linux-kernel - Re: [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush

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Date:	Fri, 20 Aug 2010 17:26:02 +0900
From:	Kiyoshi Ueda <k-ueda@...jp.nec.com>
To:	Christoph Hellwig <hch@....de>, Tejun Heo <tj@...nel.org>
CC:	jaxboe@...ionio.com, linux-fsdevel@...r.kernel.org,
	linux-scsi@...r.kernel.org, linux-ide@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
	James.Bottomley@...e.de, tytso@....edu, chris.mason@...cle.com,
	swhiteho@...hat.com, konishi.ryusuke@....ntt.co.jp,
	dm-devel@...hat.com, vst@...b.net, jack@...e.cz,
	rwheeler@...hat.com, hare@...e.de
Subject: Re: [PATCHSET block#for-2.6.36-post] block: replace barrier with
 sequenced flush

Hi Tejun, Christoph,

On Tue, Aug 17, 2010 at 06:41:47PM +0200, Tejun Heo wrote:
>>> I wasn't sure about that part.  You removed store_flush_error(), but
>>> DM_ENDIO_REQUEUE should still have higher priority than other
>>> failures, no?
>>
>> Which priority?
>
> IIUC, when any of flushes get DM_ENDIO_REQUEUE (which tells the dm
> core layer to retry the whole bio later), it trumps all other failures
> and the bio is retried later.  That was why DM_ENDIO_REQUEUE was
> prioritized over other error codes, which actually is sort of
> incorrect in that once a FLUSH fails, it _MUST_ be reported to upper
> layers as FLUSH failure implies data already lost.  So,
> DM_ENDIO_REQUEUE actually should have lower priority than other
> failures.  But, then again, the error codes still need to be
> prioritized.

I think that's correct and changing the priority of DM_ENDIO_REQUEUE
for REQ_FLUSH down to the lowest should be fine.
(I didn't know that FLUSH failure implies data loss possibility.)

But the patch is not enough, you have to change target drivers, too.
E.g. As for multipath, you need to change
     drivers/md/dm-mpath.c:do_end_io() to return error for REQ_FLUSH
     like the REQ_DISCARD support included in 2.6.36-rc1.


By the way, if these patch-set with the change above are included,
even one path failure for REQ_FLUSH on multipath configuration will
be reported to upper layer as error, although it's retried using
other paths currently.
Then, if an upper layer won't take correct recovery action for the error,
it would be seen as a regression for users. (e.g. Frequent EXT3-error
resulting in read-only mount on multipath configuration.)

Although I think the explicit error is fine rather than implicit data
corruption, please check upper layers carefully so that users won't see
such errors as much as possible.

Thanks,
Kiyoshi Ueda
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