Re: Linux disk performance.

From: Bill Davidsen
Date: Sun Dec 31 2006 - 21:00:45 EST

Phillip Susi wrote:
Bill Davidsen wrote:
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.

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 would point out that if you are looking for optimal throughput and reduced cpu overhead, and avoid blowing out the kernel fs cache, you need to couple aio with O_DIRECT. By itself O_DIRECT will lower throughput because there will be brief pauses between each IO while the application prepares the next buffer. You can overcome this by posting a few pending buffers concurrently with aio, allowing the kernel to always have a buffer ready for the next io as soon as the previous one completes.

A good point, but in this case there was no particular urgency, other than not to stop the application while doing background data moves. The best way to do it would have been to put it where it belonged in the first place :-(

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