linux-kernel - Re: O_DIRECT question

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Message-ID: <20070111124521.GN13675@harddisk-recovery.com>
Date:	Thu, 11 Jan 2007 13:45:21 +0100
From:	Erik Mouw <erik@...ddisk-recovery.com>
To:	Linus Torvalds <torvalds@...l.org>
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,
	mjt@....msk.ru
Subject: Re: O_DIRECT question

On Wed, Jan 10, 2007 at 07:05:30PM -0800, Linus Torvalds wrote:
> I should have fought back harder. There really is no valid reason for EVER 
> using O_DIRECT. You need a buffer whatever IO you do, and it might as well 
> be the page cache. There are better ways to control the page cache than 
> play games and think that a page cache isn't necessary.

There is a valid reason: you really don't want to go through the page
cache when a hard drive has bad blocks. The only way to get fast
recovery and correct error reporting to userspace is by using O_DIRECT.

> So don't use O_DIRECT. Use things like madvise() and posix_fadvise() 
> instead. 

Both don't do what I want it to do: only read the sector I request you
to read and certainly do not try to outsmart me by doing some kind of
readahead.


Erik

-- 
+-- Erik Mouw -- www.harddisk-recovery.com -- +31 70 370 12 90 --
| Lab address: Delftechpark 26, 2628 XH, Delft, The Netherlands
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