linux-kernel - Re: O_DIRECT question

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Date:	Thu, 25 Jan 2007 23:45:38 +0300
From:	Michael Tokarev <mjt@....msk.ru>
To:	Phillip Susi <psusi@....rr.com>
CC:	Denis Vlasenko <vda.linux@...glemail.com>,
	Linus Torvalds <torvalds@...l.org>, Viktor <vvp01@...ox.ru>,
	Aubrey <aubreylee@...il.com>, Hua Zhong <hzhong@...il.com>,
	Hugh Dickins <hugh@...itas.com>, linux-kernel@...r.kernel.org,
	hch@...radead.org, kenneth.w.chen@in
Subject: Re: O_DIRECT question

Phillip Susi wrote:
> Denis Vlasenko wrote:
>> You mean "You can use aio_write" ?
> 
> Exactly.  You generally don't use O_DIRECT without aio.  Combining the
> two is what gives the big win.

Well, it's not only aio.  Multithreaded I/O also helps alot -- all this,
say, to utilize a raid array with many spindles.

But even single-threaded I/O but in large quantities benefits from O_DIRECT
significantly, and I pointed this out before.  It's like enabling a write
cache on disk AND doing intensive random writes - the cache - surprizingly -
slows whole thing down by 5..10%.

/mjt
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