One way to improve the performance of Ansible is through fact caching.
Rather than gather facts in every play, we can configure Ansible to
cache them in a persistent store. An example Ansible configuration for
doing this is as follows:
[defaults]
gathering = smart
fact_caching = jsonfile
fact_caching_connection = ./facts
fact_caching_timeout = 86400
This does not affect Kolla Ansible however, since we use the setup
module which unconditionally gathers facts regardless of the state of
the cache. This gets worse with large inventories limited to a small
batch of hosts via --limit or serial, since the limited hosts must
gather facts for all others.
One way to detect whether facts exist for a host is via the
'module_setup' variable, which exists only when facts exist. This change
uses the 'module_setup' fact to determine whether facts need to be
gathered for hosts outside of the batch. For hosts in the batch, we
switch from using the setup module to gather_facts on the play, which
can use the 'smart' gathering logic.
Change-Id: I04841fb62b2e1d9e97ce4b75ce3a7349b9c74036
Partially-Implements: blueprint performance-improvements
Prior to this change, when the --limit argument is used, each host in the
limit gathers facts for every other host. This is clearly unnecessary, and
can result in up to (N-1)^2 fact gathers.
This change gathers facts for each host only once. Hosts that are not in
the limit are divided between those that are in the limit, and facts are
gathered via delegation.
This change also factors out the fact gathering logic into a separate
playbook that is imported where necessary.
Change-Id: I923df5af41a7f1b7b0142d0da185a9a0979be543
