Propose to move prep_resize to Conductor

scheduler prep_resize() is only necessary when rescheduling a cold
migration, but all of this can be done in conductor.

Implements: blueprint move-prep-resize-to-conductor

Change-Id: I4143aa011e18edcc609e1095f0054c3a8670bc86

specs/juno/move-prep-resize-to-conductor.rst

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+..
+ This work is licensed under a Creative Commons Attribution 3.0 Unported
+ License.
+
+ http://creativecommons.org/licenses/by/3.0/legalcode
+
+===============================
+ Move prep_resize to Conductor
+===============================
+
+https://blueprints.launchpad.net/nova/+spec/move-prep-resize-to-conductor
+
+So as to prepare the scheduler to be a separate project, we need to remove
+all proxy calls from the scheduler to compute nodes.
+prep_resize() is still in Scheduler V3 API, so we need to modify how cold
+migrations are retried.
+
+Problem description
+===================
+
+When a cold migration is requested, there is a direct call from conductor to
+compute.prep_resize() which is OK. The problem is when the cold migration is
+failing, where compute node is asking Scheduler to reschedule a new migration
+by calling scheduler.prep_resize(), which itself calls compute.prep_resize()
+after issuing a select_destinations().
+
+Proposed change
+===============
+
+The idea is to replace the call back by conductor.migrate_server instead of
+scheduler.prep_resize in the compute prep_resize method.
+
+
+Alternatives
+------------
+
+All prep_resize logic should be left to the conductor, but that's a bigger step
+than just moving the scheduler logic to conductor. With regards to small
+iterations, that blueprint is quicker to implement and less risky, so that
+another blueprint for placing cold and hot migrations to conductor [1] could
+use it as dependency.
+
+
+Data model impact
+-----------------
+
+None
+
+REST API impact
+---------------
+
+None
+
+Security impact
+---------------
+
+None
+
+Notifications impact
+--------------------
+
+None
+
+
+Other end user impact
+---------------------
+
+None
+
+Performance Impact
+------------------
+
+None
+
+Other deployer impact
+---------------------
+
+None
+
+Developer impact
+----------------
+
+None
+
+Implementation
+==============
+
+Assignee(s)
+-----------
+
+Primary assignee:
+  sylvain-bauza
+
+Other contributors:
+  None
+
+Work Items
+----------
+
+- Replace call to scheduler.prep_resize by call to conductor.migrate_server
+  in compute.prep_resize
+- Remove prep_resize in Scheduler RPC API and note it to be removed in manager
+
+Dependencies
+============
+
+None
+
+
+Testing
+=======
+
+Covered by existing tempest tests and CIs.
+
+
+Documentation Impact
+====================
+
+None
+
+
+References
+==========
+
+* [1] https://blueprints.launchpad.net/nova/+spec/cold-migrations-to-conductor-final