Configure Live Migration Parameters in Nova

About this task

You can configure a maximum time for a live migration to complete and/or the maximum downtime target for live migration switchover.

Parameters

  • live_migration_completion_timeout

    Time to wait, in seconds, for live migration to successfully complete transferring data before aborting the operation.

    Value is per GiB of guest RAM + disk to be transferred, with a lower boundary of 2 GiB. Set to 0 to disable timeouts.

  • live_migration_downtime

    The target maximum period of time that Nova will require the VM to be paused in order to complete the VM memory copy. If the transfer rate slows down after the VM has been paused, this value can be exceeded by a small amount of time, it will be rounded up to a minimum of 100ms.

    You may increase this value if you want to allow live-migrations to complete faster, or avoid live-migration timeout errors by allowing the guest to be paused longer during the live-migration switch over.

    Two additional live_migration_downtime_scope parameters can be modified:

    • live_migration_downtime_steps

      Sets the total number of adjustment steps until live_migration_downtime is reached. This will let nova increase live_migration_downtime gradually until either the switchover has been completed or the maximum value has been reached. The default is 10 steps.

    • live_migration_downtime_delay

      Sets the time interval between two adjustment steps in seconds. The default is 75.

Procedure

All the parameters can be modified using the Helm overrides for the StarlingX OpenStack application.

  1. Create a yaml file containing the configuration update.

    For example nova_override.yaml:

    conf:
    nova:
        libvirt:
            live_migration_completion_timeout: 300
            live_migration_downtime: 600
            live_migration_downtime_steps: 12
            live_migration_downtime_delay: 50

  2. Update the Helm overrides using the new configuration file.

    ~(keystone_admin)]$ system helm-override-update --values ./nova_override.yaml wr-openstack nova openstack --reuse-values

  3. Confirm that the user override lists the correct live migration parameters.

    ~(keystone_admin)]$ system helm-override-show wr-openstack nova openstack

  4. Apply the changes.

    ~(keystone_admin)]$ system application-apply wr-openstack

Results

If the live migration aborts because live_migration_completion_timeout has been exceeded, then the following message appears on the nova_compute logs:

WARNING nova.virt.libvirt.migration [-] [instance: 07b5842b-6324-4de0-973e-6d0ff18ef574] Live migration not completed after 300 seconds

The default behavior is to abort the operation. You can change this action by configuring live_migration_timeout_action in your yaml file.

For example:

live_migration_timeout_action: force_complete

If it is set to force_complete, the compute service will either pause the VM or trigger post-copy if post copy is enabled and available (live_migration_permit_post_copy is set to True).