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+Tejun Heo <email@example.com>, 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 contraint 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 physcial 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
+gurantee physical-medium commit order. This kind of devices can't to
+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
+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)(request_queue_t *q, struct request *rq);
+int blk_queue_ordered(request_queue_t *q, unsigned ordered,
+ prepare_flush_fn *prepare_flush_fn,
+ unsigned gfp_mask);
+int blk_queue_ordered_locked(request_queue_t *q, unsigned ordered,
+ prepare_flush_fn *prepare_flush_fn,
+ unsigned gfp_mask);
+The only difference between the two functions is whether or not the
+caller is holding q->queue_lock on entry. The latter expects the
+caller is holding the lock.
+@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
+@gfp_mask : gfp_mask used when allocating data structures
+ for ordered processing
+For example, SCSI disk driver's prepare_flush_fn looks like the
+static void sd_prepare_flush(request_queue_t *q, struct request *rq)
+ memset(rq->cmd, 0, sizeof(rq->cmd));
+ rq->flags |= REQ_BLOCK_PC;
+ rq->timeout = SD_TIMEOUT;
+ rq->cmd = SYNCHRONIZE_CACHE;
+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
+ 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
+* 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.
+QUEUE_ORDERED_TAG_FLUSH is in use.
+     [pre] [barrier] [post] <    ... >
+ still in elevator
+Let's say request ,  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  ~ [post] leaves the request queue and enters
+ low-level driver.
+ ii. After a while, unfortunately, something goes wrong and the
+ drive fails . Note that any of ,  and  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 , 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  and  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,
+* 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.