http://www.elasticsearch.org/guide/en/elasticsearch/guide/current/_important_configuration_changes.html#_minimum_master_nodes
The minimum_master_nodes
setting is extremely important to the
stability of your cluster. This setting helps prevent split brains, the existence of two masters in a single cluster.
When you have a split brain, your cluster is at danger of losing data. Because the master is considered the supreme ruler of the cluster, it decides when new indices can be created, how shards are moved, and so forth. If you have two masters, data integrity becomes perilous, since you have two nodes