Description of problem:  Hi,

this issue was first reported there https://review.opendev.org/#/c/755751/

Basically running:

openstack overcloud external-update run --tags online_upgrade

as documented doesn't run online db migration.

The problem is that online db migration are defined for upgrade, but not for update.  This is an instance of the issue[1]


      external_upgrade_tasks:
        - when: step|int == 1
          block:
            - name: Online data migration for Nova
              command: "{{ container_cli }} exec nova_conductor nova-manage db online_data_migrations"
              delegate_to: "{{ groups['nova_conductor'][0] }}"
              become: true
              tags:
                - online_upgrade
                - online_upgrade_nova


There is no equivalent external_update_tasks, so no db migration happen.

Currently we don't see any breakage related to this as we are staying on the same version.

But:
 - this is not working as expected;
 - this break 15->16.1 update new protocol.

So, not a blocker, but should be fixed asap.

[1] https://github.com/openstack/tripleo-heat-templates/blob/stable/train/deployment/nova/nova-conductor-container-puppet.yaml#L264..L274