Merge "Spec: Use in_tree getting allocation candidates"
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Zuul
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This work is licensed under a Creative Commons Attribution 3.0 Unported
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License.
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http://creativecommons.org/licenses/by/3.0/legalcode
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=========================================
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Use in_tree getting allocation candidates
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=========================================
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https://blueprints.launchpad.net/nova/+spec/use-placement-in-tree
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In Stein, we introduced ``in_tree=<rp_uuid>`` parameter in placement for the
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``GET /allocation_candidates`` endpoints, which limits the response to
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resource providers within the same tree of the specified resource provider.
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(See the `Filter Allocation Candidates by Provider Tree`_ spec for details)
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This spec proposes to use this parameter for optimization when we create or
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move instances and the target host is already picked before asking to the
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scheduler.
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Problem description
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===================
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In create and move instance operations, there are cases where the target host
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is already picked before calling the scheduler. Even in such cases, nova
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retrieves all the possible candidates from placement. This is inefficient and
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can cause, for example, `Bug#1777591`_ filtering out the pre-determined target
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resource provider by `Limiting Allocation Candidates`_ feature in placement.
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Use Cases
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---------
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* Creating an instance to a host specified by operator
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* Migrating an instance to a host specified by operator
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* Live-migrating an instance to a host specified by operator without forcing
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* Evacuating an instance to a host specified by operator without forcing
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* Rebuilding an instance in the same host with a new image.
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Proposed change
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===============
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Instead of issuing the inefficient request to placement, we will use
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``in_tree`` query with the pre-determined target host resource provider
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uuid calling the ``GET /allocation_candidates`` API.
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Alternatives
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------------
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Disable the `Limiting Allocation Candidates`_ feature calling placement.
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This is actually what we have now as workaround, but not efficient.
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(See the `unlimiting allocation candidates`_ patch for details)
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Data model impact
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-----------------
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The ``RequestGroup`` object will have a new field, ``in_tree`` in the new
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version.
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REST API impact
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---------------
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N/A
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Security impact
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---------------
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N/A
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Notifications impact
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--------------------
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N/A
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Other end user impact
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---------------------
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N/A
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Performance Impact
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------------------
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The performance would be improved because we don't need to get all
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the candidates.
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Other deployer impact
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---------------------
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This spec is proposed on the assumption that the placement code in nova
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repository will be removed in Train release and that all the deployers will
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use the extracted placement from Train release. Note that ``in_tree``
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queryparam to placement is **not** supported in placement hosted in nova.
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Developer impact
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----------------
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N/A
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Upgrade impact
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--------------
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We will have a minimum required placement API check in the
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`nova-status upgrade checks` command.
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Implementation
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==============
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Assignee(s)
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-----------
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Primary assignee:
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tetsuro0907
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Work Items
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----------
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* Changes to support the new feature
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* Add ``in_tree`` field in ``RequestGroup`` object implementing
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the translation into the placement query parameter
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* Add a database query in the scheduler to translate
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``RequestSpec.force_hosts/force_nodes`` and
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``RequestSpec.requested_destination`` to the compute node uuid
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and set it to the new ``RequestGroup.in_tree`` field
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* Revert the workaround in `unlimiting allocation candidates`_ patch
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Dependencies
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============
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The `Filter Allocation Candidates by Provider Tree`_ spec, but this has been
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completed in Stein.
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Testing
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=======
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Functional tests will be added to ensure the server operations described
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in the `Use Cases`_ section.
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Documentation Impact
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====================
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N/A
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References
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==========
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* `Filter Allocation Candidates by Provider Tree`_ spec
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* `Limiting Allocation Candidates`_ spec
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* `Bug#1777591`_ reported in the launchpad
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* `unlimiting allocation candidates`_ patch
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.. _`Filter Allocation Candidates by Provider Tree`: https://specs.openstack.org/openstack/nova-specs/specs/stein/implemented/alloc-candidates-in-tree.html
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.. _`Bug#1777591`: https://bugs.launchpad.net/nova/+bug/1777591
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.. _`Limiting Allocation Candidates`: https://specs.openstack.org/openstack/nova-specs/specs/queens/implemented/allocation-candidates-limit.html
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.. _`unlimiting allocation candidates`: https://review.openstack.org/#/c/576693/
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