Enable virtio for existing VM’s

On one of our host machine, we get around 70MB/second disk write speed, but KVM guest machine was giving us around 18MB/s write performance.

At that point we came to know about virtio driver. Here is guide to enable virtio

Backup VM config

virsh dumpxml <vmname> > ~/vmname.xml

Replace vmname with Virtual Machine (Domain) name. You can find correct name by running virsh list --all

Edit VM Config

Run following command to open editor.

virsh edit <vmname>

Find lines like below:

    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/var/lib/libvirt/images/ubuntu-kvm/tmpU2z98r.qcow2'/>
      <target dev='hda' bus='ide'/></strong>
      <address type='drive' controller='0' bus='0' unit='0'/>
    </disk>

Make following changes:

1. Replacehda withvda ANDide with virtio
2. Remove following line: <address type='drive' controller='0' bus='0' unit='0'/>
3. Add cache='none' io='native' option to driver.

Your update config will look like below:

    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none' io='native'/>
      <source file='/var/lib/libvirt/images/ubuntu-kvm/tmpU2z98r.qcow2'/>
      <target dev='vda' bus='virtio'/>
    </disk>

Save your changes and exit editor.

Changes to Guest VM’s filesystem

I am not sure if this step is necessary but one tutorial mentions this. I did not try outcome without this step as VM’s I was playing with has some critical data.

Anyway, login to Guest VM’s shell and open in editor vim /etc/fstab

Fine lines like /dev/sdX. Replace 's' with 'v'.

So /dev/sda1 will become /dev/vda1 and /dev/sda2 will become /dev/vda2 and so on.

For changes to reflect

Shutdown Guest VM for its shell. You can use command shutdown 0

Next, from Host machine, run following commands:

virsh destroy <vmname>
virsh start <vmname>

virsh reboot won’t work.

At this point you can run your disk I/O benchmarks again to check speed.

In our case performance shoot-up from 18MB/s to 40.8 MB/s.