linux-kernel - Re: O_DIRECT question

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Message-Id: <1168540875.6170.46.camel@lade.trondhjem.org>
Date:	Thu, 11 Jan 2007 13:41:15 -0500
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	Xavier Bestel <xavier.bestel@...e.fr>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	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

On Thu, 2007-01-11 at 09:04 -0800, Linus Torvalds wrote:
> That is what I think some users could do. If the main issue with O_DIRECT 
> is the page cache allocations, if we instead had better (read: "any") 
> support for POSIX_FADV_NOREUSE, one class of reasons O_DIRECT usage would 
> just go away.

For NFS, the main feature of interest when it comes to O_DIRECT is
strictly uncached I/O. Replacing it with POSIX_FADV_NOREUSE won't help
because it can't guarantee that the page will be thrown out of the page
cache before some second process tries to read it. That is particularly
true if some dopey third party process has mmapped the file.

Trond

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