ubuntu 10.4 linux-kvm poor I/O performance

Hey there

Using qemu-kvm libvirt-bin virt-manager

I’ve seen some very poor I/O performance with virtio enabled
installing an guest debian lenny 64bit took 45min
at install I’ve seen some very heavy disk load

So we need to enable this in libvirt:

virsh edit blahguest

and put that into the line “driver”

cache='writeback'
<disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='writeback'/>
      <source file='/var/lib/libvirt/images/blahguest.qcow2'/>
      .....
      .....
</disk>

Never need to set this with debian lenny using libvirt or kvm directly
I think something changed in linux-kvm code

Have fun