linux-kernel - Re: O_DIRECT question

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Message-ID: <Pine.LNX.4.61.0701110724040.19233@chaos.analogic.com>
Date:	Thu, 11 Jan 2007 07:34:00 -0500
From:	"linux-os \(Dick Johnson\)" <linux-os@...logic.com>
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>, <akpm@...l.org>, <torvalds@...l.org>,
	<mjt@....msk.ru>
Subject: Re: O_DIRECT question

On Wed, 10 Jan 2007, Aubrey 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?
>
> Thanks a lot.
> -Aubrey
> -

I don't think O_DIRECT ever did what a lot of folks expect, i.e.,
write this buffer of data to the physical device _now_. All I/O
ends up being buffered. The `man` page states that the I/O will
be synchronous, that at the conclusion of the call, data will have
been transferred. However, the data written probably will not be
in the physical device, perhaps only in a DMA-able buffer with
a promise to get it to the SCSI device, soon.

Maybe you need to say why you want to use O_DIRECT with its terrible
performance?

Cheers,
Dick Johnson
Penguin : Linux version 2.6.16.24 on an i686 machine (5592.72 BogoMips).
New book: http://www.AbominableFirebug.com/
_
..

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