Open Stack

We have an issue with live-migration if operators update OVS from a
version that does not support dpdkvhostuserclient to a version that is
supporting it.

Basically from OVS2.6 to OVS2.7 or upper.

The problem is that, for libvirt driver all the instances created that
use vhu interfaces in server mode (OVS2.6) wont be able to
live-migrate anymore. That because Neutron to select which vhu mode to
use is looking at OVS capabilities [0].

Meaning that During the live-migration port details are going to be
updated but Nova and in particular libvirt driver does not update
guests domain XML to refer the changes.

- We can fix Neutron by making it consider to always use the same vhu
  mode if the ports already exist.

- We can enhance Nova and in particular libvirt driver to update
  guests domain XML during live-migration. The benefit is that the
  instances are going to be updated for free to use vhu in client mode
  which is totally better but it's probably not so trivial to
  implement.

- We can avoid fixing it meaning that operators will have to update
  their instances to use vhu mode client that by a way like
  snapshot/rebuild. Then live-migration will be possible.

[0] https://git.openstack.org/cgit/openstack/neutron/tree/neutron/plugins/ml2/drivers/openvswitch/mech_driver/mech_openvswitch.py#n94