Add a caveat section about cellsv2 upcalls

This adds another subsection to the caveat section about operations that
require upcalls in Pike not being supported.

Change-Id: If95be4f631f929cd8c6528671433ae0fc747a6be
changes/12/490612/5
Dan Smith 5 years ago committed by Matt Riedemann
parent ea669d3488
commit 916ba166c8
  1. 32
      doc/source/user/cellsv2_layout.rst

@ -291,3 +291,35 @@ Neutron Metadata API proxy
The Neutron metadata API proxy should be global across all cells, and
thus be configured as an API-level service with access to the
``[api_database]/connection`` information.
Operations Requiring upcalls
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If you deploy multiple cells with a superconductor as described above,
computes and cell-based conductors will not have the ability to speak
to the scheduler as they are not connected to the same MQ. This is by
design for isolation, but currently the processes are not in place to
implement some features without such connectivity. Thus, anything that
requires a so-called "upcall" will not function. This impacts the
following:
- Instance reschedules during boot
- Instance affinity reporting from the compute nodes to scheduler
- The late anti-affinity check
The first is simple: if you boot an instance, it gets scheduled to a
compute node, fails, it would normally be re-scheduled to another
node. That requires scheduler intervention and thus it will not work
in Pike with a multi-cell layout. If you do not rely on reschedules
for covering up transient compute-node failures, then this will not
affect you. To ensure you do not make futile attempts at rescheduling,
you should set ``[scheduler]/max_attempts=1`` in ``nova.conf``.
The second two are related. The summary is that some of the facilities
that Nova has for ensuring that affinty/anti-affinity is preserved
between instances does not function in Pike with a multi-cell
layout. If you don't use affinity operations, then this will not
affect you. To make sure you don't make futile attempts at the
affinity check, you should set
``[workarounds]/disable_group_policy_check_upcall=True`` and
``[filter_scheduler]/track_instance_changes=False`` in ``nova.conf``.

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