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LVM write performance

Dear list,

When writing to a logical volume (/dev/sys/test) directly through the
device, I obtain a slow performance:

root@dom0-2:/dev/mapper# dd  of=/dev/sys/test if=/dev/zero
4580305+0 records in
4580305+0 records out
2345116160 bytes (2.3 GB) copied, 119.327 s, 19.7 MB/s

Making a file system on top of the LV, mounting it and write into a file
is ok:

root@dom0-2:/dev/mapper# mkfs.xfs /dev/sys/test
root@dom0-2:/mnt# mount /dev/sys/test /mnt/lv
root@dom0-2:/mnt# dd  of=/mnt/lv/out if=/dev/zero
2647510+0 records in
2647510+0 records out
1355525120 bytes (1.4 GB) copied, 11.3235 s, 120 MB/s

Furthermore, by accident I noticed that writing directly to the block
device is oke when the LV is mounted (of course destroying the file
system on it):

root@dom0-2:/mnt# dd  of=/dev/sys/test if=/dev/zero
3703375+0 records in
3703374+0 records out
1896127488 bytes (1.9 GB) copied, 15.4927 s, 122 MB/s

Does anyone know what is going on?

The configuration is as follows:

Debian 6.0.2
Kernel 2.6.32-5-xen-amd64
Tests are on a partition on one physical disk

Best regards,

Dion Kant

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