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    SubjectRe: [Xen-devel] domU is causing misaligned disk writes
    On Tue, Apr 20, 2010 at 12:39:52PM -0700, Tracy Reed wrote:
    > On Tue, Apr 20, 2010 at 11:49:55AM +0300, Pasi Kärkkäinen spake thusly:
    > > Please paste your domU partition table:
    > > sfdisk -d /dev/xvda
    >
    > I have tried many different things including dd straight to the raw
    > unpartitioned device. That should not be affected by
    > partitioning/lvm/filesystem problems right?
    >

    Yeah, partitioning doesn't affect when you use the straight/raw disk device.

    > > Are you using filesystems on normal partitions, or LVM in the domU?
    > > I'm pretty sure this is a domU partitioning problem.
    >
    > I have done all of the above. Here I am an xvdg device in my domU to
    > which I am directly doing a dd to, no partitioning or anything:
    >
    > # dd if=/dev/zero of=/dev/xvdg bs=4096 count=3000000
    > 3000000+0 records in
    > 3000000+0 records out
    > 12288000000 bytes (12 GB) copied, 449.109 seconds, 27.4 MB/s
    >

    Please try with "bs=1024k" and maybe with "bs=64k" aswell.

    4k blocksize transfer will always be slower in domU than in dom0
    since virtual disk abstraction makes some overhead, which is more
    visible with small blocksizes.


    > # /sbin/sfdisk -d /dev/xvdg
    >
    > sfdisk: ERROR: sector 0 does not have an msdos signature
    > /dev/xvdg: unrecognized partition table type
    > No partitions found
    >
    > and running iostat on the target shows the following:
    >
    > Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await svctm %util
    > sda 0.00 3474.60 1070.60 46.40 4311.20 13680.00 32.21 2.08 1.83 0.49 54.32
    > sdb 0.00 3376.00 1060.20 45.60 4289.60 13686.40 32.51 2.46 2.23 0.53 58.12
    >
    > Or I can partition it with a geometry of 248 heads and 56 sectors
    > which is a multiple of 8 which should avoid the misalignment due to
    > the extra partition table (there is a partition on the physical disk
    > on the target already then I create a logical volume to export to the
    > initiator which then puts its own partition in it which causes
    > misalignment):
    >
    > dd if=/dev/zero of=/dev/xvdg1 bs=4096 count=3000000
    > 3000000+0 records in
    > 3000000+0 records out
    > 12288000000 bytes (12 GB) copied, 445.338 seconds, 27.6 MB/s
    >

    So the speed is the same to the partitioned disk than to the raw disk?
    What disk backend are you using in dom0? phy:? tap:aio: ?


    > # /sbin/sfdisk -d /dev/xvdg
    > # partition table of /dev/xvdg
    > unit: sectors
    >
    > /dev/xvdg1 : start= 56, size=566227592, Id=8e
    > /dev/xvdg2 : start= 0, size= 0, Id= 0
    > /dev/xvdg3 : start= 0, size= 0, Id= 0
    > /dev/xvdg4 : start= 0, size= 0, Id= 0
    >
    > Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz avgqu-sz await svctm %util
    > sda 0.00 3472.20 1188.20 51.00 4805.60 14097.60 30.51 2.71 2.13 0.52 64.02
    > sdb 0.00 3472.40 1187.00 52.00 4784.00 14092.80 30.47 2.82 2.22 0.56 68.80
    >
    > Or I can take a standard partition geometry and set it to start at 64
    > instead of 63 like so many RAID alignment pages talk about:
    >
    > It is taking even longer this time and I am tired of waiting for dd
    > before sending off this email but suffice it to say it is painfully
    > slow.
    >

    You can cancel dd and it'll print the stats so far.

    -- Pasi


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