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

[NeilBrown]

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 at smartjog.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.  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.

NeilBrown