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Mark Goddard f639ad0b35 Use ansible_facts to reference facts

By default, Ansible injects a variable for every fact, prefixed with
ansible_. This can result in a large number of variables for each host,
which at scale can incur a performance penalty. Ansible provides a
configuration option [0] that can be set to False to prevent this
injection of facts. In this case, facts should be referenced via
ansible_facts.<fact>.

This change updates all references to Ansible facts within Kayobe
from using individual fact variables to using the items in the
ansible_facts dictionary. This allows users to disable fact variable
injection in their Ansible configuration, which may provide some
performance improvement.

This change disables fact variable injection in the ansible
configuration used in CI, to catch any attempts to use the injected
variables.

[0] https://docs.ansible.com/ansible/latest/reference_appendices/config.html#inject-facts-as-vars

Story: 2007993
Task: 42464
Depends-On: https://review.opendev.org/c/openstack/kolla-ansible/+/791276

Change-Id: I14db53ed6e57d37bbd28dd5819e432e3fe6628b2

2021-08-21 09:57:29 +02:00

2.7 KiB

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Ansible

Ansible configuration is described in detail in the Ansible documentation. It is explained elsewhere in this guide how to configure Ansible for Kayobe <configuration-kayobe-ansible> and Kolla Ansible <configuration-kolla-ansible-ansible>.

In this section we cover some options for tuning Ansible for performance and scale.

SSH pipelining

SSH pipelining is disabled in Ansible by default, but is generally safe to enable, and provides a reasonable performance improvement.

[ssh_connection]
pipelining = True

Forks

By default Ansible executes tasks using a fairly conservative 5 process forks. This limits the parallelism that allows Ansible to scale. Most Ansible control hosts will be able to handle far more forks than this. You will need to experiment to find out the CPU, memory and IO limits of your machine.

For example, to increase the number of forks to 20:

[defaults]
forks = 20

Fact caching

Note

Fact caching will not work correctly in Kayobe prior to the Ussuri release.

By default, Ansible gathers facts for each host at the beginning of every play, unless gather_facts is set to false. With a large number of hosts this can result in a significant amount of time spent gathering facts.

One way to improve this is through Ansible's support for fact caching. In order to make this work with Kayobe, it is necessary to change Ansible's gathering configuration option to smart. Additionally, it is necessary to use separate fact caches for Kayobe and Kolla Ansible due to some of the facts (e.g. ansible_facts.user_uid and ansible_facts.python) differing.

Example

In the following example we configure Kayobe and Kolla Ansible to use fact caching using the jsonfile cache plugin.

[defaults]
gathering = smart
fact_caching = jsonfile
fact_caching_connection = /tmp/kayobe-facts

[defaults]
gathering = smart
fact_caching = jsonfile
fact_caching_connection = /tmp/kolla-ansible-facts

You may also wish to set the expiration timeout for the cache via [defaults] fact_caching_timeout.

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