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

[Neil Brown]

On Tuesday November 3, laurent.corbes@xxxxxxxxxxxx wrote:
> 
> What is really strange is that from all the tests I did the raw md perfs never
> dropped. only a few MB of diff between kernel (~2%). This is maybe related to
> the way upper FS write datas on the md layer.

That isn't all that strange.  It just says that the problem isn't with
MD, but is in some other part of Linux closer to the filesystem.

I did some tests with a range of kernels (all 'mainline', not the
'stable' versions that you used) and while I do see a noticeable dip
at 2.6.30 (except with ext3) is see improved performance in 2.6.31 and
even greater improvements with 2.6.32-rc5.

So while I confirm that 2.6.30 is worse than earlier kernels, and that
there was a general decline leading to that point, things have become
dramatically better.  So I don't think it is worth exploring very deeply.

All the numbers in the graph come from 'bonnie' over the various
file-systems on a 5-drive RAID6.

NeilBrown