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    SubjectPROBLEM: Consistently slower 3ware RAID performance under 2.6.4
    ## Background:

    I'm doing a lot of sequential read and write performance tests on my
    3ware 7000-2 controller (RAID1 with 2xWD1200JB), because I'm getting
    very poor write performance. This is a problem that I'm currently
    working with 3ware to resolve, and they are working enthusiastically
    with me to fix it.

    Read test: time dd if=/dev/sda2 of=/dev/null bs=1024k count=1024
    Write test: time dd if=/dev/zero of=/dev/sda2 bs=1024k count=1024

    NOTE: /dev/sda2 is the swap partition which is NEAR the outer-most tracks.


    ## The new problem I discovered specific to Linux:

    Regardless of the above-mentioned problem, I am noticing a very
    significant performance drop between a 2.4 kernel and a 2.6 kernel.


    ## Performance with "2.4.25-gentoo":

    The read test here takes 21.6 seconds which is about 47MB/sec. This is
    a correct number, because I have measured the maximum read throughput
    from each drive to be 47MB/sec.

    The write test here takes 2 minutes, 2.5 seconds. This translates to
    8.35MB/sec. This is what I'm working with 3ware to correct, but let's
    call this the baseline write performance.


    ## Performance with "2.6.4-gentoo-r1":

    The read test here takes 33.9 seconds. That's down to about 30MB/sec.

    The write test here takes 2 minutes, 44.2 seconds. That is down to
    6.2MB/sec.


    ## HELP?

    How can I help kernel developers to diagnose this problem? What
    information do I need to provide that is missing?

    What is responsible for such a significant performance drop on LONG
    sequential disk accesses?


    ## My computer:

    Athlon 2800+
    1GB RAM (Corsair 2700LL)
    ABIT KD7 (KT400 chipset)


    Thanks in advance for the help!

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