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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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