Elasticsearch v5.0.0 uses allocation IDs to safely allocate primary shards whereas prior versions of ES used a version-based mode instead. Elasticsearch v5 still has support for version-based primary shard allocation as it needs to be able to load 2.x shards. ES v6 can drop the legacy support.
Store information reports on which nodes shard copies exist, the shard
copy version, indicating how recent they are, and any exceptions
encountered while opening the shard index or from earlier engine failure.
closes#10952
Tanguy Leroux
Yannick Welsch
Dongjoon Hyun
Yannick Welsch
Lee Hinman
Areek Zillur