linux-kernel - Re: O_DIRECT question

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Date:	Thu, 11 Jan 2007 17:42:14 +0000
From:	Alan <alan@...rguk.ukuu.org.uk>
To:	Linus Torvalds <torvalds@...l.org>
Cc:	Viktor <vvp01@...ox.ru>, 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

> space, just as an example) is wrong in the first place, but the really 
> subtle problems come when you realize that you can't really just "bypass" 
> the OS.

Well you can - its called SG_IO and that really does get the OS out of
the way. O_DIRECT gets crazy when you stop using it on devices directly
and use it on files

You do need some way to avoid the copy cost of caches and get data direct
to user space, it also needs to be a way that works without MMU tricks
because many of that need it are the embedded platforms.

Alan
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