============== Ansible tuning ============== 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. .. code-block:: ini :caption: ``ansible.cfg`` [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: .. code-block:: ini :caption: ``ansible.cfg`` [defaults] forks = 20 Fact caching ------------ 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 Kolla Ansible, it is necessary to change Ansible's `gathering `__ configuration option to ``smart``. Example ~~~~~~~ In the following example we configure Kolla Ansible to use fact caching using the `jsonfile cache plugin `__. .. code-block:: ini :caption: ``ansible.cfg`` [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 ``[defaults] fact_caching_timeout``. 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 ``kolla-ansible`` with a ``--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: .. code-block:: console 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 `configuration option `__ that can be set to ``False`` to prevent this injection of facts. In this case, facts should be referenced via ``ansible_facts.``. In recent releases of Kolla Ansible, facts are referenced via ``ansible_facts``, allowing users to disable fact variable injection. .. code-block:: ini :caption: ``ansible.cfg`` [defaults] inject_facts_as_vars = False Fact filtering -------------- 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: .. code-block:: yaml kolla_ansible_setup_filter: "ansible_[!qt]*" 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 ``module_setup`` and ``gather_subset`` to be filtered, but this seems to be the only way to get a good match on the interface facts. 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 ``gather_subset`` argument to the ``setup`` module. For example, if one wants to avoid collecting facts via facter: .. code-block:: yaml kolla_ansible_setup_gather_subset: "all,!facter" Max failure percentage ---------------------- It is possible to specify a `maximum failure percentage `__ using ``kolla_max_fail_percentage``. By default this is undefined, which is equivalent to a value of 100, meaning that Ansible will continue execution until all hosts have failed or completed. For example: .. code-block:: yaml kolla_max_fail_percentage: 50 A max fail percentage may be set for specific services using ``_max_fail_percentage``. For example: .. code-block:: yaml kolla_max_fail_percentage: 50 nova_max_fail_percentage: 25 Delegated fact gathering ------------------------ When Kolla Ansible is executed with a ``--limit`` argument, the scope of an operation is limited to the hosts in the limit. For example: .. code-block:: console kolla-ansible deploy --limit control Due to the nature of configuring clustered software services, there are cases where we need to know information about other hosts. Most often this is related to their hostname or network addresses. To make this work, Kolla Ansible gathers facts for hosts outside of the limit using `delegated fact gathering `__. By default, Kolla Ansible gathers facts for all hosts. Because delegated facts are gathered serially in batches by the active hosts, this can take a long time when there are not many hosts in the limit. If you know that facts are not required for all hosts, it is possible to reduce the set of hosts eligible for delegated fact gathering by setting ``kolla_ansible_delegate_facts_hosts`` to a list of hosts. This may be done permanently in ``globals.yml`` or temporarily for the duration of a command using the ``-e`` argument. The exact requirements will depend upon configuration and inventory, but here are some rules of thumb: * Facts are typically required for all controllers, regardless of which hosts are in the limit. This is due to references to RabbitMQ and Memcache connection strings etc. * Prometheus server requires facts for all other hosts to generate scrape configs for node exporter, cAdvisor, etc. Specifically it uses the IP address of the API interface. This may be avoided by hard-coding ``prometheus_target_address`` in the inventory for each host. * Configuration of ``/etc/hosts`` during the ``bootstrap-servers`` command requires facts for all other hosts. Specifically it uses the IP address of the API interface, and the ``hostname`` and ``nodename`` facts. * Noting the above exceptions, compute nodes are fairly independent. Other hosts do not need to know their facts, and they do not need to know other hosts' facts.