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block: update documentation for REQ_FLUSH / REQ_FUA

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Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
This commit is contained in:
Christoph Hellwig 2010-09-03 11:56:17 +02:00 committed by Jens Axboe
parent 09d60c701b
commit 04ccc65cd1
3 changed files with 88 additions and 263 deletions
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4
Documentation/block/00-INDEX
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@ -1,7 +1,5 @@
00-INDEX
- This file
barrier.txt
- I/O Barriers
biodoc.txt
- Notes on the Generic Block Layer Rewrite in Linux 2.5
capability.txt
@ -16,3 +14,5 @@ stat.txt
- Block layer statistics in /sys/block/<dev>/stat
switching-sched.txt
- Switching I/O schedulers at runtime
writeback_cache_control.txt
- Control of volatile write back caches

261
Documentation/block/barrier.txt
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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.

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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_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
doing:
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.