
- I/O stack. This implies that the pages added using this call
- will be modified during I/O! The first reference tag in the
- integrity metadata must have a value of bip->bip_sector.
-
- Pages can be added using bio_integrity_add_page() as long as
- there is room in the bip bio_vec array (nr_pages).
-
- Upon completion of a READ operation, the attached pages will
- contain the integrity metadata received from the storage device.
- It is up to the receiver to process them and verify data
- integrity upon completion.
-
-
-5.4 REGISTERING A BLOCK DEVICE AS CAPABLE OF EXCHANGING INTEGRITY
- METADATA
-
- To enable integrity exchange on a block device the gendisk must be
- registered as capable:
-
- int blk_integrity_register(gendisk, blk_integrity);
-
- The blk_integrity struct is a template and should contain the
- following:
-
- static struct blk_integrity my_profile = {
- .name = "STANDARDSBODY-TYPE-VARIANT-CSUM",
- .generate_fn = my_generate_fn,
- .verify_fn = my_verify_fn,
- .tuple_size = sizeof(struct my_tuple_size),
- .tag_size = <tag bytes per hw sector>,
- };
-
- 'name' is a text string which will be visible in sysfs. This is
- part of the userland API so chose it carefully and never change
- it. The format is standards body-type-variant.
- E.g. T10-DIF-TYPE1-IP or T13-EPP-0-CRC.
-
- 'generate_fn' generates appropriate integrity metadata (for WRITE).
-
- 'verify_fn' verifies that the data buffer matches the integrity
- metadata.
-
- 'tuple_size' must be set to match the size of the integrity
- metadata per sector. I.e. 8 for DIF and EPP.
-
- 'tag_size' must be set to identify how many bytes of tag space
- are available per hardware sector. For DIF this is either 2 or
- 0 depending on the value of the Control Mode Page ATO bit.
-
-----------------------------------------------------------------------
-2007-12-24 Martin K. Petersen <martin.petersen@oracle.com>
--- /dev/null
+==============================
+Deadline IO scheduler tunables
+==============================
+
+This little file attempts to document how the deadline io scheduler works.
+In particular, it will clarify the meaning of the exposed tunables that may be
+of interest to power users.
+
+Selecting IO schedulers
+-----------------------
+Refer to Documentation/block/switching-sched.rst for information on
+selecting an io scheduler on a per-device basis.
+
+------------------------------------------------------------------------------
+
+read_expire (in ms)
+-----------------------
+
+The goal of the deadline io scheduler is to attempt to guarantee a start
+service time for a request. As we focus mainly on read latencies, this is
+tunable. When a read request first enters the io scheduler, it is assigned
+a deadline that is the current time + the read_expire value in units of
+milliseconds.
+
+
+write_expire (in ms)
+-----------------------
+
+Similar to read_expire mentioned above, but for writes.
+
+
+fifo_batch (number of requests)
+------------------------------------
+
+Requests are grouped into ``batches`` of a particular data direction (read or
+write) which are serviced in increasing sector order. To limit extra seeking,
+deadline expiries are only checked between batches. fifo_batch controls the
+maximum number of requests per batch.
+
+This parameter tunes the balance between per-request latency and aggregate
+throughput. When low latency is the primary concern, smaller is better (where
+a value of 1 yields first-come first-served behaviour). Increasing fifo_batch
+generally improves throughput, at the cost of latency variation.
+
+
+writes_starved (number of dispatches)
+--------------------------------------
+
+When we have to move requests from the io scheduler queue to the block
+device dispatch queue, we always give a preference to reads. However, we
+don't want to starve writes indefinitely either. So writes_starved controls
+how many times we give preference to reads over writes. When that has been
+done writes_starved number of times, we dispatch some writes based on the
+same criteria as reads.
+
+
+front_merges (bool)
+----------------------
+
+Sometimes it happens that a request enters the io scheduler that is contiguous
+with a request that is already on the queue. Either it fits in the back of that
+request, or it fits at the front. That is called either a back merge candidate
+or a front merge candidate. Due to the way files are typically laid out,
+back merges are much more common than front merges. For some work loads, you
+may even know that it is a waste of time to spend any time attempting to
+front merge requests. Setting front_merges to 0 disables this functionality.
+Front merges may still occur due to the cached last_merge hint, but since
+that comes at basically 0 cost we leave that on. We simply disable the
+rbtree front sector lookup when the io scheduler merge function is called.
+
+
+Nov 11 2002, Jens Axboe <jens.axboe@oracle.com>
+++ /dev/null
-Deadline IO scheduler tunables
-==============================
-
-This little file attempts to document how the deadline io scheduler works.
-In particular, it will clarify the meaning of the exposed tunables that may be
-of interest to power users.
-
-Selecting IO schedulers
------------------------
-Refer to Documentation/block/switching-sched.txt for information on
-selecting an io scheduler on a per-device basis.
-
-
-********************************************************************************
-
-
-read_expire (in ms)
------------
-
-The goal of the deadline io scheduler is to attempt to guarantee a start
-service time for a request. As we focus mainly on read latencies, this is
-tunable. When a read request first enters the io scheduler, it is assigned
-a deadline that is the current time + the read_expire value in units of
-milliseconds.
-
-
-write_expire (in ms)
------------
-
-Similar to read_expire mentioned above, but for writes.
-
-
-fifo_batch (number of requests)
-----------
-
-Requests are grouped into ``batches'' of a particular data direction (read or
-write) which are serviced in increasing sector order. To limit extra seeking,
-deadline expiries are only checked between batches. fifo_batch controls the
-maximum number of requests per batch.
-
-This parameter tunes the balance between per-request latency and aggregate
-throughput. When low latency is the primary concern, smaller is better (where
-a value of 1 yields first-come first-served behaviour). Increasing fifo_batch
-generally improves throughput, at the cost of latency variation.
-
-
-writes_starved (number of dispatches)
---------------
-
-When we have to move requests from the io scheduler queue to the block
-device dispatch queue, we always give a preference to reads. However, we
-don't want to starve writes indefinitely either. So writes_starved controls
-how many times we give preference to reads over writes. When that has been
-done writes_starved number of times, we dispatch some writes based on the
-same criteria as reads.
-
-
-front_merges (bool)
-------------
-
-Sometimes it happens that a request enters the io scheduler that is contiguous
-with a request that is already on the queue. Either it fits in the back of that
-request, or it fits at the front. That is called either a back merge candidate
-or a front merge candidate. Due to the way files are typically laid out,
-back merges are much more common than front merges. For some work loads, you
-may even know that it is a waste of time to spend any time attempting to
-front merge requests. Setting front_merges to 0 disables this functionality.
-Front merges may still occur due to the cached last_merge hint, but since
-that comes at basically 0 cost we leave that on. We simply disable the
-rbtree front sector lookup when the io scheduler merge function is called.
-
-
-Nov 11 2002, Jens Axboe <jens.axboe@oracle.com>
-
-
--- /dev/null
+:orphan:
+
+=====
+Block
+=====
+
+.. toctree::
+ :maxdepth: 1
+
+ bfq-iosched
+ biodoc
+ biovecs
+ capability
+ cmdline-partition
+ data-integrity
+ deadline-iosched
+ ioprio
+ kyber-iosched
+ null_blk
+ pr
+ queue-sysfs
+ request
+ stat
+ switching-sched
+ writeback_cache_control
--- /dev/null
+===================
+Block io priorities
+===================
+
+
+Intro
+-----
+
+With the introduction of cfq v3 (aka cfq-ts or time sliced cfq), basic io
+priorities are supported for reads on files. This enables users to io nice
+processes or process groups, similar to what has been possible with cpu
+scheduling for ages. This document mainly details the current possibilities
+with cfq; other io schedulers do not support io priorities thus far.
+
+Scheduling classes
+------------------
+
+CFQ implements three generic scheduling classes that determine how io is
+served for a process.
+
+IOPRIO_CLASS_RT: This is the realtime io class. This scheduling class is given
+higher priority than any other in the system, processes from this class are
+given first access to the disk every time. Thus it needs to be used with some
+care, one io RT process can starve the entire system. Within the RT class,
+there are 8 levels of class data that determine exactly how much time this
+process needs the disk for on each service. In the future this might change
+to be more directly mappable to performance, by passing in a wanted data
+rate instead.
+
+IOPRIO_CLASS_BE: This is the best-effort scheduling class, which is the default
+for any process that hasn't set a specific io priority. The class data
+determines how much io bandwidth the process will get, it's directly mappable
+to the cpu nice levels just more coarsely implemented. 0 is the highest
+BE prio level, 7 is the lowest. The mapping between cpu nice level and io
+nice level is determined as: io_nice = (cpu_nice + 20) / 5.
+
+IOPRIO_CLASS_IDLE: This is the idle scheduling class, processes running at this
+level only get io time when no one else needs the disk. The idle class has no
+class data, since it doesn't really apply here.
+
+Tools
+-----
+
+See below for a sample ionice tool. Usage::
+
+ # ionice -c<class> -n<level> -p<pid>
+
+If pid isn't given, the current process is assumed. IO priority settings
+are inherited on fork, so you can use ionice to start the process at a given
+level::
+
+ # ionice -c2 -n0 /bin/ls
+
+will run ls at the best-effort scheduling class at the highest priority.
+For a running process, you can give the pid instead::
+
+ # ionice -c1 -n2 -p100
+
+will change pid 100 to run at the realtime scheduling class, at priority 2.
