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To: arjan@fenrus.demon.nl
Cc: linux-kernel@vger.kernel.org
Subject: Re: performance of O_DIRECT on md/lvm
In-Reply-To: <p734rlg90ga.fsf@oldwotan.suse.de.suse.lists.linux.kernel> <m16SPj2-000OVeC@amadeus.home.nl.suse.lists.linux.kernel>
From: Andi Kleen <ak@suse.de>
Date: 20 Jan 2002 23:11:35 +0100
In-Reply-To: arjan@fenrus.demon.nl's message of "20 Jan 2002 22:46:12 +0100"
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arjan@fenrus.demon.nl writes:

> In article <p734rlg90ga.fsf@oldwotan.suse.de> you wrote:
> 
> > I think an optional readahead mode for O_DIRECT would be useful. 
> 
> I disagree. O_DIRECT says "do not cache. period. I know what I'm doing"
> and the kernel should respect that imho. After all we have sys_readahead for
> the other part...

Problem with sys_readahead is that it doesn't work for big IO sizes. 
e.g. you read in big blocks. You have to do readahead(next block); 
read(directfd, ..., big-block); 
The readahead comes to early in this case; it would be better if it is
done in the middle of read of big-block based on the request size.
Otherwise you risk additional seeks when you overflow the 'read window',
which is all to easy this way. 

-Andi
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