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charm-nova-cloud-controller
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Add SSH knownhosts caching info to README 

Change-Id: I062acd8c403a6805a1b33fad5b45accb1270aa0e
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Alex Kavanagh 2019-08-02 16:41:37 +01:00
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@ -87,3 +87,41 @@ quota-injected_file_path_length - 255
quota-key_pairs - 100
quota-server_groups - 10 (only available after Icehouse)
quota-server_group_members - 10 (only available after Icehouse)
## EXPERIMENTAL: SSH knownhosts caching
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 ensures
that any given host lookup to be performed just once. The default is
`false` which effectively means that caching is not performed.
**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 could 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.