[libvirt] Qemu/KVM is 3x slower under libvirt (due to vhost=on)

Reeted reeted at shiftmail.org
Wed Sep 28 10:19:09 UTC 2011

On 09/28/11 11:53, Daniel P. Berrange wrote:
> On Wed, Sep 28, 2011 at 11:49:01AM +0200, Reeted wrote:
>> YES!
>> It's the vhost. With vhost=on it takes about 12 seconds more time to boot.
>> ...meaning? :-)
> I've no idea. I was always under the impression that 'vhost=on' was
> the 'make it go much faster' switch. So something is going wrong
> here that I cna't explain.
> Perhaps one of the network people on this list can explain...
> To turn vhost off in the libvirt XML, you should be able to use
> <driver name='qemu'/>  for the interface in question,eg
>      <interface type='user'>
>        <mac address='52:54:00:e5:48:58'/>
>        <model type='virtio'/>
>        <driver name='qemu'/>
>      </interface>

Ok that seems to work: it removes the vhost part in the virsh launch 
hence cutting down 12secs of boot time.

If nobody comes out with an explanation of why, I will open another 
thread on the kvm list for this. I would probably need to test disk 
performance on vhost=on to see if it degrades or it's for another reason 
that boot time is increased.

Thanks so much for your help Daniel,

More information about the libvir-list mailing list