linux-kernel - Re: O_DIRECT question

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Message-ID: <45A7A8F0.30200@cfl.rr.com>
Date:	Fri, 12 Jan 2007 10:27:44 -0500
From:	Phillip Susi <psusi@....rr.com>
To:	dean gaudet <dean@...tic.org>
CC:	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@...el.com, akpm@...l.org,
	mjt@....msk.ru
Subject: Re: O_DIRECT question

dean gaudet wrote:
> it seems to me that if splice and fadvise and related things are 
> sufficient for userland to take care of things "properly" then O_DIRECT 
> could be changed into splice/fadvise calls either by a library or in the 
> kernel directly...

No, because the semantics are entirely different.  An application using 
read/write with O_DIRECT expects read() to block until data is 
physically fetched from the device.  fadvise() does not FORCE the kernel 
to discard cache, it only hints that it should, so a read() or mmap() 
very well may reuse a cached page instead of fetching from the disk 
again.  The application also expects write() to block until the data is 
on the disk.  In the case of a blocking write, you could splice/msync, 
but what about aio?

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