linux-kernel - Re: [dm-devel] Re: Ext3 sequential read performance drop 2.6.29 -> 2.6.30,2.6.31,...

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Date:	Tue, 3 Nov 2009 08:50:30 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"NeilBrown" <neilb@...e.de>
Cc:	"device-mapper development" <dm-devel@...hat.com>,
	linux-fsdevel@...r.kernel.org, linux-raid@...r.kernel.org,
	"Laurent CORBES" <laurent.corbes@...rtjog.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [dm-devel] Re: Ext3 sequential read performance drop 2.6.29 ->
  2.6.30,2.6.31,...

On Tue, 3 Nov 2009 21:42:30 +1100 "NeilBrown" <neilb@...e.de> wrote:

> On Tue, November 3, 2009 9:06 pm, Christoph Hellwig wrote:
> > On Mon, Nov 02, 2009 at 01:55:54PM -0800, Andrew Morton wrote:
> >> On Tue, 13 Oct 2009 12:09:55 +0200
> >> Laurent CORBES <laurent.corbes@...rtjog.com> wrote:
> >>
> >> > Hi all,
> >> >
> >> > While benchmarking some systems I discover a big sequential read
> >> performance
> >> > drop using ext3 on ~ big files. The drop seems to be introduced in
> >> 2.6.30. I'm
> >> > testing with 2.6.28.6 -> 2.6.29.6 -> 2.6.30.4 -> 2.6.31.3.
> >>
> >> Seems that large performance regressions aren't of interest to this
> >> list :(
> >
> > No sure which list you mean, but dm-devel is for dm, not md.

bah.

>  We're also
> > seeing similarly massive performance drops with md and ext3/xfs as
> > already reported on the list.  Someone tracked it down to writeback
> > changes as usual, but there it got stuck.
> 
> I'm still looking - running some basic tests on 4 filesystems over
> half a dozen recent kernels to see what has been happening.
> 
> I have a suspicion that there a multiple problems.
> In particular, XFS has a strange degradation which was papered over
> by commit c8a4051c3731b.
> I'm beginning to wonder if it was caused by commit 17bc6c30cf6bf
> but I haven't actually tested that yet.

I think Laurent's workload involves only reads, with no writes.
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