Message-ID: <45A629E9.70502@inbox.ru>
Date: Thu, 11 Jan 2007 15:13:29 +0300
From: Viktor <vvp01@inbox.ru>
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To: Linus Torvalds <torvalds@osdl.org>
CC: Aubrey <aubreylee@gmail.com>, Hua Zhong <hzhong@gmail.com>,
       Hugh Dickins <hugh@veritas.com>, linux-kernel@vger.kernel.org,
       hch@infradead.org, kenneth.w.chen@intel.com, akpm@osdl.org,
       mjt@tls.msk.ru
Subject: Re: O_DIRECT question
References: <6d6a94c50701101857v2af1e097xde69e592135e54ae@mail.gmail.com> <Pine.LNX.4.64.0701101902270.3594@woody.osdl.org>
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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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