linux-kernel - Re: O_DIRECT question

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Date:	Thu, 11 Jan 2007 15:13:29 +0300
From:	Viktor <vvp01@...ox.ru>
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

Linus Torvalds wrote:
> 
> On Thu, 11 Jan 2007, Aubrey wrote:
> 
>>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?
> 
> 
> The right way to do it is to just not use O_DIRECT. 
> 
> The whole notion of "direct IO" is totally braindamaged. Just say no.
> 
> 	This is your brain: O
> 	This is your brain on O_DIRECT: .
> 
> 	Any questions?
> 
> 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.
> 
> So don't use O_DIRECT. Use things like madvise() and posix_fadvise() 
> instead. 

OK, madvise() used with mmap'ed file allows to have reads from a file
with zero-copy between kernel/user buffers and don't pollute cache
memory unnecessarily. But how about writes? How is to do zero-copy
writes to a file and don't pollute cache memory without using O_DIRECT?
Do I miss the appropriate interface?

> 		Linus
> -
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