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    SubjectRe: Linux disk performance.
    On 12/21/06, Bill Davidsen <> wrote:
    > >
    > > But isn't O_DIRECT supposed to bypass buffering in Kernel?
    > That's correct. But it doesn't put your write at the head of any queue,
    > it just doesn't buffer it for you.
    > > Doesn't it directly write to disk?
    > Also correct, when it's your turn to write to disk...

    But the only process accessing that disk is my application.

    > > I tried to put fdatasync() at regular intervals but there was no
    > > visible effect.
    > >
    > Quite honestly, the main place I have found O_DIRECT useful is in
    > keeping programs doing large i/o quantities from blowing the buffers and
    > making the other applications run like crap. If you application is
    > running alone, unless you are very short of CPU or memory avoiding the
    > copy to an o/s buffer will be down in the measurement noise.

    Yes... my application does large amount of I/O. It actually writes
    video data received from ethernet(IP camera) to the disk using 128 K

    > I had a news (usenet) server which normally did 120 art/sec (~480 tps),
    > which dropped to about 50 tps when doing large file copies even at low
    > priority. By using O_DIRECT the impact essentially vanished, at the cost
    > of the copy running about 10-15% slower. Changing various programs to
    > use O_DIRECT only helped when really large blocks of data were involved,
    > and only when i/o clould be done in a way to satisfy the alignment and
    > size requirements of O_DIRECT.
    > If you upgrade to a newer kernel you can try other i/o scheduler
    > options, default cfq or even deadline might be helpful.

    I tried all disk schedulers but all had timing bumps. :(

    > --
    > bill davidsen <>
    > CTO TMR Associates, Inc
    > Doing interesting things with small computers since 1979

    Manish Regmi

    UNIX without a C Compiler is like eating Spaghetti with your mouth
    sewn shut. It just doesn't make sense.
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