Comment 1 for bug 1797181

It's the expected behavior of in-used volume migration, and there already was a "_migrate_volume_completion" [1] since [2], you need call it to complete the migrations. The tmp volume will be deleted.

BTW, I test this case in my env, this api is valid in V3.

curl -g -i -X POST http://XXX/volume/v3/4be2d7fa445d4faa94fd5c722b36417f/volumes/6d9a4e2c-8e37-471f-8418-10800989a49d/action -H "Accept: application/json" -H "Content-Type: application/json" -H "User-Agent: python-cinderclient" -H "X-Auth-Token: $TOKEN" -d '{"os-migrate_volume_completion": {"new_volume": "bf5edf8d-aa89-474d-80a7-cfd6d9d51aad"}}'

So, I think what we need to do is :
1. Add this migrate_volume_completion to api-ref
2. Improve the volume migration doc [3]
3. Add something like cinder migration-complete API to cinder client

[1] https://github.com/openstack/cinder/blob/558f7b0/cinder/api/contrib/admin_actions.py#L224
[2] https://review.openstack.org/#/c/41285/
[3] https://docs.openstack.org/cinder/latest/admin/blockstorage-volume-migration.html