In this section we cover some options for tuning Ansible for performance and scale.
SSH pipelining is disabled in Ansible by default, but is generally safe to enable, and provides a reasonable performance improvement.
[ssh_connection] pipelining = True
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
By default, Ansible gathers facts for each host at the beginning of every play,
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 Kolla Ansible, it is necessary to change
configuration option to
In the following example we configure Kolla Ansible to use fact caching using the jsonfile cache plugin.
[defaults] gathering = smart fact_caching = jsonfile fact_caching_connection = /tmp/ansible-facts
You may also wish to set the expiration timeout for the cache via
Populating the cache¶
In some situations it may be helpful to populate the fact cache on demand. The
kolla-ansible gather-facts command may be used to do this.
One specific case where this may be helpful is when running
--limit argument, since in that case hosts that match the limit will
gather facts for hosts that fall outside the limit. In the extreme case of a
limit that matches only one host, it will serially gather facts for all other
hosts. To avoid this issue, run
kolla-ansible gather-facts without a limit
to populate the fact cache in parallel before running the required command with
a limit. For example:
kolla-ansible gather-facts kolla-ansible deploy --limit control01
Fact variable injection¶
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
that can be set to
False to prevent this injection of facts. In this case,
facts should be referenced via
ansible_facts.<fact>. In recent releases of
Kolla Ansible, facts are referenced via
ansible_facts, allowing users to
disable fact variable injection.
[defaults] inject_facts_as_vars = False
Ansible facts filtering can be used to speed up Ansible. Environments with
many network interfaces on the network and compute nodes can experience very
slow processing with Kolla Ansible. This happens due to the processing of the
large per-interface facts with each task. To avoid storing certain facts, we
can use the
kolla_ansible_setup_filter variable, which is used as the
filter argument to the
setup module. For example, to avoid collecting
facts for virtual interfaces beginning with q or t:
This causes Ansible to collect but not store facts matching that pattern, which
includes the virtual interface facts. Currently we are not referencing other
facts matching the pattern within Kolla Ansible. Note that including the
ansible_ prefix causes meta facts
be filtered, but this seems to be the only way to get a good match on the
The exact improvement will vary, but has been reported to be as large as 18x on systems with many virtual interfaces.
Fact gathering subsets¶
It is also possible to configure which subsets of facts are gathered, via
kolla_ansible_setup_gather_subset, which is used as the
argument to the
setup module. For example, if one wants to avoid collecting
facts via facter: