Re: XFS vs Elevators (was Re: [PATCH RFC] nilfs2: continuous snapshotting file system)

[Matthew Wilcox <matthew@wil.cx>]

On Thu, Aug 21, 2008 at 04:04:18PM +1000, Dave Chinner wrote:
> One thing I just found out - my old *laptop* is 4-5x faster than the
> 10krpm scsi disk behind an old cciss raid controller.  I'm wondering
> if the long delays in dispatch is caused by an interaction with CTQ
> but I can't change it on the cciss raid controllers. Are you using
> ctq/ncq on your machine?  If so, can you reduce the depth to
> something less than 4 and see what difference that makes?

I don't think that's going to make a difference when using CFQ.  I did
some tests that showed that CFQ would never issue more than one IO at a
time to a drive.  This was using sixteen userspace threads, each doing a
4k direct I/O to the same location.  When using noop, I would get 70k
IOPS and when using CFQ I'd get around 40k IOPS.

-- 
Matthew Wilcox				Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours.  We can't possibly take such
a retrograde step."