Re: [Xen-devel] domU is causing misaligned disk writes

[Tracy Reed <treed@ultraviolet.org>]

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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?

> 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

# /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

# /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.

# /sbin/sfdisk -d /dev/xvdg
# partition table of /dev/xvdg
unit: sectors

/dev/xvdg1 : start=       64, size=566226926, Id=83
/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  1832.73 1234.73 30.94  4991.62  7864.27    20.31     1.52    1.23   0.47  59.82
sdb               0.00  1835.13 1219.76 30.54  4916.57  7839.52    20.40     1.27    1.04   0.45  56.67

I would not be at all surprised if you are right about it being a domU
partitioning problem. But every scheme I have tried has failed to work
properly. Appreciate any pointers.

-- 
Tracy Reed
http://tracyreed.org

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