[[indices-shrink-index]]
== Shrink Index

The shrink index API allows you to shrink an existing index into a new index
with fewer primary shards. The number of primary shards in the target index
must be a factor of the shards in the source index. For example an index with
`8` primary shards can be shrunk into `4`, `2` or `1` primary shards or an index
with `15` primary shards can be shrunk into `5`, `3` or `1`. If the number
of shards in the index is a prime number it can only be shrunk into a single
primary shard. Before shrinking, a (primary or replica) copy of every shard
in the index must be present on the same node.

Shrinking works as follows:

* First, it creates a new target index with the same definition as the source
  index, but with a smaller number of primary shards.

* Then it hard-links segments from the source index into the target index. (If
  the file system doesn't support hard-linking, then all segments are copied
  into the new index, which is a much more time consuming process.)

* Finally, it recovers the target index as though it were a closed index which
  had just been re-opened.

[float]
=== Preparing an index for shrinking

In order to shrink an index, the index must be marked as read-only, and a
(primary or replica) copy of every shard in the index must be relocated to the
same node and have <<cluster-health,health>> `green`.

These two conditions can be achieved with the following request:

[source,js]
--------------------------------------------------
PUT /my_source_index/_settings
{
  "settings": {
    "index.routing.allocation.require._name": "shrink_node_name", <1>
    "index.blocks.write": true <2>
  }
}
--------------------------------------------------
<1> Forces the relocation of a copy of each shard to the node with name
    `shrink_node_name`.  See <<shard-allocation-filtering>> for more options.

<2> Prevents write operations to this index while still allowing metadata
    changes like deleting the index.

It can take a while to relocate the source index.  Progress can be tracked
with the <<cat-recovery,`_cat recovery` API>>, or the <<cluster-health,
`cluster health` API>> can be used to wait until all shards have relocated
with the `wait_for_relocating_shards` parameter.

[float]
=== Shrinking an index

To shrink `my_source_index` into a new index called `my_target_index`, issue
the following request:

[source,js]
--------------------------------------------------
POST my_source_index/_shrink/my_target_index
--------------------------------------------------

The above request returns immediately once the target index has been added to
the cluster state -- it doesn't wait for the shrink operation to start.

[IMPORTANT]
=====================================

Indices can only be shrunk if they satisfy the following requirements:

* the target index must not exist

* The index must have more primary shards than the target index.

* The number of primary shards in the target index must be a factor of the
  number of primary shards in the source index. The source index must have
  more primary shards than the target index.

* The index must not contain more than `2,147,483,519` documents in total
  across all shards that will be shrunk into a single shard on the target index
  as this is the maximum number of docs that can fit into a single shard.

* The node handling the shrink process must have sufficient free disk space to
  accommodate a second copy of the existing index.

=====================================

The `_shrink` API is similar to the <<indices-create-index, `create index` API>>
and accepts `settings` and `aliases` parameters for the target index:

[source,js]
--------------------------------------------------
POST my_source_index/_shrink/my_target_index
{
  "settings": {
    "index.number_of_replicas": 1,
    "index.number_of_shards": 1, <1>
    "index.codec": "best_compression" <2>
  },
  "aliases": {
    "my_search_indices": {}
  }
}
--------------------------------------------------

<1> The number of shards in the target index. This must be a factor of the
    number of shards in the source index.
<2> Best compression will only take affect when new writes are made to the
    index, such as when <<indices-forcemerge,force-merging>> the shard to a single
    segment.


NOTE: Mappings may not be specified in the `_shrink` request, and all
`index.analysis.*` and `index.similarity.*` settings will be overwritten with
the settings from the source index.

[float]
=== Monitoring the shrink process

The shrink process can be monitored with the <<cat-recovery,`_cat recovery`
API>>, or the <<cluster-health, `cluster health` API>> can be used to wait
until all primary shards have been allocated by setting the  `wait_for_status`
parameter to `yellow`.

The `_shrink` API returns as soon as the target index has been added to the
cluster state, before any shards have been allocated. At this point, all
shards are in the state `unassigned`. If, for any reason, the target index
can't be allocated on the shrink node, its primary shard will remain
`unassigned` until it can be allocated on that node.

Once the primary shard is allocated, it moves to state `initializing`, and the
shrink process begins. When the shrink operation completes, the shard will
become `active`. At that  point, Elasticsearch will try to allocate any
replicas and may decide to relocate the primary shard to another node.

