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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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