block: update documentation for REQ_FLUSH / REQ_FUA

00-INDEX

	- This file

barrier.txt

	- I/O Barriers

biodoc.txt

	- Notes on the Generic Block Layer Rewrite in Linux 2.5

capability.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

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.

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.