I just did these upgrades in codfw1dev and things went very smoothly. Steps:
- Merge an install_server patch moving these hosts to Buster
- Determine which host is active and which is standby:
root@cloudcontrol1003:~# neutron l3-agent-list-hosting-router d93771ba-2711-4f88-804a-8df6fd03978a
- Re-image standby host, confirm puppet stability, reboot
- Confirm (with above command) that the standby node is up and running
- Fail-over by stopping the l3-agent on the active host
service neutron-l3-agent stop
- Confirm networking still working, confirm that active/standby hosts have traded places
- Re-image remaining host