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+[[reindex-upgrade]]
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+=== Reindex to upgrade
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+
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+Elasticsearch is able to use indices created in the previous major version
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+only. For instance, Elasticsearch 6.x can use indices created in
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+Elasticsearch 5.x, but not those created in Elasticsearch 2.x or before.
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+
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+NOTE: Elasticsearch 6.x nodes will fail to start in the presence of too old indices.
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+
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+If you are running an Elasticsearch 5.x cluster which contains indices that
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+were created before 5.x, you will either need to delete those old indices or
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+to reindex them before upgrading to 6.x. See <<reindex-upgrade-inplace>>.
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+
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+If you are running an Elasticsearch 2.x cluster or older, you have two options:
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+
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+* First upgrade to Elasticsearch 5.x, reindex the old indices, then upgrade
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+ to 6.x. See <<reindex-upgrade-inplace>>.
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+
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+* Create a new 6.x cluster and use reindex-from-remote to import indices
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+ directly from the 2.x cluster. See <<reindex-upgrade-remote>>.
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+
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+[[reindex-upgrade-inplace]]
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+==== Reindex in place
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+
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+If you are running a 5.x cluster which contains indices created in
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+Elasticsearch 2.x, you will need to reindex (or delete) those indices before
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+upgrading to Elasticsearch 6.x.
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+
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+The reindex process works as follows:
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+
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+* Create a new index, copying the mappings and settings from the old index.
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+ Set the `refresh_interval` to `-1` and the `number_of_replicas` to `0` for
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+ efficient reindexing.
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+
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+* Reindex all documents from the old index to the new index using the
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+ <<docs-reindex,reindex API>>.
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+
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+* Reset the `refresh_interval` and `number_of_replicas` to the values
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+ used in the old index, and wait for the index to become green.
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+
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+* In a single <<indices-aliases,update aliases>> request:
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+
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+ * Delete the old index.
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+ * Add an alias with the old index name to the new index.
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+ * Add any aliases that existed on the old index to the new index.
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+
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+At the end of this process, you will have a new 5.x index which can be used
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+by an Elasticsearch 6.x cluster.
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+
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+[[reindex-upgrade-remote]]
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+==== Upgrading with reindex-from-remote
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+
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+If you are running a 1.x or 2.x cluster and would like to migrate directly to 6.x
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+without first migrating to 5.x, you can do so using
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+<<reindex-from-remote,reindex-from-remote>>.
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+
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+[WARNING]
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+=============================================
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+
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+Elasticsearch includes backwards compatibility code that allows indices from
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+the previous major version to be upgraded to the current major version. By
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+moving directly from Elasticsearch 2.x or before to 6.x, you will have to solve any
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+backwards compatibility issues yourself.
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+
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+=============================================
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+
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+You will need to set up a 6.x cluster alongside your existing old cluster.
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+The 6.x cluster needs to have access to the REST API of the old cluster.
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+
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+For each old index that you want to transfer to the 6.x cluster, you will need
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+to:
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+
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+* Create a new index in 6.x with the appropriate mappings and settings. Set
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+ the `refresh_interval` to `-1` and set `number_of_replicas` to `0` for
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+ faster reindexing.
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+
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+* Use <<reindex-from-remote,reindex-from-remote>> to pull documents from the
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+ old index into the new 6.x index.
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+
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+* If you run the reindex job in the background (with `wait_for_completion` set
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+ to `false`), the reindex request will return a `task_id` which can be used to
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+ monitor progress of the reindex job in the <<tasks,task API>>:
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+ `GET _tasks/TASK_ID`.
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+
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+* Once reindex has completed, set the `refresh_interval` and
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+ `number_of_replicas` to the desired values (the defaults are `30s` and `1`
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+ respectively).
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+
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+* Once the new index has finished replication, you can delete the old index.
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+
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+The 6.x cluster can start out small, and you can gradually move nodes from the
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+old cluster to the 6.x cluster as you migrate indices across.
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Clinton Gormley