+
+ionice.c tool::
+
+ #include <stdio.h>
+ #include <stdlib.h>
+ #include <errno.h>
+ #include <getopt.h>
+ #include <unistd.h>
+ #include <sys/ptrace.h>
+ #include <asm/unistd.h>
+
+ extern int sys_ioprio_set(int, int, int);
+ extern int sys_ioprio_get(int, int);
+
+ #if defined(__i386__)
+ #define __NR_ioprio_set 289
+ #define __NR_ioprio_get 290
+ #elif defined(__ppc__)
+ #define __NR_ioprio_set 273
+ #define __NR_ioprio_get 274
+ #elif defined(__x86_64__)
+ #define __NR_ioprio_set 251
+ #define __NR_ioprio_get 252
+ #elif defined(__ia64__)
+ #define __NR_ioprio_set 1274
+ #define __NR_ioprio_get 1275
+ #else
+ #error "Unsupported arch"
+ #endif
+
+ static inline int ioprio_set(int which, int who, int ioprio)
+ {
+ return syscall(__NR_ioprio_set, which, who, ioprio);
+ }
+
+ static inline int ioprio_get(int which, int who)
+ {
+ return syscall(__NR_ioprio_get, which, who);
+ }
+
+ enum {
+ IOPRIO_CLASS_NONE,
+ IOPRIO_CLASS_RT,
+ IOPRIO_CLASS_BE,
+ IOPRIO_CLASS_IDLE,
+ };
+
+ enum {
+ IOPRIO_WHO_PROCESS = 1,
+ IOPRIO_WHO_PGRP,
+ IOPRIO_WHO_USER,
+ };
+
+ #define IOPRIO_CLASS_SHIFT 13
+
+ const char *to_prio[] = { "none", "realtime", "best-effort", "idle", };
+
+ int main(int argc, char *argv[])
+ {
+ int ioprio = 4, set = 0, ioprio_class = IOPRIO_CLASS_BE;
+ int c, pid = 0;
+
+ while ((c = getopt(argc, argv, "+n:c:p:")) != EOF) {
+ switch (c) {
+ case 'n':
+ ioprio = strtol(optarg, NULL, 10);
+ set = 1;
+ break;
+ case 'c':
+ ioprio_class = strtol(optarg, NULL, 10);
+ set = 1;
+ break;
+ case 'p':
+ pid = strtol(optarg, NULL, 10);
+ break;
+ }
+ }
+
+ switch (ioprio_class) {
+ case IOPRIO_CLASS_NONE:
+ ioprio_class = IOPRIO_CLASS_BE;
+ break;
+ case IOPRIO_CLASS_RT:
+ case IOPRIO_CLASS_BE:
+ break;
+ case IOPRIO_CLASS_IDLE:
+ ioprio = 7;
+ break;
+ default:
+ printf("bad prio class %d\n", ioprio_class);
+ return 1;
+ }
+
+ if (!set) {
+ if (!pid && argv[optind])
+ pid = strtol(argv[optind], NULL, 10);
+
+ ioprio = ioprio_get(IOPRIO_WHO_PROCESS, pid);
+
+ printf("pid=%d, %d\n", pid, ioprio);
+
+ if (ioprio == -1)
+ perror("ioprio_get");
+ else {
+ ioprio_class = ioprio >> IOPRIO_CLASS_SHIFT;
+ ioprio = ioprio & 0xff;
+ printf("%s: prio %d\n", to_prio[ioprio_class], ioprio);
+ }
+ } else {
+ if (ioprio_set(IOPRIO_WHO_PROCESS, pid, ioprio | ioprio_class << IOPRIO_CLASS_SHIFT) == -1) {
+ perror("ioprio_set");
+ return 1;
+ }
+
+ if (argv[optind])
+ execvp(argv[optind], &argv[optind]);
+ }
+
+ return 0;
+ }
+
+
+March 11 2005, Jens Axboe <jens.axboe@oracle.com>
+++ /dev/null
-Block io priorities
-===================
-
-
-Intro
------
-
-With the introduction of cfq v3 (aka cfq-ts or time sliced cfq), basic io
-priorities are supported for reads on files. This enables users to io nice
-processes or process groups, similar to what has been possible with cpu
-scheduling for ages. This document mainly details the current possibilities
-with cfq; other io schedulers do not support io priorities thus far.
-
-Scheduling classes
-------------------
-
-CFQ implements three generic scheduling classes that determine how io is
-served for a process.
-
-IOPRIO_CLASS_RT: This is the realtime io class. This scheduling class is given
-higher priority than any other in the system, processes from this class are
-given first access to the disk every time. Thus it needs to be used with some
-care, one io RT process can starve the entire system. Within the RT class,
-there are 8 levels of class data that determine exactly how much time this
-process needs the disk for on each service. In the future this might change
-to be more directly mappable to performance, by passing in a wanted data
-rate instead.
-
-IOPRIO_CLASS_BE: This is the best-effort scheduling class, which is the default
-for any process that hasn't set a specific io priority. The class data
-determines how much io bandwidth the process will get, it's directly mappable
-to the cpu nice levels just more coarsely implemented. 0 is the highest
-BE prio level, 7 is the lowest. The mapping between cpu nice level and io
-nice level is determined as: io_nice = (cpu_nice + 20) / 5.
-
-IOPRIO_CLASS_IDLE: This is the idle scheduling class, processes running at this
-level only get io time when no one else needs the disk. The idle class has no
-class data, since it doesn't really apply here.
-
-Tools
------
-
-See below for a sample ionice tool. Usage:
-
-# ionice -c<class> -n<level> -p<pid>
-
-If pid isn't given, the current process is assumed. IO priority settings
-are inherited on fork, so you can use ionice to start the process at a given
-level:
-
-# ionice -c2 -n0 /bin/ls
-
-will run ls at the best-effort scheduling class at the highest priority.
-For a running process, you can give the pid instead:
-
-# ionice -c1 -n2 -p100
-
-will change pid 100 to run at the realtime scheduling class, at priority 2.
-
----> snip ionice.c tool <---
-
-#include <stdio.h>
-#include <stdlib.h>
-#include <errno.h>
-#include <getopt.h>
-#include <unistd.h>
-#include <sys/ptrace.h>
-#include <asm/unistd.h>
-
-extern int sys_ioprio_set(int, int, int);
-extern int sys_ioprio_get(int, int);
-
-#if defined(__i386__)
-#define __NR_ioprio_set 289
-#define __NR_ioprio_get 290
-#elif defined(__ppc__)
-#define __NR_ioprio_set 273
-#define __NR_ioprio_get 274
-#elif defined(__x86_64__)
-#define __NR_ioprio_set 251
-#define __NR_ioprio_get 252
-#elif defined(__ia64__)
-#define __NR_ioprio_set 1274
-#define __NR_ioprio_get 1275
-#else
-#error "Unsupported arch"
-#endif
-
-static inline int ioprio_set(int which, int who, int ioprio)
-{
- return syscall(__NR_ioprio_set, which, who, ioprio);
-}
-
-static inline int ioprio_get(int which, int who)
-{
- return syscall(__NR_ioprio_get, which, who);
-}
-
-enum {
- IOPRIO_CLASS_NONE,
- IOPRIO_CLASS_RT,
- IOPRIO_CLASS_BE,
- IOPRIO_CLASS_IDLE,
-};
-
-enum {
- IOPRIO_WHO_PROCESS = 1,
- IOPRIO_WHO_PGRP,
- IOPRIO_WHO_USER,
-};
-
-#define IOPRIO_CLASS_SHIFT 13
-
-const char *to_prio[] = { "none", "realtime", "best-effort", "idle", };
-
-int main(int argc, char *argv[])
-{
- int ioprio = 4, set = 0, ioprio_class = IOPRIO_CLASS_BE;
- int c, pid = 0;
-
- while ((c = getopt(argc, argv, "+n:c:p:")) != EOF) {
- switch (c) {
- case 'n':
- ioprio = strtol(optarg, NULL, 10);
- set = 1;
- break;
- case 'c':
- ioprio_class = strtol(optarg, NULL, 10);
- set = 1;
- break;
- case 'p':
- pid = strtol(optarg, NULL, 10);
- break;
- }
- }
-
- switch (ioprio_class) {
- case IOPRIO_CLASS_NONE:
- ioprio_class = IOPRIO_CLASS_BE;
- break;
- case IOPRIO_CLASS_RT:
- case IOPRIO_CLASS_BE:
- break;
- case IOPRIO_CLASS_IDLE:
- ioprio = 7;
- break;
- default:
- printf("bad prio class %d\n", ioprio_class);
- return 1;
- }
-
- if (!set) {
- if (!pid && argv[optind])
- pid = strtol(argv[optind], NULL, 10);
-
- ioprio = ioprio_get(IOPRIO_WHO_PROCESS, pid);
-
- printf("pid=%d, %d\n", pid, ioprio);
-
- if (ioprio == -1)
- perror("ioprio_get");
- else {
- ioprio_class = ioprio >> IOPRIO_CLASS_SHIFT;
- ioprio = ioprio & 0xff;
- printf("%s: prio %d\n", to_prio[ioprio_class], ioprio);
- }
- } else {
- if (ioprio_set(IOPRIO_WHO_PROCESS, pid, ioprio | ioprio_class << IOPRIO_CLASS_SHIFT) == -1) {
- perror("ioprio_set");
- return 1;
- }
-
- if (argv[optind])
- execvp(argv[optind], &argv[optind]);
- }
-
- return 0;
-}
-
----> snip ionice.c tool <---
-
-
-March 11 2005, Jens Axboe <jens.axboe@oracle.com>
--- /dev/null
+============================
+Kyber I/O scheduler tunables
+============================
+
+The only two tunables for the Kyber scheduler are the target latencies for
+reads and synchronous writes. Kyber will throttle requests in order to meet
+these target latencies.
