Cannot use heterogeneous nova_backend setting

[Pierre Riteau]

The nova backend (e.g. default or rbd) can be set using the variables nova_backend_ceph or nova_backend. This setting applies only to compute nodes, so it should be possible to have different values for each host or group. I tried leaving nova_backend_ceph to "no" and enabling it only for a specific group, but it fails because the following task is skipped if the first compute node in the inventory doesn't use rbd:

- name: Check nova keyring file
  stat:
    path: "{{ node_custom_config }}/nova/{{ ceph_nova_keyring }}"
  delegate_to: localhost
  run_once: True
  register: nova_cephx_keyring_file
  failed_when: not nova_cephx_keyring_file.stat.exists
  when:
    - nova_backend == "rbd"
    - external_ceph_cephx_enabled | bool

This causes a later reference to nova_cephx_keyring_file.stat.path to fail.

[Mark Goddard (mgoddard)]

Simple option: remove run_once. Performance would suffer, but we would support different keyrings per compute.

Better option: use the extract filter to pull out nova_backend from hostvars of applicable hosts.