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    SubjectRe: problem: linux and (really) fast I/O
    Davide Rossetti <> wrote:
    > DISKS="/dev/md0 /dev/md1 /dev/md2 /dev/md3 /dev/sdb1 /dev/sdd1 /dev/sde1 /dev/sdf1"
    > time dd if=/dev/zero of=$dsk bs=$blksize count=$count

    The problem is that when using the raw (?) disk interface, you will
    see at the SCSI analyzer that delay between commands is around
    milliseconds. This probably because

    write("/dev/sdX", size); -> copied in buffers
    -> sent to SCSI layer as maybe one 64k block
    -> command sent to drive. When command finished,
    at that time the user process is wakened up
    and THEN ONLY it fills again the buffer, etc.

    At least it's my understanding. NB this has been seen on the SCSI
    bus at 2.0.x times, so maybe it has improved.

    Try your benchmarks on a *filesystem*, you will see it will become
    better (I saw upto 28 MByte/s on a P200 64 MB with Bonnie ext2
    -b 4096 Adaptec 2940UW with a SCSI-to-SCSI RAID0 array with
    two IBM DGVS drives).

    I have not tried md stripping of DGVS drives yet (I have tried
    slower DCAS and it *does* double the speed, and especially the

    Even better someone has started a high performance file system suited
    for video applications under Linux (can't remember the name, though).

    Mail me if you want the Bonnie benchmark program and some more

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      Last update: 2005-03-22 13:43    [W:0.023 / U:14.076 seconds]
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