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From: Mike Snitzer <snitzer@redhat.com>
To: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Christoph Hellwig <hch@lst.de>,
	axboe@kernel.dk, agk@redhat.com, shli@kernel.org,
	philipp.reisner@linbit.com, linux-block@vger.kernel.org,
	linux-scsi@vger.kernel.org, drbd-dev@lists.linbit.com,
	dm-devel@redhat.com, linux-raid@vger.kernel.org
Subject: Re: [PATCH 22/23] drbd: implement REQ_OP_WRITE_ZEROES
Date: Thu, 30 Mar 2017 19:15:50 -0400	[thread overview]
Message-ID: <20170330231550.GA3102@redhat.com> (raw)
In-Reply-To: <yq1pogy3i6u.fsf@oracle.com>

On Thu, Mar 30 2017 at 11:20am -0400,
Martin K. Petersen <martin.petersen@oracle.com> wrote:

> Mike Snitzer <snitzer@redhat.com> writes:
> 
> > I can work on this now.  Only question I have is: should DM thinp take
> > care to zero any misaligned head and tail?  (I assume so but with all
> > the back and forth between Bart, Paolo and Martin I figured I'd ask
> > explicitly).
> 
> Yep, let's make sure our semantics match the hardware ditto.
> 
>  - So write zeroes should behave deterministically and explicitly handle
>    any blocks that can't be cleared via deprovisioning.
> 
>  - And discard can work at the discard granularity in a
>    non-deterministic fashion.

I got pretty far along with implementing the DM thinp support for
WRITE_ZEROES in terms of thinp's DISCARD support (more of an
implementation detail.. or so I thought).

But while discussing this effort with Jeff Moyer he asked: shouldn't the
zeroed blocks be provisioned?  This is a fairly embarassing question not
to be able to answer in the moment.  So I clearly need to learn what the
overall intent of WRITE_ZEROES actually is.

If it is meant as a replacement for WRITE_SAME (as hch switched dm-io
over from WRITE_SAME with a payload of 0 to WRITE_ZEROES) and for the
backing mechanism for blkdev_issue_zeroout() then I guess I have my
answer.  Unless DM thinp can guarantee that the discarded blocks will
always return zeroes (by zeroing before all partial block writes) my
discard based dm-thinp implementation of WRITE_ZEROES is a complete
throw-away (unless block zeroing is enabled.. which it never is because
performance sucks with it).  So if an upper-level of the IO stack
(e.g. ext4) were to assume that a block will _definitely_ have zeroes
then DM thinp would fall short.

This is all to say: I don't see a quick way forward on implementing
performant WRITE_ZEROES support for DM thinp.

  reply	other threads:[~2017-03-30 23:16 UTC|newest]

