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    SubjectRe: iowait problems on 2.6, not on 2.4
    Antonio Larrosa Jiménez <antlarr@tedial.com> wrote:
    >
    > On Thursday 27 May 2004 05:52, you wrote:
    > > Antonio Larrosa Jiménez <antlarr@tedial.com> wrote:
    > > > My next test will be to do the "dd tests" on one of the internal hard
    > > > disks and use it for the data instead of the external raid.
    > >
    > > That's a logical next step. The reduced read bandwith on the raid array
    > > should be fixed up before we can go any further. I don't recall any
    > > reports of qlogic fc-scsi performance regressions though.
    >
    > Ok, let's analyze that first.
    >
    > The dd tests gave the following results:

    Let me cc linux-scsi.

    Guys: poke. Does anyone know why this:

    The machine is a 4 cpu Pentium III (Cascades) system with four SCSI
    SEAGATE ST336704 hard disks connected to an Adaptec AIC-7899P U160/m, and
    a external RAID connected to a QLA2200/QLA2xxx FC-SCSI Host Bus Adapter.
    The machine has 1Gb RAM.

    got all slow at reads?


    > ext3 on the internal scsi HD:
    > 2.4.21:
    > writing : 1m14s
    > reading : 1m2s
    > reading+writing : 2m16s
    > 2.6.4:
    > writing : 1m19s
    > reading : 59s
    > reading+writing : 2m24s
    >
    > reiserfs on the internal scsi HD:
    > 2.4.21:
    > writing : 1m15s
    > reading : 1m1s
    > reading+writing : 2m22s
    > 2.6.4:
    > writing : 1m19s
    > reading : 1m
    > reading+writing : 2m25s
    >
    > ext3 on the raid using qlogic fc-scsi:
    > 2.4.21:
    > writing : 30s
    > reading : 51s
    > reading+writing : 1m29s
    > 2.6.4:
    > writing : 28s
    > reading : 1m26s
    > reading+writing : 2m19s
    >
    > reiserfs on the raid using qlogic fc-scsi:
    > 2.4.21:
    > writing : 37s
    > reading : 52s
    > reading+writing : 1m37s
    > 2.6.4:
    > writing : 25s
    > reading : 1m27s
    > reading+writing : 2m3s
    >
    > All the tests were made 3 times, and the average taken. In the cases where
    > there was too much variance, I repeated the tests some more times.
    >
    > All the tests used 2Gb reads/writes (. I tried to make 8Gb reads/writes too,
    > but they got up to a minute variance (maybe the HD slowed itself down due to
    > temperature issues sometimes? I really don't know why this happened, but in
    > any case, I couldn't make reliable tests with files of that size).
    >
    > So basically, there's no difference between 2.4.21 and 2.6.4 when using the
    > internal HD, but 2.6.4 is much slower when using the raid.
    > What I found strange is that writing to that raid is a bit faster on 2.6.4
    > while reading is much slower, which I suppose is what makes the difference.
    >
    > So yes, I suppose there's a regression on the qlogic fc-scsi module.
    >
    > Btw, the tests I timed were:
    >
    > count=2048
    > write() { dd if=/dev/zero of=x bs=1M count=$count ; sync }
    > read() { dd if=x of=/dev/null bs=1M count=$count }
    > readwrite() { dd if=x of=y bs=1M count=$count ; sync }
    >
    > In the case of read, I did the sync just before and after the timing, but
    > didn't include the sync inside the timed test.
    >
    > As I said in my other mail, I can test any patch if needed.
    >
    > Greetings and thanks for any help
    >
    > --
    > Antonio Larrosa
    > Tecnologias Digitales Audiovisuales, S.L.
    > http://www.tedial.com
    > Parque Tecnologico de Andalucia . Málaga (Spain)
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