Re: Linux disk performance.

[Phillip Susi <psusi@cfl.rr.com>]

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.