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

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Date:	Wed, 18 Aug 2010 08:35:54 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Hellwig <hch@....de>
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

Hello,

On 08/17/2010 06:59 PM, Christoph Hellwig wrote:
> I think we really need all the conversions in one tree, block layer,
> remapping drivers and filesystems.

I don't know.  If filesystem changes are really trivial maybe, but
md/dm changes seem a bit too invasive to go through the block tree.

> Btw, I've done the conversion for all filesystems and I'm running tests
> over them now.  Expect the series late today or tomorrow.

Cool. :-)

>> I might just resequence it to finish this part of discussion but what
>> does that really buy us?  It's not really gonna help bisection.
>> Bisection won't be able to tell anything in higher resolution than
>> "the new implementation doesn't work".  If you show me how it would
>> actually help, I'll happily reshuffle the patches.
> 
> It's not bisecting to find bugs in the barrier conversion.  We can't
> easily bisect it down anyway.  The problem is when we try to bisect
> other problems and get into the middle of the series barriers suddenly
> are gone.  Which is not very helpful for things like data integrity
> problems in filesystems.

Ah, okay, hmmm.... alright, I'll resequence the patches.  If the
filesystem changes can be put into a single tree somehow, we can keep
things mostly working at least for direct devices.

>> 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 something we better leave to the DM team.

Sure, but we shouldn't be ripping out the code to do that.

Thanks.

-- 
tejun
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