XFS: Abysmal write performance because of excessive seeking (allocation groups to blame?)

[Dave Chinner]

On Mon, Apr 09, 2012 at 01:39:13PM +0200, Emmanuel Florac wrote:
> Le Mon, 9 Apr 2012 10:19:43 +1000 vous écriviez:
> 
> > A decent RAID controller with a BBWC and a single LUN benefits from
> > parallelism just as much as a large disk arrays do because the BBWC
> > minimises the write IO latency and the controller to do a better job
> > of scheduling its IO.
> 
> BTW recently I've found that for storage servers, noop io scheduler
> often is the best choice, I suppose precisely because it doesn't try to
> outsmart the RAID controller logic...

We've been recommending the use of the no-op (or worst case,
deadline) scheduler for XFS on hardware RAID for quite a few years.
I only test against the no-op scheduler, because I got sick of
having to track down regressions caused by "smart" CFQ heuristics....

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com