Add mirroring_sync_batch_size to example config

Author: gerhard

docs/rabbitmq.config.example

@@ -299,6 +299,24 @@
    %% See https://www.rabbitmq.com/ha.html#queue-master-location for further details
    %% {queue_master_locator, <<"client-local">>},
 
+   %% Since RabbitMQ 3.6.0, masters perform synchronisation in batches.
+   %% Earlier versions will synchronise 1 message at a time by default.
+   %% By synchronising messages in batches, the synchronisation process can be sped up considerably.
+   %% To choose the right value, you need to consider:
+   %%   * average message size
+   %%   * network throughput between RabbitMQ nodes
+   %%   * net_ticktime value
+   %%
+   %% For example, if you set this value to 50000 (messages),
+   %% and each message in the queue is 1KB,
+   %% then each synchronisation message between nodes will be ~49MB.
+   %%
+   %% You need to make sure that your network between queue mirrors can accomodate this kind of traffic.
+   %%
+   %% If the network takes longer than net_ticktime to send one batch of messages,
+   %% then nodes in the cluster could think they are in the presence of a network partition.
+   %% {mirroring_sync_batch_size, 4096},
+
    %% To announce custom properties to clients on connection:
    %%
    %% {server_properties, []},