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