Re: READAHEAD

From: Nuno Silva (nuno.silva_at_vgertech.com)
Date: 10/31/03

  • Next message: devik: "Re: [2.6.0-test9] QoS HTB crash..." 
    Date:	Fri, 31 Oct 2003 07:43:57 +0000
To: Andrew Morton <akpm@osdl.org>

    Hi!!

    Andrew Morton wrote:
    > 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.
    >

    I know nothing about this but, FWIW, I think that what changed where the
    units. With 2.4 you specify sectors, with 2.6 you specify bytes.

    So, having -a8, in 2.4, is the same as having -a$((8*512)) [it's 4096
    :)], in 2.6.

    Not sure if it's the case, but makes sense :-)

    Regards,
    Nuno Silva

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