nova-specs/specs/stein/approved/remove-force-flag-from-live-migrate-and-evacuate.rst

Remove force flag from live-migrate and evacuate

https://blueprints.launchpad.net/nova/+spec/remove-force-flag-from-live-migrate-and-evacuate

Force live-migrate and evacuate operations cannot be meaningfully supported for servers having complex resource allocations. So this spec proposes to remove the force flag from these operations in a new REST API microversion.

Problem description

Today when force: True is specified nova tries to blindly copy the resource allocation from the source host to the target host. This only works if the the server's allocation is satisfied by the single root resource provider both on the source host and on the destination host. As soon as the allocation become more complex (e.g. it allocates from more than one provider (including sharing providers) or allocates only from a nested provider) the blind copy will fail.

Use Cases

This change removes the following use case from the system:

• The admin cannot force a live-migration to a specified destination host against the Nova scheduler and Placement agreement.
• The admin cannot force a evacuate to a specified destination host against the Nova scheduler and Placement agreement.

This does not effect the use cases when the operator specifies the destination host and let Nova and Placement verify that host before the move.

Please note that this removes the possibility to force live-migrate servers to hosts where the nova-compute is disabled as the ComputeFilter in the filter scheduler will reject such hosts.

Proposed change

Forcing the destination host in a complex allocation case cannot supported without calling Placement to get allocation candidates on the destination host as Nova does not know how to copy the complex allocation. The documentation of the force flag states that Nova will not call the scheduler to verify the destination host. This rule has already been broken since Pike by two bugfixes. Also supporting complex allocations requires to get allocation candidates from Placement. So the spec proposes to remove the force flag as it cannot be supported any more.

Note that fixing old microversions to fail cleanly without leaking resources in complex allocation scenarios is not part of this spec but handled as part of use-nested-allocation-candidates That change will make sure that the forced move operation on a server that either has complex allocation on the source host or would require complex allocation on the destination host will be rejected with a NoValidHost exception by the Nova conductor.

Alternatives

• Try to guess when the server needs a complex allocation on the destination host and only ignore the force flag in these cases.
• Do not manage resource allocations for forced move operations.

See more details in the ML thread

Data model impact

None

REST API impact

In a new microversion remove the force flag from both APIs:

• POST /servers/{server_id}/action (os-migrateLive Action)
• POST /servers/{server_id}/action (evacuate Action)

Security impact

None

Notifications impact

None

Other end user impact

Update python-novaclient and python-openstackclient to support the new microversion.

As the admin cannot skip the scheduler any more when moving servers, such move can fail with scheduler and Placement related reasons.

Performance Impact

As the admin cannot skip the scheduler when moving a server, such move will take a bit more time as Nova will call the scheduler and Placement.

Other deployer impact

Please note that this spec removes the possibility to force live-migrate servers to hosts where the nova-compute is disabled as the ComputeFilter in the filter scheduler will reject such hosts.

Developer impact

Supporting the force flag has been a detriment to maintaining nova since it's an edge case and requires workarounds like the ones made in Pike to support it. Dropping support over time will be a benefit to maintaining the project and improve consistency/reliability/usability of the API.

Upgrade impact

None

Implementation

Assignee(s)

Primary assignee:

balazs-gibizer

Work Items

• Add a new microversion to the API that removes the force flag from the payload. If the new microversion is used in the request then default force to False when calling Nova internals.
• Document the new microversion
• Add support for the new microversion in the python-novaclient and in the python-openstackclient

Dependencies

• Some part of use-nested-allocation-candidates is a dependecy of this work.

Testing

• Functional and unit test will be provided

Documentation Impact

• API reference document needs to be updated

References

History

Revisions

Release Name	Description
Stein	Introduced