+
+read_lat_nsec
+-------------
+Target latency for reads (in nanoseconds).
+
+write_lat_nsec
+--------------
+Target latency for synchronous writes (in nanoseconds).
+++ /dev/null
-Kyber I/O scheduler tunables
-===========================
-
-The only two tunables for the Kyber scheduler are the target latencies for
-reads and synchronous writes. Kyber will throttle requests in order to meet
-these target latencies.
-
-read_lat_nsec
--------------
-Target latency for reads (in nanoseconds).
-
-write_lat_nsec
---------------
-Target latency for synchronous writes (in nanoseconds).
--- /dev/null
+========================
+Null block device driver
+========================
+
+1. Overview
+===========
+
+The null block device (/dev/nullb*) is used for benchmarking the various
+block-layer implementations. It emulates a block device of X gigabytes in size.
+The following instances are possible:
+
+ Single-queue block-layer
+
+ - Request-based.
+ - Single submission queue per device.
+ - Implements IO scheduling algorithms (CFQ, Deadline, noop).
+
+ Multi-queue block-layer
+
+ - Request-based.
+ - Configurable submission queues per device.
+
+ No block-layer (Known as bio-based)
+
+ - Bio-based. IO requests are submitted directly to the device driver.
+ - Directly accepts bio data structure and returns them.
+
+All of them have a completion queue for each core in the system.
+
+2. Module parameters applicable for all instances
+=================================================
+
+queue_mode=[0-2]: Default: 2-Multi-queue
+ Selects which block-layer the module should instantiate with.
+
+ = ============
+ 0 Bio-based
+ 1 Single-queue
+ 2 Multi-queue
+ = ============
+
+home_node=[0--nr_nodes]: Default: NUMA_NO_NODE
+ Selects what CPU node the data structures are allocated from.
+
+gb=[Size in GB]: Default: 250GB
+ The size of the device reported to the system.
+
+bs=[Block size (in bytes)]: Default: 512 bytes
+ The block size reported to the system.
+
+nr_devices=[Number of devices]: Default: 1
+ Number of block devices instantiated. They are instantiated as /dev/nullb0,
+ etc.
+
+irqmode=[0-2]: Default: 1-Soft-irq
+ The completion mode used for completing IOs to the block-layer.
+
+ = ===========================================================================
+ 0 None.
+ 1 Soft-irq. Uses IPI to complete IOs across CPU nodes. Simulates the overhead
+ when IOs are issued from another CPU node than the home the device is
+ connected to.
+ 2 Timer: Waits a specific period (completion_nsec) for each IO before
+ completion.
+ = ===========================================================================
+
+completion_nsec=[ns]: Default: 10,000ns
+ Combined with irqmode=2 (timer). The time each completion event must wait.
+
+submit_queues=[1..nr_cpus]:
+ The number of submission queues attached to the device driver. If unset, it
+ defaults to 1. For multi-queue, it is ignored when use_per_node_hctx module
+ parameter is 1.
+
+hw_queue_depth=[0..qdepth]: Default: 64
+ The hardware queue depth of the device.
+
+III: Multi-queue specific parameters
+
+use_per_node_hctx=[0/1]: Default: 0
+
+ = =====================================================================
+ 0 The number of submit queues are set to the value of the submit_queues
+ parameter.
+ 1 The multi-queue block layer is instantiated with a hardware dispatch
+ queue for each CPU node in the system.
+ = =====================================================================
+
+no_sched=[0/1]: Default: 0
+
+ = ======================================
+ 0 nullb* use default blk-mq io scheduler
+ 1 nullb* doesn't use io scheduler
+ = ======================================
+
+blocking=[0/1]: Default: 0
+
+ = ===============================================================
+ 0 Register as a non-blocking blk-mq driver device.
+ 1 Register as a blocking blk-mq driver device, null_blk will set
+ the BLK_MQ_F_BLOCKING flag, indicating that it sometimes/always
+ needs to block in its ->queue_rq() function.
+ = ===============================================================
+
+shared_tags=[0/1]: Default: 0
+
+ = ================================================================
+ 0 Tag set is not shared.
+ 1 Tag set shared between devices for blk-mq. Only makes sense with
+ nr_devices > 1, otherwise there's no tag set to share.
+ = ================================================================
+
+zoned=[0/1]: Default: 0
+
+ = ======================================================================
+ 0 Block device is exposed as a random-access block device.
+ 1 Block device is exposed as a host-managed zoned block device. Requires
+ CONFIG_BLK_DEV_ZONED.
+ = ======================================================================
+
+zone_size=[MB]: Default: 256
+ Per zone size when exposed as a zoned block device. Must be a power of two.
+
+zone_nr_conv=[nr_conv]: Default: 0
+ The number of conventional zones to create when block device is zoned. If
+ zone_nr_conv >= nr_zones, it will be reduced to nr_zones - 1.
+++ /dev/null
-Null block device driver
-================================================================================
-
-I. Overview
-
-The null block device (/dev/nullb*) is used for benchmarking the various
-block-layer implementations. It emulates a block device of X gigabytes in size.
-The following instances are possible:
-
- Single-queue block-layer
- - Request-based.
- - Single submission queue per device.
- - Implements IO scheduling algorithms (CFQ, Deadline, noop).
- Multi-queue block-layer
- - Request-based.
- - Configurable submission queues per device.
- No block-layer (Known as bio-based)
- - Bio-based. IO requests are submitted directly to the device driver.
- - Directly accepts bio data structure and returns them.
-
-All of them have a completion queue for each core in the system.
-
-II. Module parameters applicable for all instances:
-
-queue_mode=[0-2]: Default: 2-Multi-queue
- Selects which block-layer the module should instantiate with.
-
- 0: Bio-based.
- 1: Single-queue.
- 2: Multi-queue.
-
-home_node=[0--nr_nodes]: Default: NUMA_NO_NODE
- Selects what CPU node the data structures are allocated from.
-
-gb=[Size in GB]: Default: 250GB
- The size of the device reported to the system.
-
-bs=[Block size (in bytes)]: Default: 512 bytes
- The block size reported to the system.
-
-nr_devices=[Number of devices]: Default: 1
- Number of block devices instantiated. They are instantiated as /dev/nullb0,
- etc.
-
-irqmode=[0-2]: Default: 1-Soft-irq
- The completion mode used for completing IOs to the block-layer.
-
- 0: None.
- 1: Soft-irq. Uses IPI to complete IOs across CPU nodes. Simulates the overhead
- when IOs are issued from another CPU node than the home the device is
- connected to.
- 2: Timer: Waits a specific period (completion_nsec) for each IO before
- completion.
-
-completion_nsec=[ns]: Default: 10,000ns
- Combined with irqmode=2 (timer). The time each completion event must wait.
-
-submit_queues=[1..nr_cpus]:
- The number of submission queues attached to the device driver. If unset, it
- defaults to 1. For multi-queue, it is ignored when use_per_node_hctx module
- parameter is 1.
-
-hw_queue_depth=[0..qdepth]: Default: 64
- The hardware queue depth of the device.
-
-III: Multi-queue specific parameters
-
-use_per_node_hctx=[0/1]: Default: 0
- 0: The number of submit queues are set to the value of the submit_queues
- parameter.
- 1: The multi-queue block layer is instantiated with a hardware dispatch
- queue for each CPU node in the system.
-
-no_sched=[0/1]: Default: 0
- 0: nullb* use default blk-mq io scheduler.
- 1: nullb* doesn't use io scheduler.
-
-blocking=[0/1]: Default: 0
- 0: Register as a non-blocking blk-mq driver device.
- 1: Register as a blocking blk-mq driver device, null_blk will set
- the BLK_MQ_F_BLOCKING flag, indicating that it sometimes/always
- needs to block in its ->queue_rq() function.
-
-shared_tags=[0/1]: Default: 0
- 0: Tag set is not shared.
- 1: Tag set shared between devices for blk-mq. Only makes sense with
- nr_devices > 1, otherwise there's no tag set to share.
-
-zoned=[0/1]: Default: 0
- 0: Block device is exposed as a random-access block device.
- 1: Block device is exposed as a host-managed zoned block device. Requires
- CONFIG_BLK_DEV_ZONED.
-
-zone_size=[MB]: Default: 256
- Per zone size when exposed as a zoned block device. Must be a power of two.
-
-zone_nr_conv=[nr_conv]: Default: 0
- The number of conventional zones to create when block device is zoned. If
- zone_nr_conv >= nr_zones, it will be reduced to nr_zones - 1.
--- /dev/null
+===============================================
+Block layer support for Persistent Reservations
+===============================================
+
+The Linux kernel supports a user space interface for simplified
+Persistent Reservations which map to block devices that support
+these (like SCSI). Persistent Reservations allow restricting
+access to block devices to specific initiators in a shared storage
+setup.
+
+This document gives a general overview of the support ioctl commands.
+For a more detailed reference please refer the the SCSI Primary
+Commands standard, specifically the section on Reservations and the
+"PERSISTENT RESERVE IN" and "PERSISTENT RESERVE OUT" commands.
+
+All implementations are expected to ensure the reservations survive
+a power loss and cover all connections in a multi path environment.
+These behaviors are optional in SPC but will be automatically applied
+by Linux.
+
+
+The following types of reservations are supported:
+--------------------------------------------------
+
+ - PR_WRITE_EXCLUSIVE
+ Only the initiator that owns the reservation can write to the
+ device. Any initiator can read from the device.
+
+ - PR_EXCLUSIVE_ACCESS
+ Only the initiator that owns the reservation can access the
+ device.
