Comment 2 for bug 1867144

So, the standlone_upgrade by itself does not trigger the external_upgrade_tasks with --tags ceph_systemd (which is the piece of code needed to change the ceph_playbook in ceph-ansible: https://opendev.org/openstack/tripleo-heat-templates/src/branch/master/deployment/ceph-ansible/ceph-base.yaml#L675-L681)

If you see the pythontripleo-client code, when calling "openstack tripleo upgrade" we call to all these operations: https://github.com/openstack/python-tripleoclient/blob/735acf94e3791c468ee543b50c94f359b97814f5/tripleoclient/v1/tripleo_deploy.py#L1292-L1308 which translates into this structure https://github.com/openstack/python-tripleoclient/blob/735acf94e3791c468ee543b50c94f359b97814f5/tripleoclient/constants.py#L110-L132:

1st: upgrade_steps_playbook.yaml
2nd: deploy_steps_playbook.yaml
3th: post_upgrade_steps_playbook.yaml
4th: external_upgrade_steps_playbook.yaml --tags online_upgrade

So, no sign of external_upgrade_steps_playbook passing --tags ceph_systemd or --tags ceph which is the part we need to execute the ceph ugprade bits https://github.com/openstack/tripleo-heat-templates/blob/master/deployment/ceph-ansible/ceph-base.yaml#L694

However, as John mentioned, I'm really wondering the purpose of this scenario with a single ceph-mon. We disabled ceph from our upgrades jobs long time ago as the upgrade required having at least three ceph-mons, so maybe you could pass the upgrades part but it would probably fail when running the "external_upgrade_steps_playbook --tags ceph". If the team considers this could work and it's a valid scenario I'm happy to help implementing this feature.