Before upgrading Elasticsearch:

Indices created in Elasticsearch 1.x or before

Elasticsearch is able to read indices created in the previous major version only. For instance, Elasticsearch 5.x can use indices created in Elasticsearch 2.x, but not those created in Elasticsearch 1.x or before.

This condition also applies to indices backed up with snapshot and restore. If an index was originally created in 1.x, it cannot be restored into a 5.x cluster even if the snapshot was made by a 2.x cluster.

Elasticsearch 5.x nodes will fail to start in the presence of too old indices.

See Reindex to upgrade for more information about how to upgrade old indices.

！！！回滚升级就是一次升级一个节点！！！！

 A rolling upgrade allows the Elasticsearch cluster to be upgraded one node at a time, with no downtime for end users. Running multiple versions of Elasticsearch in the same cluster for any length of time beyond that required for an upgrade is not supported, as shards will not be replicated from the more recent version to the older version.

Elasticsearch is able to use indices created in the previous major version only. For instance, Elasticsearch 5.x can use indices created in Elasticsearch 2.x, but not those created in Elasticsearch 1.x or before.

Elasticsearch 5.x nodes will fail to start in the presence of too old indices.

If you are running an Elasticsearch 2.x cluster which contains indices that were created before 2.x, you will either need to delete those old indices or to reindex them before upgrading to 5.x. See Reindex in place.

If you are running an Elasticsearch 1.x cluster, you have two options:

转自：https://www.elastic.co/guide/en/elasticsearch/reference/current/setup-upgrade.html#setup-upgrade

本文转自张昺华-sky博客园博客，原文链接：http://www.cnblogs.com/bonelee/p/7443844.html，如需转载请自行联系原作者