linux-kernel - Re: O_DIRECT question

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Message-Id: <200701270035.31285.vda.linux@googlemail.com>
Date:	Sat, 27 Jan 2007 00:35:31 +0100
From:	Denis Vlasenko <vda.linux@...glemail.com>
To:	Bill Davidsen <davidsen@....com>
Cc:	Michael Tokarev <mjt@....msk.ru>, Phillip Susi <psusi@....rr.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

On Friday 26 January 2007 19:23, Bill Davidsen wrote:
> Denis Vlasenko wrote:
> > On Thursday 25 January 2007 21:45, Michael Tokarev wrote:
> >> 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.
> > 
> > Which shouldn't be true. There is no fundamental reason why
> > ordinary writes should be slower than O_DIRECT.
> > 
> Other than the copy to buffer taking CPU and memory resources.

It is not required by any standard that I know. Kernel can be smarter
and avoid that if it can.
--
vda
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