Nova supports setting allocation ratios at the nova-compute level from
Liberty onwards. Prior to this allocation ratios were set at the
nova-scheduler level.
Newton introduced the Placement API, and Ocata introduced the ability to
have compute resources (Core/RAM/Disk) precomputed before passing
candidates to the FilterScheduler [0]. Pike removed CoreFilter,
RAMFilter and DiskFilter scheduler filters.
From Pike onwards valid methods for settings these allocation ratios are via:
- A call to the Placement API [1].
- Config values to supplied to nova-compute (xxx_allocation_ratio).
Stein introduced initial_xxx_allocation_ratio in response to the runtime
behaviour of the ResourceTracker [2].
Currently, the precedence of resource ratio values are:
xxx_allocation_ratio > Placement API call > initial_xxx_allocation_ratio
That is a (compute) resource provider's allocation ratios will default
to initial_xxx_allocation_ratio which may be overridden at run time by a
call to the Placement API. If xxx_allocation_ratio is set it will
override all configurations for that provider.
When not otherwise configured, we set initial_xxx_allocation_ratio to
the values provided by ncc to maintain backwards compatibility. Where
initial_xxx_allocation_ratio is not available we set
xxx_allocation_ratio.
[0] https://specs.openstack.org/openstack/nova-specs/specs/ocata/implemented/resource-providers-scheduler-db-filters.html
[1] https://docs.openstack.org/api-ref/placement/#update-resource-provider-inventories
[2] https://specs.openstack.org/openstack/nova-specs/specs/stein/implemented/initial-allocation-ratios.html
Change-Id: Ifa314e9e23e0ae5d16113cd91a7507e61f9de704
Closes-Bug: #1677223
43acdd4f80