+
+ - PR_WRITE_EXCLUSIVE_REG_ONLY
+ Only initiators with a registered key can write to the device,
+ Any initiator can read from the device.
+
+ - PR_EXCLUSIVE_ACCESS_REG_ONLY
+ Only initiators with a registered key can access the device.
+
+ - PR_WRITE_EXCLUSIVE_ALL_REGS
+
+ Only initiators with a registered key can write to the device,
+ Any initiator can read from the device.
+ All initiators with a registered key are considered reservation
+ holders.
+ Please reference the SPC spec on the meaning of a reservation
+ holder if you want to use this type.
+
+ - PR_EXCLUSIVE_ACCESS_ALL_REGS
+ Only initiators with a registered key can access the device.
+ All initiators with a registered key are considered reservation
+ holders.
+ Please reference the SPC spec on the meaning of a reservation
+ holder if you want to use this type.
+
+
+The following ioctl are supported:
+----------------------------------
+
+1. IOC_PR_REGISTER
+^^^^^^^^^^^^^^^^^^
+
+This ioctl command registers a new reservation if the new_key argument
+is non-null. If no existing reservation exists old_key must be zero,
+if an existing reservation should be replaced old_key must contain
+the old reservation key.
+
+If the new_key argument is 0 it unregisters the existing reservation passed
+in old_key.
+
+
+2. IOC_PR_RESERVE
+^^^^^^^^^^^^^^^^^
+
+This ioctl command reserves the device and thus restricts access for other
+devices based on the type argument. The key argument must be the existing
+reservation key for the device as acquired by the IOC_PR_REGISTER,
+IOC_PR_REGISTER_IGNORE, IOC_PR_PREEMPT or IOC_PR_PREEMPT_ABORT commands.
+
+
+3. IOC_PR_RELEASE
+^^^^^^^^^^^^^^^^^
+
+This ioctl command releases the reservation specified by key and flags
+and thus removes any access restriction implied by it.
+
+
+4. IOC_PR_PREEMPT
+^^^^^^^^^^^^^^^^^
+
+This ioctl command releases the existing reservation referred to by
+old_key and replaces it with a new reservation of type for the
+reservation key new_key.
+
+
+5. IOC_PR_PREEMPT_ABORT
+^^^^^^^^^^^^^^^^^^^^^^^
+
+This ioctl command works like IOC_PR_PREEMPT except that it also aborts
+any outstanding command sent over a connection identified by old_key.
+
+6. IOC_PR_CLEAR
+^^^^^^^^^^^^^^^
+
+This ioctl command unregisters both key and any other reservation key
+registered with the device and drops any existing reservation.
+
+
+Flags
+-----
+
+All the ioctls have a flag field. Currently only one flag is supported:
+
+ - PR_FL_IGNORE_KEY
+ Ignore the existing reservation key. This is commonly supported for
+ IOC_PR_REGISTER, and some implementation may support the flag for
+ IOC_PR_RESERVE.
+
+For all unknown flags the kernel will return -EOPNOTSUPP.
+++ /dev/null
-
-Block layer support for Persistent Reservations
-===============================================
-
-The Linux kernel supports a user space interface for simplified
-Persistent Reservations which map to block devices that support
-these (like SCSI). Persistent Reservations allow restricting
-access to block devices to specific initiators in a shared storage
-setup.
-
-This document gives a general overview of the support ioctl commands.
-For a more detailed reference please refer the the SCSI Primary
-Commands standard, specifically the section on Reservations and the
-"PERSISTENT RESERVE IN" and "PERSISTENT RESERVE OUT" commands.
-
-All implementations are expected to ensure the reservations survive
-a power loss and cover all connections in a multi path environment.
-These behaviors are optional in SPC but will be automatically applied
-by Linux.
-
-
-The following types of reservations are supported:
---------------------------------------------------
-
- - PR_WRITE_EXCLUSIVE
-
- Only the initiator that owns the reservation can write to the
- device. Any initiator can read from the device.
-
- - PR_EXCLUSIVE_ACCESS
-
- Only the initiator that owns the reservation can access the
- device.
-
- - PR_WRITE_EXCLUSIVE_REG_ONLY
-
- Only initiators with a registered key can write to the device,
- Any initiator can read from the device.
-
- - PR_EXCLUSIVE_ACCESS_REG_ONLY
-
- Only initiators with a registered key can access the device.
-
- - PR_WRITE_EXCLUSIVE_ALL_REGS
-
- Only initiators with a registered key can write to the device,
- Any initiator can read from the device.
- All initiators with a registered key are considered reservation
- holders.
- Please reference the SPC spec on the meaning of a reservation
- holder if you want to use this type.
-
- - PR_EXCLUSIVE_ACCESS_ALL_REGS
-
- Only initiators with a registered key can access the device.
- All initiators with a registered key are considered reservation
- holders.
- Please reference the SPC spec on the meaning of a reservation
- holder if you want to use this type.
-
-
-The following ioctl are supported:
-----------------------------------
-
-1. IOC_PR_REGISTER
-
-This ioctl command registers a new reservation if the new_key argument
-is non-null. If no existing reservation exists old_key must be zero,
-if an existing reservation should be replaced old_key must contain
-the old reservation key.
-
-If the new_key argument is 0 it unregisters the existing reservation passed
-in old_key.
-
-
-2. IOC_PR_RESERVE
-
-This ioctl command reserves the device and thus restricts access for other
-devices based on the type argument. The key argument must be the existing
-reservation key for the device as acquired by the IOC_PR_REGISTER,
-IOC_PR_REGISTER_IGNORE, IOC_PR_PREEMPT or IOC_PR_PREEMPT_ABORT commands.
-
-
-3. IOC_PR_RELEASE
-
-This ioctl command releases the reservation specified by key and flags
-and thus removes any access restriction implied by it.
-
-
-4. IOC_PR_PREEMPT
-
-This ioctl command releases the existing reservation referred to by
-old_key and replaces it with a new reservation of type for the
-reservation key new_key.
-
-
-5. IOC_PR_PREEMPT_ABORT
-
-This ioctl command works like IOC_PR_PREEMPT except that it also aborts
-any outstanding command sent over a connection identified by old_key.
-
-6. IOC_PR_CLEAR
-
-This ioctl command unregisters both key and any other reservation key
-registered with the device and drops any existing reservation.
-
-
-Flags
------
-
-All the ioctls have a flag field. Currently only one flag is supported:
-
- - PR_FL_IGNORE_KEY
-
- Ignore the existing reservation key. This is commonly supported for
- IOC_PR_REGISTER, and some implementation may support the flag for
- IOC_PR_RESERVE.
-
-For all unknown flags the kernel will return -EOPNOTSUPP.
--- /dev/null
+=================
+Queue sysfs files
+=================
+
+This text file will detail the queue files that are located in the sysfs tree
+for each block device. Note that stacked devices typically do not export
+any settings, since their queue merely functions are a remapping target.
+These files are the ones found in the /sys/block/xxx/queue/ directory.
+
+Files denoted with a RO postfix are readonly and the RW postfix means
+read-write.
+
+add_random (RW)
+---------------
+This file allows to turn off the disk entropy contribution. Default
+value of this file is '1'(on).
+
+chunk_sectors (RO)
+------------------
+This has different meaning depending on the type of the block device.
+For a RAID device (dm-raid), chunk_sectors indicates the size in 512B sectors
+of the RAID volume stripe segment. For a zoned block device, either host-aware
+or host-managed, chunk_sectors indicates the size in 512B sectors of the zones
+of the device, with the eventual exception of the last zone of the device which
+may be smaller.
+
+dax (RO)
+--------
+This file indicates whether the device supports Direct Access (DAX),
+used by CPU-addressable storage to bypass the pagecache. It shows '1'
+if true, '0' if not.
+
+discard_granularity (RO)
+------------------------
+This shows the size of internal allocation of the device in bytes, if
+reported by the device. A value of '0' means device does not support
+the discard functionality.
+
+discard_max_hw_bytes (RO)
+-------------------------
+Devices that support discard functionality may have internal limits on
+the number of bytes that can be trimmed or unmapped in a single operation.
+The discard_max_bytes parameter is set by the device driver to the maximum
+number of bytes that can be discarded in a single operation. Discard
+requests issued to the device must not exceed this limit. A discard_max_bytes
+value of 0 means that the device does not support discard functionality.
+
+discard_max_bytes (RW)
+----------------------
+While discard_max_hw_bytes is the hardware limit for the device, this
+setting is the software limit. Some devices exhibit large latencies when
+large discards are issued, setting this value lower will make Linux issue
+smaller discards and potentially help reduce latencies induced by large
+discard operations.
+
+discard_zeroes_data (RO)
+------------------------
+Obsolete. Always zero.
+
+fua (RO)
+--------
+Whether or not the block driver supports the FUA flag for write requests.
+FUA stands for Force Unit Access. If the FUA flag is set that means that
+write requests must bypass the volatile cache of the storage device.
+
+hw_sector_size (RO)
+-------------------
+This is the hardware sector size of the device, in bytes.
+
+io_poll (RW)
+------------
+When read, this file shows whether polling is enabled (1) or disabled
+(0). Writing '0' to this file will disable polling for this device.
+Writing any non-zero value will enable this feature.
+
+io_poll_delay (RW)
+------------------
+If polling is enabled, this controls what kind of polling will be
+performed. It defaults to -1, which is classic polling. In this mode,
+the CPU will repeatedly ask for completions without giving up any time.
