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Add nova-cc ssh changes to 1907 release notes 

Change-Id: Iaf249e603c79d28913dee1ba9860ae72439e033d
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Alex Kavanagh 2019-07-24 17:19:46 +01:00
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@ -89,6 +89,47 @@ Feature 1
Feature description...
nova-cloud-controller: improved SSH host key handling
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The Nova compute service uses direct (machine-to-machine) SSH connections to
perform instance migrations. Each compute host must therefore be in possession
of every other compute host's SSH host key via the knownhosts file.
This release of the nova-cloud-controller charm has improved the host key
discovery and distribution algorithm, the net effect being that the addition of
a nova-compute unit will be faster than before and the nova-cloud-controller
upgrade-charm hook will be significantly improved for large deployments.
The rest of this section covers an EXPERIMENTAL option involving the caching of
SSH host lookups (knownhosts) on each nova-compute unit.
There is a new Boolean configuration key ``cache-known-hosts`` that allows any
given host lookup to be performed just once.
.. note:: A cloud can be deployed with the ``cache-known-hosts`` key set to ``false``,
and be set to ``true`` post-deployment. At that point the hosts will have
been cached. The key only controls *whether* the cache is used or not.
If the above key is set, a new Juju action ``clear-unit-knownhost-cache`` is provided
to clear the cache. This can be applied to a unit, service, or an entire
nova-cloud-controller application. This would be needed if DNS resolution had
changed in an existing cloud or during a cloud deployment. Not clearing the
cache in such cases would result in an inconsistent set of knownhosts files.
This action will cause DNS resolution to be performed (for unit/service/application),
thus potentially triggering a relation-set on the nova-cloud-controller unit(s) and
subsequent changed hook on the related nova-compute units.
The action is used as follows, based on unit, service, or application, respectively:
juju run-action nova-cloud-controller/0 clear-unit-knownhost-cache target=nova-compute/2
juju run-action nova-cloud-controller/0 clear-unit-knownhost-cache target=nova-compute
juju run-action nova-cloud-controller/0 clear-unit-knownhost-cache
In a high-availability setup, the action **must** be run on all
nova-cloud-controller units.
Preview Charm Features
======================