Enable cold migration with target host - Ocata


The aim of this feature is to let operators cold migrate instances with target host manually. In addition, provide a way to make sure that resource allocation is consistent for the cold migration operation when a destination host is specified.

Problem description

A target host can be specified on the live migration operation. And there is a scheduler rule check and ‘force’ flag in REST API when a target host is specified on the live migration operation.

But a target host cannot be specified on the cold migration operation. It is inconsistent with them.

Use Cases

It is same as the live migration use case. Sometimes an operator or a script decides which host is the best suited to accept a cold migration and then wants to perform it. A consistency with a live migration case should be ensured.

Proposed change

Modify the API and the current resize_instance flow to be able to specify the target host for cold migration.

Add the function to check whether a destination host is in accordance with scheduler rules or not in cold migration as a default behaviour. Specifically to say, add setting ‘requested_destination’ of the RequestSpec object in nova/compute/api.py. The field has already been supported in the scheduler, so it just need to be filled in.

This blueprint also provides a way for operators to bypass the scheduler, we will make the API for cold migration including a destination host by adding an request body argument called ‘force’ (accepting True or False, defaulted to False) and the corresponding CLI methods will expose that force option. If the microversion asked by the client is older than the version providing the field, then it won’t be passed (neither True or False, rather the key won’t exist) to the conductor so the conductor won’t call the scheduler.



Data model impact


REST API impact

  • URL: POST /v2.1/servers/{server_id}/action

    JSON request body:

        "migrate": {
            "host": "target-host",
            "force": True

The ‘host’ parameter to specify a target host is required (not optional) because of consistency with a live migration API.

If ‘force’ is True, do not check the destination. If ‘force’ is False or null or not provided, do check the destination.

If ‘force’ is supplied in the request body and its value is true but the ‘host’ parameter is null, then an HTTP 400 Bad Request will be served to the user.

Microversion is bumped up.

Security impact


Notifications impact


Other end user impact

python-novaclient will be modified to have a target host argument as optional. And add the ‘force’ argument as optional.

nova migrate [–force True] <server> [<host>]

Performance Impact


Other deployer impact


Developer impact




Primary assignee:


Work Items

  • Add logic to specify target host for cold migration

  • Add processing checking destination host in the cold migration

  • Add API with bumping a new microversion

  • Add a target host argument and ‘force’ argument on novaclient

  • Add nova functional tests

  • Add tempest tests




Add the following tests.

  • Unit tests

  • Functional tests

  • Tempest tests

Documentation Impact

  • API Reference

  • CLI Reference

  • Admin User Guide on cold migration topic.



The function to specify a target host was previously approved in juno[1]. The function to check whether a destination host is in accordance with scheduler rules or not and the ‘force’ flag are added for ocata.


Release Name