+If set to 0, a hybrid polling mode is used, where the kernel will attempt
+to make an educated guess at when the IO will complete. Based on this
+guess, the kernel will put the process issuing IO to sleep for an amount
+of time, before entering a classic poll loop. This mode might be a
+little slower than pure classic polling, but it will be more efficient.
+If set to a value larger than 0, the kernel will put the process issuing
+IO to sleep for this amount of microseconds before entering classic
+polling.
+
+io_timeout (RW)
+---------------
+io_timeout is the request timeout in milliseconds. If a request does not
+complete in this time then the block driver timeout handler is invoked.
+That timeout handler can decide to retry the request, to fail it or to start
+a device recovery strategy.
+
+iostats (RW)
+-------------
+This file is used to control (on/off) the iostats accounting of the
+disk.
+
+logical_block_size (RO)
+-----------------------
+This is the logical block size of the device, in bytes.
+
+max_discard_segments (RO)
+-------------------------
+The maximum number of DMA scatter/gather entries in a discard request.
+
+max_hw_sectors_kb (RO)
+----------------------
+This is the maximum number of kilobytes supported in a single data transfer.
+
+max_integrity_segments (RO)
+---------------------------
+Maximum number of elements in a DMA scatter/gather list with integrity
+data that will be submitted by the block layer core to the associated
+block driver.
+
+max_sectors_kb (RW)
+-------------------
+This is the maximum number of kilobytes that the block layer will allow
+for a filesystem request. Must be smaller than or equal to the maximum
+size allowed by the hardware.
+
+max_segments (RO)
+-----------------
+Maximum number of elements in a DMA scatter/gather list that is submitted
+to the associated block driver.
+
+max_segment_size (RO)
+---------------------
+Maximum size in bytes of a single element in a DMA scatter/gather list.
+
+minimum_io_size (RO)
+--------------------
+This is the smallest preferred IO size reported by the device.
+
+nomerges (RW)
+-------------
+This enables the user to disable the lookup logic involved with IO
+merging requests in the block layer. By default (0) all merges are
+enabled. When set to 1 only simple one-hit merges will be tried. When
+set to 2 no merge algorithms will be tried (including one-hit or more
+complex tree/hash lookups).
+
+nr_requests (RW)
+----------------
+This controls how many requests may be allocated in the block layer for
+read or write requests. Note that the total allocated number may be twice
+this amount, since it applies only to reads or writes (not the accumulated
+sum).
+
+To avoid priority inversion through request starvation, a request
+queue maintains a separate request pool per each cgroup when
+CONFIG_BLK_CGROUP is enabled, and this parameter applies to each such
+per-block-cgroup request pool. IOW, if there are N block cgroups,
+each request queue may have up to N request pools, each independently
+regulated by nr_requests.
+
+nr_zones (RO)
+-------------
+For zoned block devices (zoned attribute indicating "host-managed" or
+"host-aware"), this indicates the total number of zones of the device.
+This is always 0 for regular block devices.
+
+optimal_io_size (RO)
+--------------------
+This is the optimal IO size reported by the device.
+
+physical_block_size (RO)
+------------------------
+This is the physical block size of device, in bytes.
+
+read_ahead_kb (RW)
+------------------
+Maximum number of kilobytes to read-ahead for filesystems on this block
+device.
+
+rotational (RW)
+---------------
+This file is used to stat if the device is of rotational type or
+non-rotational type.
+
+rq_affinity (RW)
+----------------
+If this option is '1', the block layer will migrate request completions to the
+cpu "group" that originally submitted the request. For some workloads this
+provides a significant reduction in CPU cycles due to caching effects.
+
+For storage configurations that need to maximize distribution of completion
+processing setting this option to '2' forces the completion to run on the
+requesting cpu (bypassing the "group" aggregation logic).
+
+scheduler (RW)
+--------------
+When read, this file will display the current and available IO schedulers
+for this block device. The currently active IO scheduler will be enclosed
+in [] brackets. Writing an IO scheduler name to this file will switch
+control of this block device to that new IO scheduler. Note that writing
+an IO scheduler name to this file will attempt to load that IO scheduler
+module, if it isn't already present in the system.
+
+write_cache (RW)
+----------------
+When read, this file will display whether the device has write back
+caching enabled or not. It will return "write back" for the former
+case, and "write through" for the latter. Writing to this file can
+change the kernels view of the device, but it doesn't alter the
+device state. This means that it might not be safe to toggle the
+setting from "write back" to "write through", since that will also
+eliminate cache flushes issued by the kernel.
+
+write_same_max_bytes (RO)
+-------------------------
+This is the number of bytes the device can write in a single write-same
+command. A value of '0' means write-same is not supported by this
+device.
+
+wbt_lat_usec (RW)
+-----------------
+If the device is registered for writeback throttling, then this file shows
+the target minimum read latency. If this latency is exceeded in a given
+window of time (see wb_window_usec), then the writeback throttling will start
+scaling back writes. Writing a value of '0' to this file disables the
+feature. Writing a value of '-1' to this file resets the value to the
+default setting.
+
+throttle_sample_time (RW)
+-------------------------
+This is the time window that blk-throttle samples data, in millisecond.
+blk-throttle makes decision based on the samplings. Lower time means cgroups
+have more smooth throughput, but higher CPU overhead. This exists only when
+CONFIG_BLK_DEV_THROTTLING_LOW is enabled.
+
+write_zeroes_max_bytes (RO)
+---------------------------
+For block drivers that support REQ_OP_WRITE_ZEROES, the maximum number of
+bytes that can be zeroed at once. The value 0 means that REQ_OP_WRITE_ZEROES
+is not supported.
+
+zoned (RO)
+----------
+This indicates if the device is a zoned block device and the zone model of the
+device if it is indeed zoned. The possible values indicated by zoned are
+"none" for regular block devices and "host-aware" or "host-managed" for zoned
+block devices. The characteristics of host-aware and host-managed zoned block
+devices are described in the ZBC (Zoned Block Commands) and ZAC
+(Zoned Device ATA Command Set) standards. These standards also define the
+"drive-managed" zone model. However, since drive-managed zoned block devices
+do not support zone commands, they will be treated as regular block devices
+and zoned will report "none".
+
+Jens Axboe <jens.axboe@oracle.com>, February 2009
+++ /dev/null
-Queue sysfs files
-=================
-
-This text file will detail the queue files that are located in the sysfs tree
-for each block device. Note that stacked devices typically do not export
-any settings, since their queue merely functions are a remapping target.
-These files are the ones found in the /sys/block/xxx/queue/ directory.
-
-Files denoted with a RO postfix are readonly and the RW postfix means
-read-write.
-
-add_random (RW)
-----------------
-This file allows to turn off the disk entropy contribution. Default
-value of this file is '1'(on).
-
-chunk_sectors (RO)
-------------------
-This has different meaning depending on the type of the block device.
-For a RAID device (dm-raid), chunk_sectors indicates the size in 512B sectors
-of the RAID volume stripe segment. For a zoned block device, either host-aware
-or host-managed, chunk_sectors indicates the size in 512B sectors of the zones
-of the device, with the eventual exception of the last zone of the device which
-may be smaller.
-
-dax (RO)
---------
-This file indicates whether the device supports Direct Access (DAX),
-used by CPU-addressable storage to bypass the pagecache. It shows '1'
-if true, '0' if not.
-
-discard_granularity (RO)
------------------------
-This shows the size of internal allocation of the device in bytes, if
-reported by the device. A value of '0' means device does not support
-the discard functionality.
-
-discard_max_hw_bytes (RO)
-----------------------
-Devices that support discard functionality may have internal limits on
-the number of bytes that can be trimmed or unmapped in a single operation.
-The discard_max_bytes parameter is set by the device driver to the maximum
-number of bytes that can be discarded in a single operation. Discard
-requests issued to the device must not exceed this limit. A discard_max_bytes
-value of 0 means that the device does not support discard functionality.
-
-discard_max_bytes (RW)
-----------------------
-While discard_max_hw_bytes is the hardware limit for the device, this
-setting is the software limit. Some devices exhibit large latencies when
-large discards are issued, setting this value lower will make Linux issue
-smaller discards and potentially help reduce latencies induced by large
-discard operations.
-
-discard_zeroes_data (RO)
-------------------------
-Obsolete. Always zero.
-
-fua (RO)
---------
-Whether or not the block driver supports the FUA flag for write requests.
-FUA stands for Force Unit Access. If the FUA flag is set that means that
-write requests must bypass the volatile cache of the storage device.
-
-hw_sector_size (RO)
--------------------
-This is the hardware sector size of the device, in bytes.
-
-io_poll (RW)
-------------
-When read, this file shows whether polling is enabled (1) or disabled
-(0). Writing '0' to this file will disable polling for this device.
-Writing any non-zero value will enable this feature.
-
-io_poll_delay (RW)
-------------------
-If polling is enabled, this controls what kind of polling will be
-performed. It defaults to -1, which is classic polling. In this mode,
-the CPU will repeatedly ask for completions without giving up any time.
-If set to 0, a hybrid polling mode is used, where the kernel will attempt
-to make an educated guess at when the IO will complete. Based on this
-guess, the kernel will put the process issuing IO to sleep for an amount
-of time, before entering a classic poll loop. This mode might be a
-little slower than pure classic polling, but it will be more efficient.
-If set to a value larger than 0, the kernel will put the process issuing
-IO to sleep for this amount of microseconds before entering classic
-polling.
-
-io_timeout (RW)
----------------
-io_timeout is the request timeout in milliseconds. If a request does not
-complete in this time then the block driver timeout handler is invoked.
-That timeout handler can decide to retry the request, to fail it or to start
-a device recovery strategy.
