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KVM disk performance


Hi List,

I have a question about KVM disk performance. We are using Openstack Nova on three machines. These machines have a SSD drive with a dd write performance of about 130mb/s

Within the instance the write performance is down to about 5 mb/s. When using the allocate trick (dd zero to disk before newfs) we get a performance of 20 mb/s.

Things a have tried but don't give any extra results are:
- Settings the disklayout from qcow2 to raw
- Settings the cache type in libvirt.xml (writeback, writethrough, none)
- Switching KSM on and off.
- Tested with different guests OS, Linux, FreeBSD, Windows.

Is there someone who had some extra info i can check? Or are there more people with this issue?

Snippet from libvirt.xml
<driver type='qcow2'/ cache='writeback'/>
<source file='/var/lib/nova/instances/instance-00000113/disk'/>
<target dev='vda' bus='virtio'/>

Kind regards,


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