[nova][ops] Live migration and CPU features

Belmiro Moreira moreira.belmiro.email.lists at gmail.com
Tue Aug 18 14:49:37 UTC 2020

in our infrastructure we have always compute nodes that need a hardware
intervention and as a consequence they are rebooted, bringing a new kernel,
kvm, ...

In order to have a good compromise between performance and flexibility
(live migration) we have been using "host-model" for the "cpu_mode"
configuration of our service VMs. We didn't expect to have CPU
compatibility issues because we have the same hardware type per cell.

The problem is that when a compute node is rebooted the instance domain is
recreated with the new cpu features that were introduced because of the
reboot (using centOS).

If there are new CPU features exposed, this basically blocks live migration
to all the non rebooted compute nodes (those cpu features are not exposed,
yet). The nova-scheduler doesn't know about them when scheduling the live
migration destination.

I wonder how other operators are solving this issue.
I don't like stopping OS upgrades.
What I'm considering is to define a "custom" cpu_mode for each hardware

I would appreciate your comments and learn how you are solving this problem.

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