linux-kernel - Re: O_DIRECT question

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Message-Id: <20070110205157.4aca3689.akpm@osdl.org>
Date:	Wed, 10 Jan 2007 20:51:57 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Aubrey <aubreylee@...il.com>
Cc:	"Hua Zhong" <hzhong@...il.com>, "Hugh Dickins" <hugh@...itas.com>,
	linux-kernel@...r.kernel.org, hch@...radead.org,
	kenneth.w.chen@...el.com, torvalds@...l.org, mjt@....msk.ru
Subject: Re: O_DIRECT question

On Thu, 11 Jan 2007 10:57:06 +0800
Aubrey <aubreylee@...il.com> wrote:

> Hi all,
> 
> Opening file with O_DIRECT flag can do the un-buffered read/write access.
> So if I need un-buffered access, I have to change all of my
> applications to add this flag. What's more, Some scripts like "cp
> oldfile newfile" still use pagecache and buffer.
> Now, my question is, is there a existing way to mount a filesystem
> with O_DIRECT flag? so that I don't need to change anything in my
> system. If there is no option so far, What is the right way to achieve
> my purpose?

Not possible, basically.

O_DIRECT reads and writes must be aligned to the device's block size
(usually 512 bytes) in memory addresses, file offsets and read/write request
sizes.  Very few applications will bother to do that and will hence fail if
their files are automagically opened with O_DIRECT.

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