-
-iostats (RW)
--------------
-This file is used to control (on/off) the iostats accounting of the
-disk.
-
-logical_block_size (RO)
------------------------
-This is the logical block size of the device, in bytes.
-
-max_discard_segments (RO)
--------------------------
-The maximum number of DMA scatter/gather entries in a discard request.
-
-max_hw_sectors_kb (RO)
-----------------------
-This is the maximum number of kilobytes supported in a single data transfer.
-
-max_integrity_segments (RO)
----------------------------
-Maximum number of elements in a DMA scatter/gather list with integrity
-data that will be submitted by the block layer core to the associated
-block driver.
-
-max_sectors_kb (RW)
--------------------
-This is the maximum number of kilobytes that the block layer will allow
-for a filesystem request. Must be smaller than or equal to the maximum
-size allowed by the hardware.
-
-max_segments (RO)
------------------
-Maximum number of elements in a DMA scatter/gather list that is submitted
-to the associated block driver.
-
-max_segment_size (RO)
----------------------
-Maximum size in bytes of a single element in a DMA scatter/gather list.
-
-minimum_io_size (RO)
---------------------
-This is the smallest preferred IO size reported by the device.
-
-nomerges (RW)
--------------
-This enables the user to disable the lookup logic involved with IO
-merging requests in the block layer. By default (0) all merges are
-enabled. When set to 1 only simple one-hit merges will be tried. When
-set to 2 no merge algorithms will be tried (including one-hit or more
-complex tree/hash lookups).
-
-nr_requests (RW)
-----------------
-This controls how many requests may be allocated in the block layer for
-read or write requests. Note that the total allocated number may be twice
-this amount, since it applies only to reads or writes (not the accumulated
-sum).
-
-To avoid priority inversion through request starvation, a request
-queue maintains a separate request pool per each cgroup when
-CONFIG_BLK_CGROUP is enabled, and this parameter applies to each such
-per-block-cgroup request pool. IOW, if there are N block cgroups,
-each request queue may have up to N request pools, each independently
-regulated by nr_requests.
-
-nr_zones (RO)
--------------
-For zoned block devices (zoned attribute indicating "host-managed" or
-"host-aware"), this indicates the total number of zones of the device.
-This is always 0 for regular block devices.
-
-optimal_io_size (RO)
---------------------
-This is the optimal IO size reported by the device.
-
-physical_block_size (RO)
-------------------------
-This is the physical block size of device, in bytes.
-
-read_ahead_kb (RW)
-------------------
-Maximum number of kilobytes to read-ahead for filesystems on this block
-device.
-
-rotational (RW)
----------------
-This file is used to stat if the device is of rotational type or
-non-rotational type.
-
-rq_affinity (RW)
-----------------
-If this option is '1', the block layer will migrate request completions to the
-cpu "group" that originally submitted the request. For some workloads this
-provides a significant reduction in CPU cycles due to caching effects.
-
-For storage configurations that need to maximize distribution of completion
-processing setting this option to '2' forces the completion to run on the
-requesting cpu (bypassing the "group" aggregation logic).
-
-scheduler (RW)
---------------
-When read, this file will display the current and available IO schedulers
-for this block device. The currently active IO scheduler will be enclosed
-in [] brackets. Writing an IO scheduler name to this file will switch
-control of this block device to that new IO scheduler. Note that writing
-an IO scheduler name to this file will attempt to load that IO scheduler
-module, if it isn't already present in the system.
-
-write_cache (RW)
-----------------
-When read, this file will display whether the device has write back
-caching enabled or not. It will return "write back" for the former
-case, and "write through" for the latter. Writing to this file can
-change the kernels view of the device, but it doesn't alter the
-device state. This means that it might not be safe to toggle the
-setting from "write back" to "write through", since that will also
-eliminate cache flushes issued by the kernel.
-
-write_same_max_bytes (RO)
--------------------------
-This is the number of bytes the device can write in a single write-same
-command. A value of '0' means write-same is not supported by this
-device.
-
-wbt_lat_usec (RW)
------------------
-If the device is registered for writeback throttling, then this file shows
-the target minimum read latency. If this latency is exceeded in a given
-window of time (see wb_window_usec), then the writeback throttling will start
-scaling back writes. Writing a value of '0' to this file disables the
-feature. Writing a value of '-1' to this file resets the value to the
-default setting.
-
-throttle_sample_time (RW)
--------------------------
-This is the time window that blk-throttle samples data, in millisecond.
-blk-throttle makes decision based on the samplings. Lower time means cgroups
-have more smooth throughput, but higher CPU overhead. This exists only when
-CONFIG_BLK_DEV_THROTTLING_LOW is enabled.
-
-write_zeroes_max_bytes (RO)
----------------------------
-For block drivers that support REQ_OP_WRITE_ZEROES, the maximum number of
-bytes that can be zeroed at once. The value 0 means that REQ_OP_WRITE_ZEROES
-is not supported.
-
-zoned (RO)
-----------
-This indicates if the device is a zoned block device and the zone model of the
-device if it is indeed zoned. The possible values indicated by zoned are
-"none" for regular block devices and "host-aware" or "host-managed" for zoned
-block devices. The characteristics of host-aware and host-managed zoned block
-devices are described in the ZBC (Zoned Block Commands) and ZAC
-(Zoned Device ATA Command Set) standards. These standards also define the
-"drive-managed" zone model. However, since drive-managed zoned block devices
-do not support zone commands, they will be treated as regular block devices
-and zoned will report "none".
-
-Jens Axboe <jens.axboe@oracle.com>, February 2009
--- /dev/null
+============================
+struct request documentation
+============================
+
+Jens Axboe <jens.axboe@oracle.com> 27/05/02
+
+
+.. FIXME:
+ No idea about what does mean - seems just some noise, so comment it
+
+ 1.0
+ Index
+
+ 2.0 Struct request members classification
+
+ 2.1 struct request members explanation
+
+ 3.0
+
+
+ 2.0
+
+
+
+Short explanation of request members
+====================================
+
+Classification flags:
+
+ = ====================
+ D driver member
+ B block layer member
+ I I/O scheduler member
+ = ====================
+
+Unless an entry contains a D classification, a device driver must not access
+this member. Some members may contain D classifications, but should only be
+access through certain macros or functions (eg ->flags).
+
+<linux/blkdev.h>
+
+=============================== ======= =======================================
+Member Flag Comment
+=============================== ======= =======================================
+struct list_head queuelist BI Organization on various internal
+ queues
+
+``void *elevator_private`` I I/O scheduler private data
+
+unsigned char cmd[16] D Driver can use this for setting up
+ a cdb before execution, see
+ blk_queue_prep_rq
+
+unsigned long flags DBI Contains info about data direction,
+ request type, etc.
+
+int rq_status D Request status bits
+
+kdev_t rq_dev DBI Target device
+
+int errors DB Error counts
+
+sector_t sector DBI Target location
+
+unsigned long hard_nr_sectors B Used to keep sector sane
+
+unsigned long nr_sectors DBI Total number of sectors in request
+
+unsigned long hard_nr_sectors B Used to keep nr_sectors sane
+
+unsigned short nr_phys_segments DB Number of physical scatter gather
+ segments in a request
+
+unsigned short nr_hw_segments DB Number of hardware scatter gather
+ segments in a request
+
+unsigned int current_nr_sectors DB Number of sectors in first segment
+ of request
+
+unsigned int hard_cur_sectors B Used to keep current_nr_sectors sane
+
+int tag DB TCQ tag, if assigned
+
+``void *special`` D Free to be used by driver
+
+``char *buffer`` D Map of first segment, also see
+ section on bouncing SECTION
+
+``struct completion *waiting`` D Can be used by driver to get signalled
+ on request completion
+
+``struct bio *bio`` DBI First bio in request
+
+``struct bio *biotail`` DBI Last bio in request
+
+``struct request_queue *q`` DB Request queue this request belongs to
+
+``struct request_list *rl`` B Request list this request came from
+=============================== ======= =======================================
+++ /dev/null
-
-struct request documentation
-
-Jens Axboe <jens.axboe@oracle.com> 27/05/02
-
-1.0
-Index
-
-2.0 Struct request members classification
-
- 2.1 struct request members explanation
-
-3.0
-
-
-2.0
-Short explanation of request members
-
-Classification flags:
-
- D driver member
- B block layer member
- I I/O scheduler member
-
-Unless an entry contains a D classification, a device driver must not access
-this member. Some members may contain D classifications, but should only be
-access through certain macros or functions (eg ->flags).
-
-<linux/blkdev.h>
-
-2.1
-Member Flag Comment
------- ---- -------
-
-struct list_head queuelist BI Organization on various internal
- queues
-
-void *elevator_private I I/O scheduler private data
-
-unsigned char cmd[16] D Driver can use this for setting up
- a cdb before execution, see
- blk_queue_prep_rq
-
-unsigned long flags DBI Contains info about data direction,
- request type, etc.
-
-int rq_status D Request status bits
-
-kdev_t rq_dev DBI Target device
-
-int errors DB Error counts
-
-sector_t sector DBI Target location
-
-unsigned long hard_nr_sectors B Used to keep sector sane
-
-unsigned long nr_sectors DBI Total number of sectors in request
-
-unsigned long hard_nr_sectors B Used to keep nr_sectors sane
-
-unsigned short nr_phys_segments DB Number of physical scatter gather
- segments in a request
-
-unsigned short nr_hw_segments DB Number of hardware scatter gather
- segments in a request
-
-unsigned int current_nr_sectors DB Number of sectors in first segment
- of request
-
-unsigned int hard_cur_sectors B Used to keep current_nr_sectors sane
-
-int tag DB TCQ tag, if assigned
-
-void *special D Free to be used by driver
-
-char *buffer D Map of first segment, also see
- section on bouncing SECTION
-
-struct completion *waiting D Can be used by driver to get signalled
- on request completion
-
-struct bio *bio DBI First bio in request
-
-struct bio *biotail DBI Last bio in request
-
-struct request_queue *q DB Request queue this request belongs to
-
-struct request_list *rl B Request list this request came from
--- /dev/null
+===============================================
+Block layer statistics in /sys/block/<dev>/stat
+===============================================
+
+This file documents the contents of the /sys/block/<dev>/stat file.
