mariadb: Tweak query cache settings

We used to set query_cache_type to 0 in the default settings but we were
also setting query_cache_size to a non-zero/non-default value, which was
in turn re-enabling the query cache. Update the configuration to reflect
the actual cache state and make sure query_cache_size is set to zero for
the "query_cache_type = 0" case.

Now that the setting controls the real state of the query cache, disable
it for bbs.archlinux.org; its hit rate is small compared to insert rate.

playbooks/bbs.archlinux.org.yml

@@ -10,7 +10,7 @@
     - { role: root_ssh }
     - { role: certbot }
     - { role: nginx }
-    - { role: mariadb }
+    - { role: mariadb, mariadb_query_cache_type: '0' }
     - { role: sudo }
     - { role: php_fpm, php_extensions: ['apcu', 'iconv', 'intl', 'mysqli'], zend_extensions: ['opcache'] }
     - { role: fluxbb }

roles/mariadb/defaults/main.yml

@@ -9,7 +9,7 @@ mariadb_net_buffer_length: '8K'
 mariadb_read_buffer_size: '256K'
 mariadb_read_rnd_buffer_size: '512K'
 mariadb_myisam_sort_buffer_size: '8M'
-mariadb_query_cache_type: '0'
+mariadb_query_cache_type: '1'
 mariadb_query_cache_limit: '16M'
 mariadb_query_cache_size: '32M'
 mariadb_max_connections: '400'

roles/mariadb/templates/server.cnf.j2

@@ -23,13 +23,19 @@ net_buffer_length = {{ mariadb_net_buffer_length }}
 read_buffer_size = {{ mariadb_read_buffer_size }}
 read_rnd_buffer_size = {{ mariadb_read_rnd_buffer_size }}
 myisam_sort_buffer_size = {{ mariadb_myisam_sort_buffer_size }}
-query_cache_type = {{ mariadb_query_cache_type }}
-query_cache_limit = {{ mariadb_query_cache_limit }}
-query_cache_size = {{ mariadb_query_cache_size }}
 max_connections = {{ mariadb_max_connections }}
 thread_cache_size = {{ mariadb_thread_cache_size }}
 userstat = 1
 
+# Query cache settings
+query_cache_type = {{ mariadb_query_cache_type }}
+query_cache_limit = {{ mariadb_query_cache_limit }}
+{% if mariadb_query_cache_type == '0' %}
+query_cache_size = 0
+{% else %}
+query_cache_size = {{ mariadb_query_cache_size }}
+{% endif %}
+
 # Single server setup
 server-id       = 1