Improve Mnesia IO performances

With this configuration, the database Mnesia reduces the activity to the
disk. It is useful when a huge of queues/exchanges/bindings are created
and destroyed.
With the default value (100), RabbitMQ could log {Mnesia is overloaded}.
Moved to 300. The range should be between 100 and 1000.
Read [1] for more info.

[1] http://erlang.org/doc/man/mnesia.html#dump_log_write_threshold

Change-Id: I6dcfc9db02bcd8c8f0a1ebf58d9c3ceb84cae10a

defaults/main.yml

@@ -130,6 +130,7 @@ rabbitmq_hipe_compile: False
 # Disable non-TLS listeners
 rabbitmq_disable_non_tls_listeners: False
 
+
 # RabbitMQ policies
 # Used to tune performance characteristics of OpenStack messaging
 #
@@ -166,3 +167,13 @@ rabbitmq_port_bindings:
     "0.0.0.0": 5671
   tcp_listeners:
     "0.0.0.0": 5672
+
+# Mnesia configuration
+# The Mnesia dump_log_write_threshold option controls
+# how often the dumping occurs
+# Increase this value can increase the performances,
+# reducing the IO.
+# Increase it in case of:
+# Mnesia is overloaded: {dump_log,write_threshold}.
+# The default value is 100
+mnesia_dump_log_write_threshold: 300

releasenotes/notes/rabbitmq-server-mnesia-io-d9067e226c7cb19c.yaml

@@ -0,0 +1,12 @@
+---
+upgrade:
+  - |
+    The default Mnesia ``dump_log_write_threshold`` value
+    has changed to ``300`` instead of ``100`` for efficiency.
+    ``dump_log_write_threshold`` specifies the maximum number of writes
+    allowed to the transaction log before a new dump of the log is performed.
+    Increasing this value can increase the performances
+    during the queues/exchanges/bindings creation/destroying.
+    The values should be between 100 and 1000. More detail [1].
+
+    [1] http://erlang.org/doc/man/mnesia.html#dump_log_write_threshold

templates/rabbitmq.config.j2

@@ -40,5 +40,6 @@
   { rabbitmq_management, [
       { rates_mode, {{ rabbitmq_management_rates_mode }} }
     ]
-  }
+  },
+  {mnesia, [{dump_log_write_threshold, {{ mnesia_dump_log_write_threshold }} }]}
 ].