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Date: Thu, 30 Oct 2003 13:44:07 -0800
From: Andrew Morton <akpm@osdl.org>
To: age <ahuisman@cistron.nl>
Cc: linux-kernel@vger.kernel.org
Subject: Re: READAHEAD
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age <ahuisman@cistron.nl> wrote:
>
> I have a problem which i don`t understand and i hope that you
>  will and can  help me. The problem is that i experience strange disk
>  read performance. I have to set hdparm -m16 -u1 -c1 -d1 -a4096 /dev/hde
>  to get  timing buffered disk reads of 56 MB/SEC.
>  When i disable readahead i get 17 MB/SEC
>  When i enable readahead with -a8 i get  17 MB/SEC
>  When i enable readahead with -a16 i get 24,5 MB/SEC
>  When i enable readahead with -a32 i get 30,5 MB/SEC
>  When i enable readahead with -a64 i get 35 MB/SEC
>  When i enable readahead with -a128 i get 39 MB/SEC
>  When i enable readahead with -a256 i get 39 MB/SEC
>  When i enable readahead with -a512 i get 41 MB/SEC
>  When i enable readahead with -a1024 i get 50 MB/SEC
>  When i enable readahead with -a2048 i get 50 MB/SEC
>  When i enable readahead with -a4096 i get 56 MB/SEC
>  With -a8192,-a16384 and -a32768 i get also 56MB/SEC
> 
>  Before, i never had to set readahead so high
>  Please could you tell me, what is going on here ?

Lots of people have been reporting this.  It's rather weird.

Is the same effect observable when reading a large file, or is it only
observable via `hdparm -t'?