post-copy and auto-converge are mutually exclusive

[Andrea Ieri]

live_migration_permit_post_copy and live_migration_permit_auto_converge cannot be true at the same time. Or rather, they can, but post-copy will win over auto-converge. See the config reference[0].

I think the charm should go in a blocked state if both are set to true, because although it is a valid configuration, the resulting behavior is not obvious, and has bitten us several times in the past.

[0] https://docs.openstack.org/nova/stein/configuration/config.html#libvirt.live_migration_permit_post_copy

[Andrea Ieri (aieri)]

On second thought, I have a better suggestion: since live_migration_permit_post_copy and live_migration_permit_auto_converge effectively implement a three-way switch, it would be more idiomatic to deprecate them in favor of a new setting. For example:

  live-migration-force-strategy:
    type: string
    default: stop
    description: |
      Selects which strategy will be used when migrations are force-completed.
      Valid options: stop, post-copy, auto-converge
      Only used if the deprecated live-migration-permit-post-copy and
      live-migration-permit-auto-converge options are unset
  live-migration-permit-post-copy:
    type: boolean
    default: False
    description: |
      DEPRECATED: use live-migration-force-strategy instead
      If live-migration is enabled, this option allows Nova to switch an on-
      going live migration to post-copy mode.
  live-migration-permit-auto-converge:
    type: boolean
    default: False
    description: |
      DEPRECATED: use live-migration-force-strategy instead
      If live-migration is enabled, this option allows Nova to throttle down
      CPU when an on-going live migration is slow.
      WARNING: auto-converge will only be used if post-copy is disabled

[Adam Dyess (addyess)]

I agree that these two options
 - live-migration-permit-post-copy
 - live-migration-permit-auto-converge

are entagled in that an enabled permit-post-copy ignores an enabled auto-converge.

See: https://github.com/openstack/nova/blob/master/nova/virt/libvirt/driver.py#L1151

a charmed config of this would be nice to capture that.

[Andrea Ieri (aieri)]

Yeah, Adam helped me clear up my confusion: auto-converge is not a force-completion strategy, it kicks in immediately if allowed.

Perhaps a better UI would be:

  live-migration-strategy:
    type: string
    default: abort
    description: |
      Selects which strategy will be used during live migrations.
      Valid options: abort, auto-converge
      Only used if the deprecated live-migration-permit-post-copy and
      live-migration-permit-auto-converge options are unset
  live-migration-force-completion-strategy:
    type: string
    default: pause
    description: |
      Selects which strategy will be used when migrations are force-completed.
      Valid options: pause, post-copy
      Only used if the deprecated live-migration-permit-post-copy and
      live-migration-permit-auto-converge options are unset
  live-migration-permit-post-copy:
    type: boolean
    default: False
    description: |
      DEPRECATED: use live-migration-force-strategy instead
      If live-migration is enabled, this option allows Nova to switch an on-
      going live migration to post-copy mode.
  live-migration-permit-auto-converge:
    type: boolean
    default: False
    description: |
      DEPRECATED: use live-migration-force-strategy instead
      If live-migration is enabled, this option allows Nova to throttle down
      CPU when an on-going live migration is slow.
      WARNING: auto-converge will only be used if post-copy is disabled

I think the above would make it easier for a user to select how they want standard live migrations to be handled vs how they want force-completions to happen.
If both post-copy and auto-converge are selected the charm should go in a blocked state.