[[indices-upgrade]]
== Upgrade

The upgrade API allows to upgrade one or more indices to the latest format
through an API. The upgrade process converts any segments written
with previous formats.

[float]
=== Start an upgrade

[source,sh]
--------------------------------------------------
$ curl -XPOST 'http://localhost:9200/twitter/_upgrade'
--------------------------------------------------

NOTE: Upgrading is an I/O intensive operation, and is limited to processing a
single shard per node at a time.  It also is not allowed to run at the same
time as optimize.

This call will block until the upgrade is complete. If the http connection
is lost, the request will continue in the background, and
any new requests will block until the previous upgrade is complete.

[float]
[[upgrade-parameters]]
==== Request Parameters

The `upgrade` API accepts the following request parameters:

[horizontal]
`only_ancient_segments`:: If true, only very old segments (from a
previous Lucene major release) will be upgraded.  While this will do
the minimal work to ensure the next major release of Elasticsearch can
read the segments, it's dangerous because it can leave other very old
segments in sub-optimal formats.  Defaults to `false`.

[float]
=== Check upgrade status

Use a `GET` request to monitor how much of an index is upgraded.  This
can also be used prior to starting an upgrade to identify which
indices you want to upgrade at the same time.

The `ancient` byte values that are returned indicate total bytes of
segments whose version is extremely old (Lucene major version is
different from the current version), showing how much upgrading is
necessary when you run with `only_ancient_segments=true`.

[source,sh]
--------------------------------------------------
curl 'http://localhost:9200/twitter/_upgrade?pretty&human'
--------------------------------------------------

[source,js]
--------------------------------------------------
{
   "twitter": {
      "size": "21gb",
      "size_in_bytes": "21000000000",
      "size_to_upgrade": "10gb",
      "size_to_upgrade_in_bytes": "10000000000"
      "size_to_upgrade_ancient": "1gb",
      "size_to_upgrade_ancient_in_bytes": "1000000000"
   }
}
--------------------------------------------------