Merge "Support member_of param for 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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Support filtering by aggregate membership to allocation candidates
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==================================================================
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https://blueprints.launchpad.net/nova/+spec/alloc-candidates-member-of
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Provide support for filtering allocation candidates by the underlying resource
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provider's membership in one or more aggregates.
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Problem description
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===================
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The list of resource providers that the placement API's ``GET
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/allocation_candidates`` returns can be very large, particularly when many
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compute hosts are empty. Sometimes nova may have information that would allow
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the number of compute hosts to be dramatically reduced. For instance, if nova
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knows that a particular project is "pinned" to a host aggregate, currently nova
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asks placement for all the resource providers that meet the resource
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requirements of the flavor and then promptly discards any compute hosts that
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are not in that particular host aggregate (in the aggregate multi-tenancy
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isolation filter).
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This process could be much more efficient if the nova scheduler were to simply
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ask placement to only return compute hosts that are associated with a nova host
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aggregate.
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Use Cases
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---------
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Simple post-processing scheduler filters like the aggregate multi-tenancy
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isolation filter can be replaced with more efficient placement-side filtering.
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Proposed change
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===============
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The existing ``GET /resource_providers`` placement REST API call supports a
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``member_of`` query `parameter_`. This parameter is "a string representing an
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aggregate uuid; or the prefix in: followed by a comma-separated list of strings
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representing aggregate uuids. The returned resource providers must be
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associated with at least one of the aggregates identified by uuid."
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We propose to support this exact same parameter for the ``GET
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/allocation_candidates`` placement REST API call.
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.. _parameter: https://developer.openstack.org/api-ref/placement/#list-resource-providers
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Alternatives
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------------
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We can continue to do post-processing of compute hosts by looking at host
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aggregate relationships in the scheduler filters. As noted, however, this is
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inefficient.
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Data model impact
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-----------------
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None.
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REST API impact
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---------------
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Add the ``member_of`` parameter to the ``GET /allocation_candidates`` REST API
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call. Make the behavior and specification identical to the same-named parameter
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for the ``GET /resource_providers`` REST API call. A new microversion will be
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used to indicate to clients that the new parameter is available.
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Security impact
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---------------
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None.
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Notifications impact
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--------------------
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None.
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Other end user impact
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---------------------
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None.
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Performance Impact
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------------------
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Expected increase in overall scheduler performance for use cases where the
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scheduler can limit the number of compute hosts it operates on.
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Other deployer impact
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---------------------
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We should be able to deprecate the aggregate multi-tenancy isolation scheduler
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filter after the "S" release.
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Developer impact
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----------------
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None.
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Upgrade impact
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--------------
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None.
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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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jaypipes
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Other contributors:
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cdent
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Work Items
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----------
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* Add support to the ``nova.objects.AllocationCandidates.get_by_requests()``
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method for the ``member_of`` filter. This will require changes to the
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``RequestGroup`` object as well
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* Add new microversion to the placement REST API to support the ``member_of``
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query parameter
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Dependencies
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============
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In order for this functionality to be useful, nova host aggregates should be
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"mirrored" into the placement service. Currently, nova host aggregates are not
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yet showing up automatically in the placement service. A separate `blueprint_`
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for this will be a soft dependency for this work.
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.. blueprint_: https://blueprints.launchpad.net/nova/+spec/placement-mirror-host-aggregates
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Testing
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=======
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Normal functional and unit testing.
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Documentation Impact
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====================
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Document the REST API microversion in the appropriate reference docs.
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References
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==========
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placement-req-filter blueprint (use case): https://review.openstack.org/544585
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