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Subject[PATCH] block: update documentation for REQ_FLUSH / REQ_FUA

Signed-off-by: Christoph Hellwig <>

Index: linux-2.6/Documentation/block/barrier.txt
--- linux-2.6.orig/Documentation/block/barrier.txt 2010-08-26 06:46:20.993253858 -0300
+++ /dev/null 1970-01-01 00:00:00.000000000 +0000
@@ -1,261 +0,0 @@
-I/O Barriers
-Tejun Heo <>, July 22 2005
-I/O barrier requests are used to guarantee ordering around the barrier
-requests. Unless you're crazy enough to use disk drives for
-implementing synchronization constructs (wow, sounds interesting...),
-the ordering is meaningful only for write requests for things like
-journal checkpoints. All requests queued before a barrier request
-must be finished (made it to the physical medium) before the barrier
-request is started, and all requests queued after the barrier request
-must be started only after the barrier request is finished (again,
-made it to the physical medium).
-In other words, I/O barrier requests have the following two properties.
-1. Request ordering
-Requests cannot pass the barrier request. Preceding requests are
-processed before the barrier and following requests after.
-Depending on what features a drive supports, this can be done in one
-of the following three ways.
-i. For devices which have queue depth greater than 1 (TCQ devices) and
-support ordered tags, block layer can just issue the barrier as an
-ordered request and the lower level driver, controller and drive
-itself are responsible for making sure that the ordering constraint is
-met. Most modern SCSI controllers/drives should support this.
-NOTE: SCSI ordered tag isn't currently used due to limitation in the
- SCSI midlayer, see the following random notes section.
-ii. For devices which have queue depth greater than 1 but don't
-support ordered tags, block layer ensures that the requests preceding
-a barrier request finishes before issuing the barrier request. Also,
-it defers requests following the barrier until the barrier request is
-finished. Older SCSI controllers/drives and SATA drives fall in this
-iii. Devices which have queue depth of 1. This is a degenerate case
-of ii. Just keeping issue order suffices. Ancient SCSI
-controllers/drives and IDE drives are in this category.
-2. Forced flushing to physical medium
-Again, if you're not gonna do synchronization with disk drives (dang,
-it sounds even more appealing now!), the reason you use I/O barriers
-is mainly to protect filesystem integrity when power failure or some
-other events abruptly stop the drive from operating and possibly make
-the drive lose data in its cache. So, I/O barriers need to guarantee
-that requests actually get written to non-volatile medium in order.
-There are four cases,
-i. No write-back cache. Keeping requests ordered is enough.
-ii. Write-back cache but no flush operation. There's no way to
-guarantee physical-medium commit order. This kind of devices can't to
-I/O barriers.
-iii. Write-back cache and flush operation but no FUA (forced unit
-access). We need two cache flushes - before and after the barrier
-iv. Write-back cache, flush operation and FUA. We still need one
-flush to make sure requests preceding a barrier are written to medium,
-but post-barrier flush can be avoided by using FUA write on the
-barrier itself.
-How to support barrier requests in drivers
-All barrier handling is done inside block layer proper. All low level
-drivers have to are implementing its prepare_flush_fn and using one
-the following two functions to indicate what barrier type it supports
-and how to prepare flush requests. Note that the term 'ordered' is
-used to indicate the whole sequence of performing barrier requests
-including draining and flushing.
-typedef void (prepare_flush_fn)(struct request_queue *q, struct request *rq);
-int blk_queue_ordered(struct request_queue *q, unsigned ordered,
- prepare_flush_fn *prepare_flush_fn);
-@q : the queue in question
-@ordered : the ordered mode the driver/device supports
-@prepare_flush_fn : this function should prepare @rq such that it
- flushes cache to physical medium when executed
-For example, SCSI disk driver's prepare_flush_fn looks like the
-static void sd_prepare_flush(struct request_queue *q, struct request *rq)
- memset(rq->cmd, 0, sizeof(rq->cmd));
- rq->cmd_type = REQ_TYPE_BLOCK_PC;
- rq->timeout = SD_TIMEOUT;
- rq->cmd[0] = SYNCHRONIZE_CACHE;
- rq->cmd_len = 10;
-The following seven ordered modes are supported. The following table
-shows which mode should be used depending on what features a
-device/driver supports. In the leftmost column of table,
-QUEUE_ORDERED_ prefix is omitted from the mode names to save space.