Thread overview: 70+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-03-23 14:33 RFC: always use REQ_OP_WRITE_ZEROES for zeroing offload Christoph Hellwig
2017-03-23 14:33 ` [PATCH 01/23] block: renumber REQ_OP_WRITE_ZEROES Christoph Hellwig
2017-03-28 16:12   ` Bart Van Assche
2017-03-30  8:53     ` hch
2017-03-23 14:33 ` [PATCH 02/23] block: implement splitting of REQ_OP_WRITE_ZEROES bios Christoph Hellwig
2017-03-23 14:33 ` [PATCH 03/23] sd: implement REQ_OP_WRITE_ZEROES Christoph Hellwig
2017-03-28 18:50   ` Bart Van Assche
2017-03-28 19:33     ` Mike Snitzer
2017-03-29 14:51     ` Paolo Bonzini
2017-03-29 16:28       ` Bart Van Assche
2017-03-29 16:53         ` Paolo Bonzini
2017-03-30  2:25     ` Martin K. Petersen
2017-03-23 14:33 ` [PATCH 04/23] md: support REQ_OP_WRITE_ZEROES Christoph Hellwig
2017-03-23 14:33 ` [PATCH 05/23] dm: " Christoph Hellwig
2017-03-23 14:33 ` [PATCH 06/23] dm-kcopyd: switch to use REQ_OP_WRITE_ZEROES Christoph Hellwig
2017-03-23 14:55   ` Mike Snitzer
2017-03-23 14:56     ` Christoph Hellwig
2017-03-23 15:10       ` Mike Snitzer
2017-03-27  9:12         ` Christoph Hellwig
2017-03-23 14:33 ` [PATCH 07/23] block: stop using blkdev_issue_write_same for zeroing Christoph Hellwig
2017-03-23 14:33 ` [PATCH 08/23] block: add a flags argument to (__)blkdev_issue_zeroout Christoph Hellwig
2017-03-23 14:33 ` [PATCH 09/23] block: add a REQ_UNMAP flag for REQ_OP_WRITE_ZEROES Christoph Hellwig
2017-03-23 14:33 ` [PATCH 10/23] block: add a new BLKDEV_ZERO_NOFALLBACK flag Christoph Hellwig
2017-03-23 14:33 ` [PATCH 11/23] block_dev: use blkdev_issue_zerout for hole punches Christoph Hellwig
2017-03-28 16:50   ` Bart Van Assche
2017-03-30  8:59     ` hch
2017-03-23 14:33 ` [PATCH 12/23] sd: handle REQ_UNMAP Christoph Hellwig
2017-03-28 16:48   ` Bart Van Assche
2017-03-29 14:57     ` Paolo Bonzini
2017-03-30  9:02     ` hch
2017-03-30 15:28       ` Martin K. Petersen
2017-03-30 17:30         ` hch
2017-03-31  2:19           ` Martin K. Petersen
2017-03-31  7:18             ` hch
2017-03-23 14:33 ` [PATCH 13/23] nvme: implement REQ_OP_WRITE_ZEROES Christoph Hellwig
2017-03-23 14:33 ` [PATCH 14/23] zram: " Christoph Hellwig
2017-03-23 14:33 ` [PATCH 15/23] loop: " Christoph Hellwig
2017-03-23 14:33 ` [PATCH 16/23] brd: remove discard support Christoph Hellwig
2017-03-23 14:33 ` [PATCH 17/23] rbd: remove the discard_zeroes_data flag Christoph Hellwig
2017-03-23 14:33 ` [PATCH 18/23] rsxx: " Christoph Hellwig
2017-03-23 14:33 ` [PATCH 19/23] mmc: " Christoph Hellwig
2017-03-23 14:33 ` [PATCH 20/23] block: stop using discards for zeroing Christoph Hellwig
2017-03-23 14:33 ` [PATCH 21/23] drbd: make intelligent use of blkdev_issue_zeroout Christoph Hellwig
2017-03-23 14:33 ` [PATCH 22/23] drbd: implement REQ_OP_WRITE_ZEROES Christoph Hellwig
2017-03-30 10:06   ` Lars Ellenberg
2017-03-30 11:44     ` Christoph Hellwig
2017-03-30 12:50       ` [Drbd-dev] " Lars Ellenberg
2017-03-30 13:49       ` Mike Snitzer
2017-03-30 15:20         ` Martin K. Petersen
2017-03-30 23:15           ` Mike Snitzer [this message]
2017-03-31  2:34             ` Martin K. Petersen
2017-03-31  7:17             ` Christoph Hellwig
2017-03-23 14:33 ` [PATCH 23/23] block: remove the discard_zeroes_data flag Christoph Hellwig
2017-03-28 17:00   ` Bart Van Assche
2017-03-29 14:52     ` Paolo Bonzini
2017-03-30  9:06     ` hch
2017-03-30 15:29       ` Martin K. Petersen
2017-03-30 17:29         ` hch
2017-03-23 15:54 ` RFC: always use REQ_OP_WRITE_ZEROES for zeroing offload Lars Ellenberg
2017-03-23 17:02   ` Mike Snitzer
2017-03-23 22:53     ` Lars Ellenberg
2017-03-29 14:57       ` Paolo Bonzini
2017-03-27  9:10   ` Christoph Hellwig
2017-03-27 14:03     ` Mike Snitzer
2017-03-27 14:57       ` Christoph Hellwig
2017-03-27 15:08       ` [Drbd-dev] " Bart Van Assche
2017-03-30  9:04   ` Christoph Hellwig
2017-03-30 15:12 ` Mike Snitzer
2017-03-30 15:22   ` Martin K. Petersen
2017-03-30 15:38     ` Mike Snitzer

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