[[indices-upgrade]]
== Upgrade

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

.When to use the `upgrade` API
**************************************************

Newer versions of Lucene often come with a new index format which provides bug
fixes and performance improvements. In order to take advantage of these
improvements, the segments in each shard need to be rewritten using the latest
Lucene format.

.Automatic upgrading

Indices that are actively being written to will automatically write new
segments in the latest format.  The background merge process which combines
multiple small segments into a single bigger segment will also write the new
merged segment in the latest format.

.Optional manual upgrades

Some old segments may never be merged away because they are already too big to
be worth merging, and indices that no longer receive changes will not be
upgraded automatically.  Upgrading segments is not required for most
Elasticsearch upgrades because it can read older formats from the current and
previous major version of Lucene.

You can, however, choose to upgrade old segments manually to take advantage of
the latest format. The `upgrade` API will rewrite any old segments in the
latest Lucene format.  It can be run on one index, multiple or all indices, so
you can control when it is run and how many indices it should upgrade.

.When you must use the `upgrade` API

Elasticsearch can only read formats from the current and previous major
version of Lucene.  For instance, Elasticsearch 2.x (Lucene 5) can read disk
formats from Elasticsearch 0.90 and 1.x (Lucene 4), but not from Elasticsearch
0.20 and before (Lucene 3).

In fact, an Elasticsearch 2.0 cluster will refuse to start if any indices
created before Elasticsearch 0.90 are present, and it will refuse to open them
if they are imported as dangling indices later on.  It will not be possible to
restore an index created with Elasticsearch 0.20.x and before into a 2.0
cluster.

These ancient indices must either be deleted or upgraded before migrating to
Elasticsearch 2.0.  Upgrading will:

* Rewrite old segments in the latest Lucene format.
* Add the `index.version.minimum_compatible` setting to the index, to mark it as
  2.0 compatible added[1.6.0].

Instead of upgrading all segments that weren't written with the most recent
version of Lucene, you can choose to do the minimum work required before
moving to Elasticsearch 2.0, by specifying the `only_ancient_segments` option,
which will only rewrite segments written by Lucene 3.

**************************************************

[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]
--------------------------------------------------
{
  "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"
  "indices": {
    "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"
    }
  }
}
--------------------------------------------------

The level of details in the upgrade status command can be controlled by
setting `level` parameter to `cluster`, `index` (default) or `shard` levels.
For example, you can run the upgrade status command with `level=shard` to
get detailed upgrade information of each individual shard.