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    SubjectRaw i/o usage wrecks block device performance??


    Here's a very strange (and repeatable) result. Affects 2.2.x + raw device
    patches (i.e. RH7.0). Also had a similar effect on 2.4.0test9!

    The problem is best described with a little sequence. After using raw i/o
    facilities, streamed block device reads from the same underlying device
    exhibit much poorer performance than before the raw i/o.


    [root@localhost /root]# hdparm -t /dev/hda

    Timing buffered disk reads: 64 MB in 3.81 seconds = 16.80 MB/sec
    [root@localhost /root]# time dd if=/dev/raw/raw1 of=/dev/null bs=1024k
    64+0 records in
    64+0 records out

    real 0m2.990s
    user 0m0.010s
    sys 0m0.450s
    [root@localhost /root]# hdparm -t /dev/hda

    Timing buffered disk reads: 64 MB in 6.12 seconds = 10.46 MB/sec

    The read figures before and after the raw i/o are repeatable with only
    little jitter.

    Raw device reads are consistent and not affected by this phenomena.

    Anyone know what's going on? Looks like a bug or inefficiency somewhere in
    the kernel.


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      Last update: 2005-03-22 12:39    [W:11.890 / U:0.004 seconds]
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