Re: ext4 write performance regression in 3.6-rc1 on RAID0/5

Theodore Ts'o
2012-08-17T15:38:01Z

On Fri, Aug 17, 2012 at 11:13:18PM +0800, Fengguang Wu wrote:
>
> Obviously the major regressions happen to the 100dd over raid cases.
> Some 10dd cases are also impacted.
>
> The attached graphs show that everything becomes more fluctuated in
> 3.6.0-rc1 for the lkp-nex04/RAID0-12HDD-thresh=8G/ext4-100dd-1 case.

Hmm... I'm not seeing any differences in the block allocation code, or
in ext4's buffered writeback code paths, which would be the most
likely cause of such problems. Maybe a quick eyeball of the blktrace
to see if we're doing something pathalogically stupid?

You could also try running a filefrag -v on a few of the dd files to
see if there's any significant difference, although as I said, there
doesn't look like there was any significant changes in the block
allocation code between v3.5 and v3.6-rc1 --- although I suppose
changes in timeing could have have caused the block allocation
decisions to be different, so it's worth checking that out.

Thanks, regards,

- Ted