linux-ext4 - Re: Filesystem writes on RAID5 too slow

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Message-ID: <20131122224038.GH6502@dastard>
Date:	Sat, 23 Nov 2013 09:40:38 +1100
From:	Dave Chinner <david@...morbit.com>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Martin Boutin <martboutin@...il.com>,
	"Kernel.org-Linux-RAID" <linux-raid@...r.kernel.org>,
	Eric Sandeen <sandeen@...hat.com>,
	"Kernel.org-Linux-EXT4" <linux-ext4@...r.kernel.org>,
	xfs-oss <xfs@....sgi.com>
Subject: Re: Filesystem writes on RAID5 too slow

On Fri, Nov 22, 2013 at 01:21:36AM -0800, Christoph Hellwig wrote:
> > From: Dave Chinner <dchinner@...hat.com>
> > 
> > The function xfs_bmap_isaeof() is used to indicate that an
> > allocation is occurring at or past the end of file, and as such
> > should be aligned to the underlying storage geometry if possible.
> > 
> > Commit 27a3f8f ("xfs: introduce xfs_bmap_last_extent") changed the
> > behaviour of this function for empty files - it turned off
> > allocation alignment for this case accidentally. Hence large initial
> > allocations from direct IO are not getting correctly aligned to the
> > underlying geometry, and that is cause write performance to drop in
> > alignment sensitive configurations.
> > 
> > Fix it by considering allocation into empty files as requiring
> > aligned allocation again.
> > 
> > Signed-off-by: Dave Chinner <dchinner@...hat.com>
> 
> Ooops.  The fix looks good,
> 
> Reviewed-by: Christoph Hellwig <hch@....de>
> 
> 
> Might be worth cooking up a test for this, scsi_debug can expose
> geometry, and we already have it wired to to large sector size
> testing in xfstests.

We don't need to screw around with the sector size - that is
irrelevant to the problem, and we have an allocation alignment
test that is supposed to catch these issues: generic/223.

As I said, I have seen occasional failures of that test (once a
month, on average) as a result of this bug. It was simply not often
enough - running in a hard loop didn't increase the frequency of
failures - to be able debug it or to reach my "there's a regression
I need to look at" threshold. Perhaps we need to revisit that test
and see if we can make it more likely to trigger failures...

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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