Comment 4 for bug 1768876

So before that change in Pike, the instance.availability_zone was set to CONF.default_availability_zone which defaults to None:

https://docs.openstack.org/nova/pike/configuration/config.html#DEFAULT.default_schedule_zone

And reading the help on that, it sounds like we maybe regressed some functionality / contract in Pike, see:

"None, which means that the instance can move from one availability zone to another during its lifetime if it is moved from one compute node to another."

So I think that means before Pike, I could create an instance without a specific AZ and then the operator could live migrate it to any other compute node regardless of AZ, but after Pike the operator can only live migrate the instance to a compute node in the same AZ, which is a regression. It should be fairly easy to reproduce this with an in-tree functional test that has 2 compute nodes in separate AZs, create an instance w/o a specific AZ, and then try to live migrate the instance - if that works in Ocata but not after Ocata, it's a regression.