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