In my experience, it is possible to perform in-place migration from ML2/LXB -> ML2/OVN, albeit with a shutdown or hard reboot of the instance(s) to complete the VIF plugging and some other needed operations. I have a very rough outline of required steps if you’re interested, but they’re geared towards an openstack-ansible based deployment. I’ll try to put a writeup together in the next week or two demonstrating the process in a multi-node environment; the only one I have done recently was an all-in-one.

 

James Denton

Rackspace Private Cloud

Thanks for replying, I'd really love to see your outline / list of steps. BTW, we are actively working on switching to openstack-ansible - so that would suit us well.
We also came to the conclusion that a shutdown of all instances might be required. Question is, if that has to happen instantly or if one could do that on a project by project base.
Our cloud is small enough to still make this feasible, but I suppose this topic is or will become more important  to other, larger clouds as well.

Regards

Christian