47 lines
2.5 KiB
YAML
47 lines
2.5 KiB
YAML
---
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upgrade:
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- |
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The ``caching_scheduler`` scheduler driver, which was deprecated in the
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16.0.0 Pike release, has now been removed. Unlike the default
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``filter_scheduler`` scheduler driver which creates resource allocations
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in the placement service during scheduling, the ``caching_scheduler``
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driver did not interface with the placement service. As more and more
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functionality within nova relies on managing (sometimes complex) resource
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allocations in the placement service, compatibility with the
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``caching_scheduler`` driver is difficult to maintain, and seldom tested.
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The original reasons behind the need for the CachingScheduler should now
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be resolved with the FilterScheduler and the placement service, notably:
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* resource claims (allocations) are made atomically during scheduling to
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alleviate the potential for racing to concurrently build servers on the
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same compute host which could lead to failures
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* because of the atomic allocation claims made during scheduling by the
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``filter_scheduler`` driver, it is safe [1]_ to run multiple scheduler
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workers and scale horizontally
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.. [1] There are still known race issues with concurrently building some
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types of resources and workloads, such as anything that requires
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PCI/NUMA or (anti-)affinity groups. However, those races also existed
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with the ``caching_scheduler`` driver.
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To migrate from the CachingScheduler to the FilterScheduler, operators can
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leverage the ``nova-manage placement heal_allocations`` command:
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https://docs.openstack.org/nova/latest/cli/nova-manage.html#placement
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Finally, it is still technically possible to load an out-of-tree scheduler
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driver using the ``nova.scheduler.driver`` entry-point. However,
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out-of-tree driver interfaces are not guaranteed to be stable:
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https://docs.openstack.org/nova/latest/contributor/policies.html#out-of-tree-support
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And as noted above, as more of the code base evolves to rely on resource
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allocations being tracked in the placement service (created during
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scheduling), out-of-tree scheduler driver support may be severely impacted.
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If you rely on the ``caching_scheduler`` driver or your own out-of-tree
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driver which sets ``USES_ALLOCATION_CANDIDATES = False`` to bypass the
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placement service, please communicate with the nova development team in
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the openstack-dev mailing list and/or #openstack-nova OFTC IRC channel
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to determine what prevents you from using the ``filter_scheduler`` driver.
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