+
+The stat file provides several statistics about the state of block
+device <dev>.
+
+Q.
+ Why are there multiple statistics in a single file? Doesn't sysfs
+ normally contain a single value per file?
+
+A.
+ By having a single file, the kernel can guarantee that the statistics
+ represent a consistent snapshot of the state of the device. If the
+ statistics were exported as multiple files containing one statistic
+ each, it would be impossible to guarantee that a set of readings
+ represent a single point in time.
+
+The stat file consists of a single line of text containing 11 decimal
+values separated by whitespace. The fields are summarized in the
+following table, and described in more detail below.
+
+
+=============== ============= =================================================
+Name units description
+=============== ============= =================================================
+read I/Os requests number of read I/Os processed
+read merges requests number of read I/Os merged with in-queue I/O
+read sectors sectors number of sectors read
+read ticks milliseconds total wait time for read requests
+write I/Os requests number of write I/Os processed
+write merges requests number of write I/Os merged with in-queue I/O
+write sectors sectors number of sectors written
+write ticks milliseconds total wait time for write requests
+in_flight requests number of I/Os currently in flight
+io_ticks milliseconds total time this block device has been active
+time_in_queue milliseconds total wait time for all requests
+discard I/Os requests number of discard I/Os processed
+discard merges requests number of discard I/Os merged with in-queue I/O
+discard sectors sectors number of sectors discarded
+discard ticks milliseconds total wait time for discard requests
+=============== ============= =================================================
+
+read I/Os, write I/Os, discard I/0s
+===================================
+
+These values increment when an I/O request completes.
+
+read merges, write merges, discard merges
+=========================================
+
+These values increment when an I/O request is merged with an
+already-queued I/O request.
+
+read sectors, write sectors, discard_sectors
+============================================
+
+These values count the number of sectors read from, written to, or
+discarded from this block device. The "sectors" in question are the
+standard UNIX 512-byte sectors, not any device- or filesystem-specific
+block size. The counters are incremented when the I/O completes.
+
+read ticks, write ticks, discard ticks
+======================================
+
+These values count the number of milliseconds that I/O requests have
+waited on this block device. If there are multiple I/O requests waiting,
+these values will increase at a rate greater than 1000/second; for
+example, if 60 read requests wait for an average of 30 ms, the read_ticks
+field will increase by 60*30 = 1800.
+
+in_flight
+=========
+
+This value counts the number of I/O requests that have been issued to
+the device driver but have not yet completed. It does not include I/O
+requests that are in the queue but not yet issued to the device driver.
+
+io_ticks
+========
+
+This value counts the number of milliseconds during which the device has
+had I/O requests queued.
+
+time_in_queue
+=============
+
+This value counts the number of milliseconds that I/O requests have waited
+on this block device. If there are multiple I/O requests waiting, this
+value will increase as the product of the number of milliseconds times the
+number of requests waiting (see "read ticks" above for an example).
+++ /dev/null
-Block layer statistics in /sys/block/<dev>/stat
-===============================================
-
-This file documents the contents of the /sys/block/<dev>/stat file.
-
-The stat file provides several statistics about the state of block
-device <dev>.
-
-Q. Why are there multiple statistics in a single file? Doesn't sysfs
- normally contain a single value per file?
-A. By having a single file, the kernel can guarantee that the statistics
- represent a consistent snapshot of the state of the device. If the
- statistics were exported as multiple files containing one statistic
- each, it would be impossible to guarantee that a set of readings
- represent a single point in time.
-
-The stat file consists of a single line of text containing 11 decimal
-values separated by whitespace. The fields are summarized in the
-following table, and described in more detail below.
-
-Name units description
----- ----- -----------
-read I/Os requests number of read I/Os processed
-read merges requests number of read I/Os merged with in-queue I/O
-read sectors sectors number of sectors read
-read ticks milliseconds total wait time for read requests
-write I/Os requests number of write I/Os processed
-write merges requests number of write I/Os merged with in-queue I/O
-write sectors sectors number of sectors written
-write ticks milliseconds total wait time for write requests
-in_flight requests number of I/Os currently in flight
-io_ticks milliseconds total time this block device has been active
-time_in_queue milliseconds total wait time for all requests
-discard I/Os requests number of discard I/Os processed
-discard merges requests number of discard I/Os merged with in-queue I/O
-discard sectors sectors number of sectors discarded
-discard ticks milliseconds total wait time for discard requests
-
-read I/Os, write I/Os, discard I/0s
-===================================
-
-These values increment when an I/O request completes.
-
-read merges, write merges, discard merges
-=========================================
-
-These values increment when an I/O request is merged with an
-already-queued I/O request.
-
-read sectors, write sectors, discard_sectors
-============================================
-
-These values count the number of sectors read from, written to, or
-discarded from this block device. The "sectors" in question are the
-standard UNIX 512-byte sectors, not any device- or filesystem-specific
-block size. The counters are incremented when the I/O completes.
-
-read ticks, write ticks, discard ticks
-======================================
-
-These values count the number of milliseconds that I/O requests have
-waited on this block device. If there are multiple I/O requests waiting,
-these values will increase at a rate greater than 1000/second; for
-example, if 60 read requests wait for an average of 30 ms, the read_ticks
-field will increase by 60*30 = 1800.
-
-in_flight
-=========
-
-This value counts the number of I/O requests that have been issued to
-the device driver but have not yet completed. It does not include I/O
-requests that are in the queue but not yet issued to the device driver.
-
-io_ticks
-========
-
-This value counts the number of milliseconds during which the device has
-had I/O requests queued.
-
-time_in_queue
-=============
-
-This value counts the number of milliseconds that I/O requests have waited
-on this block device. If there are multiple I/O requests waiting, this
-value will increase as the product of the number of milliseconds times the
-number of requests waiting (see "read ticks" above for an example).
--- /dev/null
+===================
+Switching Scheduler
+===================
+
+To choose IO schedulers at boot time, use the argument 'elevator=deadline'.
+'noop' and 'cfq' (the default) are also available. IO schedulers are assigned
+globally at boot time only presently.
+
+Each io queue has a set of io scheduler tunables associated with it. These
+tunables control how the io scheduler works. You can find these entries
+in::
+
+ /sys/block/<device>/queue/iosched
+
+assuming that you have sysfs mounted on /sys. If you don't have sysfs mounted,
+you can do so by typing::
+
+ # mount none /sys -t sysfs
+
+It is possible to change the IO scheduler for a given block device on
+the fly to select one of mq-deadline, none, bfq, or kyber schedulers -
+which can improve that device's throughput.
+
+To set a specific scheduler, simply do this::
+
+ echo SCHEDNAME > /sys/block/DEV/queue/scheduler
+
+where SCHEDNAME is the name of a defined IO scheduler, and DEV is the
+device name (hda, hdb, sga, or whatever you happen to have).
+
+The list of defined schedulers can be found by simply doing
+a "cat /sys/block/DEV/queue/scheduler" - the list of valid names
+will be displayed, with the currently selected scheduler in brackets::
+
+ # cat /sys/block/sda/queue/scheduler
+ [mq-deadline] kyber bfq none
+ # echo none >/sys/block/sda/queue/scheduler
+ # cat /sys/block/sda/queue/scheduler
+ [none] mq-deadline kyber bfq
+++ /dev/null
-To choose IO schedulers at boot time, use the argument 'elevator=deadline'.
-'noop' and 'cfq' (the default) are also available. IO schedulers are assigned
-globally at boot time only presently.
-
-Each io queue has a set of io scheduler tunables associated with it. These
-tunables control how the io scheduler works. You can find these entries
-in:
-
-/sys/block/<device>/queue/iosched
-
-assuming that you have sysfs mounted on /sys. If you don't have sysfs mounted,
-you can do so by typing:
-
-# mount none /sys -t sysfs
-
-It is possible to change the IO scheduler for a given block device on
-the fly to select one of mq-deadline, none, bfq, or kyber schedulers -
-which can improve that device's throughput.
-
-To set a specific scheduler, simply do this:
-
-echo SCHEDNAME > /sys/block/DEV/queue/scheduler
-
-where SCHEDNAME is the name of a defined IO scheduler, and DEV is the
-device name (hda, hdb, sga, or whatever you happen to have).
-
-The list of defined schedulers can be found by simply doing
-a "cat /sys/block/DEV/queue/scheduler" - the list of valid names
-will be displayed, with the currently selected scheduler in brackets:
-
-# cat /sys/block/sda/queue/scheduler
-[mq-deadline] kyber bfq none
-# echo none >/sys/block/sda/queue/scheduler
-# cat /sys/block/sda/queue/scheduler
-[none] mq-deadline kyber bfq
--- /dev/null
+==========================================
+Explicit volatile write back cache control
+==========================================
+
+Introduction
+------------
+
+Many storage devices, especially in the consumer market, come with volatile
+write back caches. That means the devices signal I/O completion to the
+operating system before data actually has hit the non-volatile storage. This
+behavior obviously speeds up various workloads, but it means the operating
+system needs to force data out to the non-volatile storage when it performs
+a data integrity operation like fsync, sync or an unmount.
