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    SubjectRe: Why is NCQ enabled by default by libata? (2.6.20)

    On Tue, 27 Mar 2007, Tejun Heo wrote:

    > Justin Piszcz wrote:
    >> Checking the benchmarks on various hardware websites, anandtech,
    >> hothardware and others, they generally all come to the same conclusion if
    >> there is only 1 thread using I/O (single user system) then NCQ off is the
    >> best.
    > Are they testing using Linux? I/O performance is highly dependent on
    > workload and scheduling, so result on windows wouldn't be very useful.
    > Posting some links here would be nice.
    >> I see 30-50MB/s faster speeds with NCQ turned off on two different SW
    >> RAID5s.
    > You're testing raptors, right? If the performance drop is that drastic and
    > consistent over different workloads, we'll have to disable NCQ for raptors.
    > I'm not sure about other drives. Care to perform tests over more popular
    > ones (e.g. recent seagates or 7200rpm wds)?
    > --
    > tejun

    You are correct, it definitely depends upon the workload, and a lot of the
    benchmarks do use Windows; however, I will have to check later, I recall
    finding a few that did test under Linux.

    For a plain untar with lots of small files, the benefit is not as big as
    sequential reads/writes of big files; however, there is still an

    Raid5 Quad 150 Raptor (NCQ)
    # time sh -c 'tar xf linux-2.6.20.tar; sync'

    real 0m21.721s
    user 0m0.174s
    sys 0m1.541s

    Raid5 Quad 150 Raptor (NO NCQ)
    # time sh -c 'tar xf linux-2.6.20.tar; sync'

    real 0m16.761s
    user 0m0.195s
    sys 0m1.361s

    Raid5 Six 400GB Sata Drives (NO NCQ)
    # time sh -c 'tar xf linux-2.6.20.tar; sync'
    real 0m54.844s
    user 0m0.189s
    sys 0m1.432s

    Raid5 Six 400GB Sata Drives (NCQ)
    # time sh -c 'tar xf linux-2.6.20.tar; sync'
    real 1m7.322s
    user 0m0.194s
    sys 0m1.492s

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