openstack/nova
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doc: provide more details on scheduling with placement

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This provides a more detailed, but still high level, explanation
of how the FilterScheduler is using allocation candidates to pick
hosts during scheduling, and how the allocations are handled during
a move operation, including a resize to the same host.

Ideally this content will live somewhere else in the devref, or
would be updated in the spec for blueprint placement-claims, but
for now this should suffice for Pike release notes that can point
at this upgrade doc.

Change-Id: I274c684f829bc310ebb17faabf498d36f4ceea0c
This commit is contained in:
Matt Riedemann
2017-08-08 15:24:35 -04:00
parent 3f12c8badd
commit 204e7d7f42
1 changed files with 27 additions and 5 deletions
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32
doc/source/user/placement.rst
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@@ -195,11 +195,33 @@ Pike (16.0.0)
* The ``nova.scheduler.filter_scheduler.FilterScheduler`` in Pike will * The ``nova.scheduler.filter_scheduler.FilterScheduler`` in Pike will
no longer fall back to not using the Placement Service, even if older no longer fall back to not using the Placement Service, even if older
computes are running in the deployment. computes are running in the deployment.
* The scheduler now requests allocation candidates from the Placement service * The FilterScheduler now requests allocation candidates from the Placement
during scheduling. The allocation candidates information was introduced in service during scheduling. The allocation candidates information was
the Placement API 1.10 microversion, so you should upgrade the placement introduced in the Placement API 1.10 microversion, so you should upgrade the
service **before** the Nova scheduler service so that the scheduler can take placement service **before** the Nova scheduler service so that the scheduler
advantage of the allocation candidate information. can take advantage of the allocation candidate information.
The scheduler gets the allocation candidates from the placement API and
uses those to get the compute nodes, which come from the cell(s). The
compute nodes are passed through the enabled scheduler filters and weighers.
The scheduler then iterates over this filtered and weighed list of hosts and
attempts to claim resources in the placement API for each instance in the
request. Claiming resources involves finding an allocation candidate that
contains an allocation against the selected host's UUID and asking the
placement API to allocate the requested instance resources. We continue
performing this claim request until success or we run out of allocation
candidates, resulting in a NoValidHost error.
For a move operation, such as migration, allocations are made in Placement
against both the source and destination compute node. Once the
move operation is complete, the resource tracker in the *nova-compute*
service will adjust the allocations in Placement appropriately.
For a resize to the same host, allocations are summed on the single compute
node. This could pose a problem if the compute node has limited capacity.
Since resizing to the same host is disabled by default, and generally only
used in testing, this is mentioned for completeness but should not be a
concern for production deployments.
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