Live-Migration force after timeout

https://blueprints.launchpad.net/nova/+spec/live-migration-force-after-timeout

Replace the existing flawed automatic post-copy logic with the option to force-complete live-migrations on completion timeout, instead of aborting.

Problem description

In an ideal world, we could tell when a VM looks unable to move, and warn the operator sooner than the completion timeout. This was the idea with the progress timeout. Sadly we do not get enough information from QEMU and libvirt to correctly detect this case. As we were sampling a saw tooth wave, it was possible for us to think little progress was being made, when in fact that was not the case. In addition, only memory was being monitored, so large block_migrations always looked like they were making no progress. Refer to the References section for details.

In Ocata we deprecated that progress timeout, and disabled it by default. Given there is no quick way to make that work, it should be removed now. The automatic post-copy is using the same flawed data, so that logic should also be removed.

Nova currently optimizes for limited guest downtime, over ensuring the live-migration operation always succeeds. When performing a host maintenance, operators may want to move all VMs from the affected host to an unaffected host. In some cases, the VM could be too busy to move before the completion timeout, and currently that means the live-migration will fail with a timeout error.

Automatic post-copy used to be able to help with this use case, ensuring Nova does its best to ensure the live-migration completes, at the cost of a little more VM downtime. We should look at a replacement for automatic post-copy.

Use Cases

  • Operators want to patch a host and want to move all the VMs out of that host, with minimal impact to the VMs, so they use live-migration. If the VM isn’t live-migrated there will be significant VM downtime, so its better to take a little more VM downtime during the live-migration so the VM is able to avoid the much larger amount of downtime should the VM not get moved by the live-migration.

Proposed change

  • Config option libvirt.live_migration_progress_timeout was deprecated in Ocata, and can now be removed.

  • Current logic in libvirt driver to auto trigger post-copy will be removed.

  • A new configuration option libvirt.live_migration_timeout_action will be added. This new option will have choice to abort (default) or force_complete. This option will determine what actions will be taken against a VM after live_migration_completion_timeout expires. Currently nova just aborts the LM operation after completion timeout expires. By default, we keep the same behavior of aborting after completion timeout.

Please note the abort and force_complete actions that are options in live_migration_timeout_action config option are the same as if you were to call the existing REST APIs of the same name. In particular, force_complete will either pause the VM or trigger post_copy depending on if post copy is enabled and available.

Alternatives

We could just remove the automatic post copy logic and not replace it, but this stops us helping operators with the above use case.

Data model impact

None

REST API impact

None

Security impact

None

Notifications impact

None

Other end user impact

None

Performance Impact

None

Other deployer impact

None

Developer impact

None

Upgrade impact

None

Implementation

Assignee(s)

Primary assignee:

Kevin Zheng

Other contributors:

Yikun Jiang

Work Items

  • Remove libvirt.live_migration_progress_timeout and auto post copy logic.

  • Add a new libvirt conf option live_migration_timeout_action.

Dependencies

None

Testing

Add in-tree functional and unit tests to test new logic. Testing these types of scenarios in Tempest is not really possible given the unpredictable nature of a timeout test. Therefore we can simulate and test the logic in functional tests like those that already exist.

Documentation Impact

Document new config options.

History

Revisions

Release Name

Description

Pike

Approved but not implemented

Stein

Reproposed