On 12/21/06, Bill Davidsen <[email protected]> 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
chunks.
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 <[email protected]>
CTO TMR Associates, Inc
Doing interesting things with small computers since 1979
--
---------------------------------------------------------------
regards
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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