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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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