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    SubjectRe: Why is MEGASAS_SAS_QD set to 256?
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    Hi,

    在 2022/11/27 17:42, Ming Lei 写道:
    > On Sat, Nov 26, 2022 at 02:08:02PM +0800, Yu Kuai wrote:
    >> Hi, Ming
    >>
    >> 在 2022/11/26 10:18, Ming Lei 写道:
    >>>
    >>> If you want aggressive merge on sequential IO workload, the queue depth need
    >>> to be a bit less, then more requests can be staggered into scheduler queue,
    >>> and merge chance is increased.
    >>
    >> But if nr_requests >= queue_depth, it seems to me elevator will have no
    >> effect, no request can be merged or sorted by scheduler, right?
    >
    > Yeah.
    >
    > If nr_requests <= queue_depth, every request can be queued to
    > driver/device, so requests won't be merged by scheduler.
    >
    > But plug merge still works if IOs are submitted as batch.

    Yes, io can still be merged by plug. I just find it a little werid to
    set default elevator as deadline, and default queue_depth to 256. Which
    means deadline here is useless.

    >
    >>>
    >>> If you want good perf on random IO perf, the queue depth needs to
    >>> be deep enough to have enough parallelism for saturating SSD internal.
    >>>
    >>> But we don't recognize sequential/random IO pattern, and usually fixed
    >>> queue depth is used.
    >>
    >> Is it possible to use none elevator and set large queue_depth if nvme is
    >> used in this case?
    >
    > Yeah, if the storage is SSD, usually none with bigger queue_depth should
    > help, and usually 256 should be enough to saturate one single SSD for
    > one well implemented driver.

    Yes, I'm testing with multiple SSDs / NVMEs, and I can't get optimal
    performance by default.

    Thanks,
    Kuai
    >
    >
    > Thanks
    > Ming
    >
    > .
    >

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