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    SubjectRe: raid io requests not parallel?
    On Saturday July 15, jbaccash@gmail.com wrote:
    > I'm using kernel linux-2.6.15-gentoo-r1, and I noticed performance of
    > the software RAID-1 is not as good as I would have expected on my two
    > SATA drives, and I was wondering if anyone has an idea what may be
    > happening. The test I run is 1024 16k direct-IO reads/writes from
    > random locations within a 1GB file (on a RAID-1 partition), with my
    > disk caches set to
    > write-through mode. In the MT (multi-threaded) case, I issue them from
    > 8 threads (so it's 128 requests per thread):
    >
    > Random read: 10.295 sec
    > Random write: 19.142 sec

    Odd. I would expect these two numbers to be a lot closer together.

    Try changing the IO scheduler on the drives and see if it makes a
    difference.
    e.g.
    cat /sys/block/XXX/queue/scheduler
    echo cfq > /sys/block/XXX/queue/scheduler
    echo deadline > /sys/block/XXX/queue/scheduler

    See what works best.

    NeilBrown



    > MT Random read: 5.276 sec
    > MT Random write: 19.839 sec
    >
    > As expected, the multi-threaded reads are 2x as fast as single-threaded
    > reads. But I would have expected (assuming the write to both disks can
    > occur in parallel) that the random writes are about the same speed (10
    > seconds) as the single-threaded random reads, for both the
    > single-threaded and multi-threaded write cases. The fact that the
    > multi-threaded reads were
    > twice as fast indicates to me that read requests can occur in parallel.
    >
    > So.... why doesn't the raid issue the writes in parallel? Thanks in
    > advance for any help.
    >
    > Jon.
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