linux-kernel - Re: O_DIRECT question

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Message-ID: <20070111141537.GD2277@kernel.dk>
Date:	Thu, 11 Jan 2007 15:15:38 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	"linux-os (Dick Johnson)" <linux-os@...logic.com>
Cc:	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,
	torvalds@...l.org, mjt@....msk.ru
Subject: Re: O_DIRECT question

On Thu, Jan 11 2007, linux-os (Dick Johnson) wrote:
> 
> 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.

Thanks for your guessing, but O_DIRECT is with the physical drive once
the call returns. Whether it's in drive cache or on drive platters is a
different story, but from the OS point of view, it's definitely with the
drive.

-- 
Jens Axboe

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