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[ovn] Document the OVN L3 scheduler: Anti-Affinity 

Document the ``OVNGatewayLeastLoadedScheduler`` behavior when
there are multiple gateway ports.

Partial-Bug: #2002687
Change-Id: I99cf269e35619a2e1fb680d8decbc613267ca62e
Signed-off-by: Frode Nordahl <frode.nordahl@canonical.com>
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Frode Nordahl 2024-02-27 20:36:13 +01:00
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doc/source/admin/ovn/l3_scheduler.rst
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@ -99,9 +99,37 @@ list will belong to another AZ.
This improvement was implemented in [3]_.
Soft Anti-Affinity for ``Logical_Router`` with multiple ``Logical_Router_Port``
-------------------------------------------------------------------------------
Support for multiple gateway ports [4]_ was implemented to support
configurations that provide resiliency and load sharing across multiple router
ports at the layer 3 level.
In addition to external dependencies such as BFD for liveness detection and
ECMP for load sharing accross default routes, the feature required changes to
the scheduler, the goal being that each ``Logical_Router_Port`` record for a
``Logical_Router`` would have a different set of ``Chassis`` for each priority.
The Anti-Affinity is accomplished by having the OVN driver provide the router
object subject to scheduling to the scheduler. The scheduler then checks
whether there already exists ``Logical_Router_Port`` records for the target
router, and makes any ``Chassis`` involed in the already existing ports
appear as having higher load, making it less likely that the already used
``Chassis`` gets picked for a new ``Logical_Router_Port``.
Since the algorithm is based on load and priority, Anti-Affinity is only
supported for the ``OVNGatewayLeastLoadedScheduler``.
This improvement was implemented in [5]_.
References
----------
.. [1] https://review.opendev.org/c/openstack/neutron/+/893653
.. [2] https://review.opendev.org/c/openstack/neutron/+/893654
.. [3] https://review.opendev.org/c/openstack/neutron/+/892604
.. [4] https://review.opendev.org/c/openstack/neutron-specs/+/870030
.. [5] https://review.opendev.org/c/openstack/neutron/+/873699