linux-ext4 - Re: [PATCH] ext4: allow inode_readahead

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Message-ID: <20110222023211.GF2924@thunk.org>
Date:	Mon, 21 Feb 2011 21:32:11 -0500
From:	Ted Ts'o <tytso@....edu>
To:	"Alexander V. Lukyanov" <lav@...is.ru>
Cc:	linux-ext4@...r.kernel.org, Andreas Dilger <adilger@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ext4: allow inode_readahead_blks=0 (linux-2.6.37)

On Tue, Feb 08, 2011 at 09:39:25AM +0300, Alexander V. Lukyanov wrote:
> Hello!
> 
> I cannot disable inode-read-ahead feature of ext4 (on 2.6.37):
> 
> # echo 0 > /sys/fs/ext4/sda2/inode_readahead_blks 
> bash: echo: write error: Invalid argument
> 
> On a server with lots of small files and random access this read-ahead makes
> performance worse, and I'd like to disable it. I work around this problem
> by using value of 1, but it still reads an extra block.

So I'm curious --- have you actually benchmarked a performance
decrease?  What sort of hardware are you using?

The readahead should be changing a 4k read to a 8k read with a value
of 1, which shouldn't take a much of a difference to a HDD.

I can apply this patch, but is it really making a difference for you?

      	    	 	       	  	 	- Ted
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