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    SubjectRe: [Xen-devel] domU is causing misaligned disk writes
    On Tue, Apr 20, 2010 at 01:41:51PM -0700, Brendan Cully spake thusly:
    > You could also be limited by the size of the block request ring (I
    > believe the ring is normally only one page) -- the ring needs to be
    > large enough to handle the bandwidth delay product, and AoE means the
    > delay is probably higher than normal.

    Interesting. Any easy way to increase this as a test?

    > Do you get better performance against a local partition?

    You mean a local partition on local disk in the dom0 given to a domU
    as xvd? Let's see...

    I just created a 20G logical volume on the dom0:

    # /usr/sbin/lvcreate -n test -L20G sysvg

    Added it to the domain config file to be /dev/xvdi and rebooted.

    "phy:/dev/sysvg/test,xvdi,w"

    I know you can attach block devices on the fly but this has not been
    entirely reliable for me in the past so I reboot now.

    In domU against xvdi which is /dev/sysvg/test in dom0:

    # dd if=/dev/zero of=/dev/xvdi bs=4096 count=1000000
    1000000+0 records in
    1000000+0 records out
    4096000000 bytes (4.1 GB) copied, 99.3749 seconds, 41.2 MB/s

    And iostat on dom0 shows:
    Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz
    avgqu-sz await svctm %util
    sda 0.00 824.00 4.00 101.00 16.00 39936.00
    760.99 3.19 31.31 9.52 100.00

    In dom0 against the local disk to demonstrate native performance:

    # dd if=/dev/zero of=/dev/sysvg/test bs=4096 count=1000000
    1000000+0 records in
    1000000+0 records out
    4096000000 bytes (4.1 GB) copied, 84.9047 seconds, 48.2 MB/s

    Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz
    avgqu-sz await svctm %util
    sda 7.00 11104.00 5.00 96.00 48.00 48144.00 954.30
    133.92 1172.40 9.94 100.40

    Virtually no reads happening. This disk seems a bit slow (older 80G
    sata disk) but otherwise normal. I don't see anything indicating
    alignment issues.

    --
    Tracy Reed
    http://tracyreed.org
    [unhandled content-type:application/pgp-signature]
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