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diff --git a/Documentation/block/barrier.txt b/Documentation/block/barrier.txt deleted file mode 100644 index 2c2f24f634e..00000000000 --- a/Documentation/block/barrier.txt +++ /dev/null @@ -1,261 +0,0 @@ -I/O Barriers -============ -Tejun Heo <htejun@gmail.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 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 -category. - -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 -request. - -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 -following. - -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 - - -QUEUE_ORDERED_NONE - I/O barriers are not needed and/or supported. - - Sequence: N/A - -QUEUE_ORDERED_DRAIN - Requests are ordered by draining the request queue and cache - flushing isn't needed. - - Sequence: drain => barrier - -QUEUE_ORDERED_DRAIN_FLUSH - Requests are ordered by draining the request queue and both - pre-barrier and post-barrier cache flushings are needed. - - Sequence: drain => preflush => barrier => postflush - -QUEUE_ORDERED_DRAIN_FUA - 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 - -QUEUE_ORDERED_TAG - Requests are ordered by ordered tag and cache flushing isn't - needed. - - Sequence: barrier - -QUEUE_ORDERED_TAG_FLUSH - Requests are ordered by ordered tag and both pre-barrier and - post-barrier cache flushings are needed. - - Sequence: preflush -> barrier -> postflush - -QUEUE_ORDERED_TAG_FUA - 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. -QUEUE_ORDERED_TAG_FLUSH is in use. - - [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. |