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Currently we only update the resource usage with Placement API in the
instance claim and the available resource update periodic task. But there
is no claim for migration with placement API yet. This works is tracked by
https://bugs.launchpad.net/nova/+bug/1621709. In newton, we only fix one
bit which make the resource update periodic task works correctly, then it
will auto-heal everything. For the migration claim part, that isn't the
goal for newton release.

So the first question is do we want to fix it in this release? If the
answer is yes, there have a concern need to discuss.

In order to implement the drop of migration claim, the RT needs to remove
allocation records on the specific RP(on the source/destination compute
node). But there isn't any API can do that. The API about remove allocation
records is 'DELETE /allocations/{consumer_uuid}', but it will delete all
the allocation records for the consumer. So the initial fix(
https://review.openstack.org/#/c/369172/) adds new API 'DELETE
/resource_providers/{rp_uuid}/allocations/{consumer_id}'. But Chris Dent
pointed out this against the original design. All the allocations for the
specific consumer only can be dropped together.

There also have suggestion from Andrew, we can update all the allocation
records for the consumer each time. That means the RT will build the
original allocation records and new allocation records for the claim
together, and put into one API. That API should be 'PUT
/allocations/{consumer_uuid}'. Unfortunately that API doesn't replace all
the allocation records for the consumer, it always amends the new
allocation records for the consumer.

So which directly we should go at here?

Thanks
Alex
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