[Xen-users] Xen DomU with high IOWAIT and low disk performance (lvm raid1)

[Andreas Nitsche <andi@xxxxxxxxx>]

Hello list!

My setup:

Dom0: Debain 5.0.3 with xen-hypervisor-3.2-1-i386 (2.6.26-2-xen-686)
DomU: Ubuntu 8.04 2.6.26-2-xen-686

System is running on two hard drives mirrored with raid1 and organized
by LVM. Dom0 and DomU are running on logical volumes.

Partitions for DomUs are connected via 'phy:/dev/lvm/disk1,sda1,w' for
example.

Here are some scenarios I testet, where you should easily can see that
DomU is much much slower than Dom0 when having read and write access at
the same time on the harddrive.

dd if=/dev/zero of=test.img bs=1M count=1000
Dom0:
$ dd if=/dev/zero of=test.img bs=1M count=1000
1000+0 DatensÃtze ein
1000+0 DatensÃtze aus
1048576000 Bytes (1,0 GB) kopiert, 12,88 s, 81,4 MB/s
DomU:
$ dd if=/dev/zero of=test.img bs=1M count=1000
1000+0 DatensÃtze ein
1000+0 DatensÃtze aus
1048576000 Bytes (1,0 GB) kopiert, 16,8728 s, 62,1 MB/s


dd if=test.img of=/dev/null
Dom0:
$ dd if=test.img of=/dev/null
2048000+0 DatensÃtze ein
2048000+0 DatensÃtze aus
1048576000 Bytes (1,0 GB) kopiert, 5,47333 s, 192 MB/s
DomU:
$ dd if=test.img of=/dev/null
2048000+0 DatensÃtze ein
2048000+0 DatensÃtze aus
1048576000 Bytes (1,0 GB) kopiert, 10,2963 s, 102 MB/s

Reading and writing from and to the hard drives works fast.

date; cp test.img test2.img; date
Dom0:
$ date; cp test.img test2.img; date
Do 3. Dez 08:22:44 CET 2009
Do 3. Dez 08:22:57 CET 2009

~76,92 MB/s

DomU:
$ date; cp test.img test2.img; date
Do 3. Dez 07:23:16 UTC 2009
Do 3. Dez 07:28:24 UTC 2009

~3,25 MB/s

date; mv test.img /backup/; date
Dom0:
$ date; mv test.img /backup/; date
Do 3. Dez 08:28:45 CET 2009
Do 3. Dez 08:28:58 CET 2009

~76,92 MB/s

DomU:
$ date; sudo mv test.img /backup/; date
Do 3. Dez 07:31:15 UTC 2009
Do 3. Dez 07:36:16 UTC 2009

~3,32 MB/s

/backup is a second logical volume inside the same lvm.

Each test was made time by time, so no process was running which
produces high io at testing time.

My question: Why is DomU that slow on read and write access at the same
time? And how can I get that faster?

-- 
regards
Andi

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