+
+The Linux block layer provides two simple mechanisms that let filesystems
+control the caching behavior of the storage device. These mechanisms are
+a forced cache flush, and the Force Unit Access (FUA) flag for requests.
+
+
+Explicit cache flushes
+----------------------
+
+The REQ_PREFLUSH flag can be OR ed into the r/w flags of a bio submitted from
+the filesystem and will make sure the volatile cache of the storage device
+has been flushed before the actual I/O operation is started. This explicitly
+guarantees that previously completed write requests are on non-volatile
+storage before the flagged bio starts. In addition the REQ_PREFLUSH flag can be
+set on an otherwise empty bio structure, which causes only an explicit cache
+flush without any dependent I/O. It is recommend to use
+the blkdev_issue_flush() helper for a pure cache flush.
+
+
+Forced Unit Access
+------------------
+
+The REQ_FUA flag can be OR ed into the r/w flags of a bio submitted from the
+filesystem and will make sure that I/O completion for this request is only
+signaled after the data has been committed to non-volatile storage.
+
+
+Implementation details for filesystems
+--------------------------------------
+
+Filesystems can simply set the REQ_PREFLUSH and REQ_FUA bits and do not have to
+worry if the underlying devices need any explicit cache flushing and how
+the Forced Unit Access is implemented. The REQ_PREFLUSH and REQ_FUA flags
+may both be set on a single bio.
+
+
+Implementation details for make_request_fn based block drivers
+--------------------------------------------------------------
+
+These drivers will always see the REQ_PREFLUSH and REQ_FUA bits as they sit
+directly below the submit_bio interface. For remapping drivers the REQ_FUA
+bits need to be propagated to underlying devices, and a global flush needs
+to be implemented for bios with the REQ_PREFLUSH bit set. For real device
+drivers that do not have a volatile cache the REQ_PREFLUSH and REQ_FUA bits
+on non-empty bios can simply be ignored, and REQ_PREFLUSH requests without
+data can be completed successfully without doing any work. Drivers for
+devices with volatile caches need to implement the support for these
+flags themselves without any help from the block layer.
+
+
+Implementation details for request_fn based block drivers
+---------------------------------------------------------
+
+For devices that do not support volatile write caches there is no driver
+support required, the block layer completes empty REQ_PREFLUSH requests before
+entering the driver and strips off the REQ_PREFLUSH and REQ_FUA bits from
+requests that have a payload. For devices with volatile write caches the
+driver needs to tell the block layer that it supports flushing caches by
+doing::
+
+ blk_queue_write_cache(sdkp->disk->queue, true, false);
+
+and handle empty REQ_OP_FLUSH requests in its prep_fn/request_fn. Note that
+REQ_PREFLUSH requests with a payload are automatically turned into a sequence
+of an empty REQ_OP_FLUSH request followed by the actual write by the block
+layer. For devices that also support the FUA bit the block layer needs
+to be told to pass through the REQ_FUA bit using::
+
+ blk_queue_write_cache(sdkp->disk->queue, true, true);
+
+and the driver must handle write requests that have the REQ_FUA bit set
+in prep_fn/request_fn. If the FUA bit is not natively supported the block
+layer turns it into an empty REQ_OP_FLUSH request after the actual write.
+++ /dev/null
-
-Explicit volatile write back cache control
-=====================================
-
-Introduction
-------------
-
-Many storage devices, especially in the consumer market, come with volatile
-write back caches. That means the devices signal I/O completion to the
-operating system before data actually has hit the non-volatile storage. This
-behavior obviously speeds up various workloads, but it means the operating
-system needs to force data out to the non-volatile storage when it performs
-a data integrity operation like fsync, sync or an unmount.
-
-The Linux block layer provides two simple mechanisms that let filesystems
-control the caching behavior of the storage device. These mechanisms are
-a forced cache flush, and the Force Unit Access (FUA) flag for requests.
-
-
-Explicit cache flushes
-----------------------
-
-The REQ_PREFLUSH flag can be OR ed into the r/w flags of a bio submitted from
-the filesystem and will make sure the volatile cache of the storage device
-has been flushed before the actual I/O operation is started. This explicitly
-guarantees that previously completed write requests are on non-volatile
-storage before the flagged bio starts. In addition the REQ_PREFLUSH flag can be
-set on an otherwise empty bio structure, which causes only an explicit cache
-flush without any dependent I/O. It is recommend to use
-the blkdev_issue_flush() helper for a pure cache flush.
-
-
-Forced Unit Access
------------------
-
-The REQ_FUA flag can be OR ed into the r/w flags of a bio submitted from the
-filesystem and will make sure that I/O completion for this request is only
-signaled after the data has been committed to non-volatile storage.
-
-
-Implementation details for filesystems
---------------------------------------
-
-Filesystems can simply set the REQ_PREFLUSH and REQ_FUA bits and do not have to
-worry if the underlying devices need any explicit cache flushing and how
-the Forced Unit Access is implemented. The REQ_PREFLUSH and REQ_FUA flags
-may both be set on a single bio.
-
-
-Implementation details for make_request_fn based block drivers
---------------------------------------------------------------
-
-These drivers will always see the REQ_PREFLUSH and REQ_FUA bits as they sit
-directly below the submit_bio interface. For remapping drivers the REQ_FUA
-bits need to be propagated to underlying devices, and a global flush needs
-to be implemented for bios with the REQ_PREFLUSH bit set. For real device
-drivers that do not have a volatile cache the REQ_PREFLUSH and REQ_FUA bits
-on non-empty bios can simply be ignored, and REQ_PREFLUSH requests without
-data can be completed successfully without doing any work. Drivers for
-devices with volatile caches need to implement the support for these
-flags themselves without any help from the block layer.
-
-
-Implementation details for request_fn based block drivers
---------------------------------------------------------------
-
-For devices that do not support volatile write caches there is no driver
-support required, the block layer completes empty REQ_PREFLUSH requests before
-entering the driver and strips off the REQ_PREFLUSH and REQ_FUA bits from
-requests that have a payload. For devices with volatile write caches the
-driver needs to tell the block layer that it supports flushing caches by
-doing:
-
- blk_queue_write_cache(sdkp->disk->queue, true, false);
-
-and handle empty REQ_OP_FLUSH requests in its prep_fn/request_fn. Note that
-REQ_PREFLUSH requests with a payload are automatically turned into a sequence
-of an empty REQ_OP_FLUSH request followed by the actual write by the block
-layer. For devices that also support the FUA bit the block layer needs
-to be told to pass through the REQ_FUA bit using:
-
- blk_queue_write_cache(sdkp->disk->queue, true, true);
-
-and the driver must handle write requests that have the REQ_FUA bit set
-in prep_fn/request_fn. If the FUA bit is not natively supported the block
-layer turns it into an empty REQ_OP_FLUSH request after the actual write.
File /sys/block/zram<id>/stat
-Represents block layer statistics. Read Documentation/block/stat.txt for
+Represents block layer statistics. Read Documentation/block/stat.rst for
details.
File /sys/block/zram<id>/io_stat
L: linux-block@vger.kernel.org
S: Maintained
F: block/bfq-*
-F: Documentation/block/bfq-iosched.txt
+F: Documentation/block/bfq-iosched.rst
BFS FILE SYSTEM
M: "Tigran A. Aivazian" <aivazian.tigran@gmail.com>
which don't otherwise have any standardized method for listing the
partitions on a block device.
- See Documentation/block/cmdline-partition.txt for more information.
+ See Documentation/block/cmdline-partition.rst for more information.
config BLK_WBT
bool "Enable support for block device writeback throttling"
regardless of the device parameters and with any workload. It
also guarantees a low latency to interactive and soft
real-time applications. Details in
- Documentation/block/bfq-iosched.txt
+ Documentation/block/bfq-iosched.rst
config BFQ_GROUP_IOSCHED
bool "BFQ hierarchical scheduling support"
* low-latency capabilities. BFQ also supports full hierarchical
* scheduling through cgroups. Next paragraphs provide an introduction
* on BFQ inner workings. Details on BFQ benefits, usage and
- * limitations can be found in Documentation/block/bfq-iosched.txt.
+ * limitations can be found in Documentation/block/bfq-iosched.rst.
*
* BFQ is a proportional-share storage-I/O scheduling algorithm based
* on the slice-by-slice service scheme of CFQ. But BFQ assigns
* send/receive integrity metadata it must use this function to register
* the capability with the block layer. The template is a blk_integrity
* struct with values appropriate for the underlying hardware. See
- * Documentation/block/data-integrity.txt.
+ * Documentation/block/data-integrity.rst.
*/
void blk_integrity_register(struct gendisk *disk, struct blk_integrity *template)
{
*
* ioprio_set(PRIO_PROCESS, pid, prio);
*
- * See also Documentation/block/ioprio.txt
+ * See also Documentation/block/ioprio.rst
*
*/
#include <linux/gfp.h>
#include "blk-mq-sched.h"
/*
- * See Documentation/block/deadline-iosched.txt
+ * See Documentation/block/deadline-iosched.rst
*/
static const int read_expire = HZ / 2; /* max time before a read is submitted. */
static const int write_expire = 5 * HZ; /* ditto for writes, these limits are SOFT! */
*
* The format for the command line is just like mtdparts.
*
- * For further information, see "Documentation/block/cmdline-partition.txt"
+ * For further information, see "Documentation/block/cmdline-partition.rst"
*
*/
projects / openwrt / staging / blogic.git / commitdiff