When a node joins the cluster, any shards stored in its local data directory which do not already exist in the cluster will be imported into the cluster. This functionality is intended as a best effort to help users who lose all master nodes. If a new master node is started which is unaware of the other indices in the cluster, adding the old nodes will cause the old indices to be imported, instead of being deleted.
IMPORTANT: No additional bug fixes or documentation updates will be released for this version. For the latest information, see the current release documentation.