Merge "Support reducing scope of delegated fact gathering"

ansible/gather-facts.yml

@@ -44,7 +44,7 @@
     batch_size: "{{ ansible_play_batch | length }}"
     # Use a python list slice to divide the group up.
     # Syntax: [<start index>:<end index>:<step size>]
-    delegate_hosts: "{{ groups['all'][batch_index | int::batch_size | int] }}"
+    delegate_hosts: "{{ kolla_ansible_delegate_facts_hosts[batch_index | int::batch_size | int] }}"
   tasks:
     - name: Gather facts
       setup:

ansible/group_vars/all.yml

@@ -36,6 +36,12 @@ kolla_ansible_setup_filter: "{{ omit }}"
 # By default, we do not provide a gather subset.
 kolla_ansible_setup_gather_subset: "{{ omit }}"
 
+# This variable determines which hosts require facts when using --limit. Facts
+# will be gathered using delegation for hosts in this list that are not
+# included in the limit.
+# By default, this list includes all hosts.
+kolla_ansible_delegate_facts_hosts: "{{ groups['all'] }}"
+
 ###################
 # Kolla options
 ###################

doc/source/user/ansible-tuning.rst

@@ -160,3 +160,44 @@ A max fail percentage may be set for specific services using
 
    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
+<https://docs.ansible.com/ansible/latest/playbook_guide/playbooks_delegation.html#delegating-facts>`__.
+
+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.

etc/kolla/globals.yml

@@ -5,6 +5,9 @@
 # commented parameters are shown here, To override the default value uncomment
 # the parameter and change its value.
 
+# Dummy variable to allow Ansible to accept this file.
+workaround_ansible_issue_8743: yes
+
 ###################
 # Ansible options
 ###################
@@ -21,15 +24,18 @@
 # By default, we do not provide a gather subset.
 #kolla_ansible_setup_gather_subset: "{{ omit }}"
 
-# Dummy variable to allow Ansible to accept this file.
-workaround_ansible_issue_8743: yes
-
 # This variable may be used to set the maximum failure percentage for all
 # plays. More fine-grained control is possible via per-service variables, e.g.
 # nova_max_fail_percentage. The default behaviour is to set a max fail
 # percentage of 100, which is equivalent to not setting it.
 #kolla_max_fail_percentage:
 
+# This variable determines which hosts require facts when using --limit. Facts
+# will be gathered using delegation for hosts in this list that are not
+# included in the limit.
+# By default, this list includes all hosts.
+#kolla_ansible_delegate_facts_hosts: "{{ groups['all'] }}"
+
 ###############
 # Kolla options
 ###############

releasenotes/notes/delegate-facts-hosts-a3c8bd588c805ffa.yaml

@@ -0,0 +1,6 @@
+---
+features:
+  - |
+    Adds a new variable, ``kolla_ansible_delegate_facts_hosts``, that may be
+    used to control which hosts require facts when using ``--limit``. Its
+    default value is ``groups['all']``.