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

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