-The table is followed by description of each mode. Note that in the
-descriptions of QUEUE_ORDERED_DRAIN*, '=>' is used whereas '->' is
-used for QUEUE_ORDERED_TAG* descriptions. '=>' indicates that the
-preceding step must be complete before proceeding to the next step.
-'->' indicates that the next step can start as soon as the previous
-step is issued.
- write-back cache ordered tag flush FUA
-NONE yes/no N/A no N/A
-DRAIN no no N/A N/A
-DRAIN_FLUSH yes no yes no
-DRAIN_FUA yes no yes yes
-TAG no yes N/A N/A
-TAG_FLUSH yes yes yes no
-TAG_FUA yes yes yes yes
- I/O barriers are not needed and/or supported.
- Sequence: N/A
- Requests are ordered by draining the request queue and cache
- flushing isn't needed.
- Sequence: drain => barrier
- Requests are ordered by draining the request queue and both
- pre-barrier and post-barrier cache flushings are needed.
- Sequence: drain => preflush => barrier => postflush
- Requests are ordered by draining the request queue and
- pre-barrier cache flushing is needed. By using FUA on barrier
- request, post-barrier flushing can be skipped.
- Sequence: drain => preflush => barrier
- Requests are ordered by ordered tag and cache flushing isn't
- needed.
- Sequence: barrier
- Requests are ordered by ordered tag and both pre-barrier and
- post-barrier cache flushings are needed.
- Sequence: preflush -> barrier -> postflush
- Requests are ordered by ordered tag and pre-barrier cache
- flushing is needed. By using FUA on barrier request,
- post-barrier flushing can be skipped.
- Sequence: preflush -> barrier
-Random notes/caveats
-* SCSI layer currently can't use TAG ordering even if the drive,
-controller and driver support it. The problem is that SCSI midlayer
-request dispatch function is not atomic. It releases queue lock and
-switch to SCSI host lock during issue and it's possible and likely to
-happen in time that requests change their relative positions. Once
-this problem is solved, TAG ordering can be enabled.
-* Currently, no matter which ordered mode is used, there can be only
-one barrier request in progress. All I/O barriers are held off by
-block layer until the previous I/O barrier is complete. This doesn't
-make any difference for DRAIN ordered devices, but, for TAG ordered
-devices with very high command latency, passing multiple I/O barriers
-to low level *might* be helpful if they are very frequent. Well, this
-certainly is a non-issue. I'm writing this just to make clear that no
-two I/O barrier is ever passed to low-level driver.
-* Completion order. Requests in ordered sequence are issued in order
-but not required to finish in order. Barrier implementation can
-handle out-of-order completion of ordered sequence. IOW, the requests
-MUST be processed in order but the hardware/software completion paths
-are allowed to reorder completion notifications - eg. current SCSI
-midlayer doesn't preserve completion order during error handling.
-* Requeueing order. Low-level drivers are free to requeue any request
-after they removed it from the request queue with
-blkdev_dequeue_request(). As barrier sequence should be kept in order
-when requeued, generic elevator code takes care of putting requests in
-order around barrier. See blk_ordered_req_seq() and
-ELEVATOR_INSERT_REQUEUE handling in __elv_add_request() for details.
-Note that block drivers must not requeue preceding requests while
-completing latter requests in an ordered sequence. Currently, no
-error checking is done against this.
-* Error handling. Currently, block layer will report error to upper
-layer if any of requests in an ordered sequence fails. Unfortunately,
-this doesn't seem to be enough. Look at the following request flow.
- [0] [1] [2] [3] [pre] [barrier] [post] < [4] [5] [6] ... >
- still in elevator
-Let's say request [2], [3] are write requests to update file system
-metadata (journal or whatever) and [barrier] is used to mark that
-those updates are valid. Consider the following sequence.
- i. Requests [0] ~ [post] leaves the request queue and enters
- low-level driver.
- ii. After a while, unfortunately, something goes wrong and the
- drive fails [2]. Note that any of [0], [1] and [3] could have
- completed by this time, but [pre] couldn't have been finished
- as the drive must process it in order and it failed before
- processing that command.
- iii. Error handling kicks in and determines that the error is
- unrecoverable and fails [2], and resumes operation.
- iv. [pre] [barrier] [post] gets processed.
- v. *BOOM* power fails
-The problem here is that the barrier request is *supposed* to indicate
-that filesystem update requests [2] and [3] made it safely to the
-physical medium and, if the machine crashes after the barrier is
-written, filesystem recovery code can depend on that. Sadly, that
-isn't true in this case anymore. IOW, the success of a I/O barrier
-should also be dependent on success of some of the preceding requests,
-where only upper layer (filesystem) knows what 'some' is.
-This can be solved by implementing a way to tell the block layer which
-requests affect the success of the following barrier request and
-making lower lever drivers to resume operation on error only after
-block layer tells it to do so.
-As the probability of this happening is very low and the drive should
-be faulty, implementing the fix is probably an overkill. But, still,
-it's there.
-* In previous drafts of barrier implementation, there was fallback
-mechanism such that, if FUA or ordered TAG fails, less fancy ordered
-mode can be selected and the failed barrier request is retried
-automatically. The rationale for this feature was that as FUA is
-pretty new in ATA world and ordered tag was never used widely, there
-could be devices which report to support those features but choke when
-actually given such requests.
- This was removed for two reasons 1. it's an overkill 2. it's
-impossible to implement properly when TAG ordering is used as low
-level drivers resume after an error automatically. If it's ever
-needed adding it back and modifying low level drivers accordingly
-shouldn't be difficult.
Index: linux-2.6/Documentation/block/writeback_cache_control.txt
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6/Documentation/block/writeback_cache_control.txt 2010-08-26 06:46:14.555017932 -0300
@@ -0,0 +1,86 @@
+Explicit volatile write back cache control
+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_FLUSH 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_FLUSH 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_FLUSH 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_FLUSH 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_FLUSH 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_FLUSH bit set. For real device
+drivers that do not have a volatile cache the REQ_FLUSH and REQ_FUA bits
+on non-empty bios can simply be ignored, and REQ_FLUSH 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_FLUSH requests before
+entering the driver and strips off the REQ_FLUSH 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
+ blk_queue_flush(sdkp->disk->queue, REQ_FLUSH);
+and handle empty REQ_FLUSH requests in its prep_fn/request_fn. Note that
+REQ_FLUSH requests with a payload are automatically turned into a sequence
+of an empty REQ_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_flush(sdkp->disk->queue, REQ_FLUSH | REQ_FUA);
+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_FLUSH request after the actual write.
Index: linux-2.6/Documentation/block/00-INDEX
--- linux-2.6.orig/Documentation/block/00-INDEX 2010-08-26 06:46:33.723023240 -0300
+++ linux-2.6/Documentation/block/00-INDEX 2010-08-26 06:46:54.932004457 -0300
@@ -1,7 +1,5 @@
- This file
- - I/O Barriers
- Notes on the Generic Block Layer Rewrite in Linux 2.5
@@ -16,3 +14,5 @@ stat.txt
- Block layer statistics in /sys/block/<dev>/stat
- Switching I/O schedulers at runtime
+ - Control of volatile write back caches

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