OpenSearch/docs/reference/ilm/actions/ilm-forcemerge.asciidoc

[role="xpack"]
[[ilm-forcemerge]]
=== Force merge

Phases allowed: hot, warm.

<<indices-forcemerge,Force merges>> the index into 
the specified maximum number of <<indices-segments,segments>>.

This action makes the index <<dynamic-index-settings,read-only>>.

To use the `forcemerge` action in the `hot` phase, the `rollover` action *must* be present.
If no rollover action is configured, {ilm-init} will reject the policy. 

[NOTE]
The `forcemerge` action is best effort. It might happen that some of the
shards are relocating, in which case they will not be merged.

[[ilm-forcemerge-options]]
==== Options

`max_num_segments`::
(Required, integer) 
Number of segments to merge to. To fully merge the index, set to `1`.

`codec`::
(Optional, string)   
Use the `best_compression` codec. Valid values: `best_compression`.
+
[WARNING]
======
Setting `"codec": "best_compression"` in the {ilm-init} forcemerge action causes {ilm-int} to
<<indices-close,close>> and then <<indices-open-close,re-open>> the index prior to the force merge.
During this time, the index is unavailable for both read and write operations.
======

[[ilm-forcemerge-action-ex]]
==== Example

[source,console]
--------------------------------------------------
PUT _ilm/policy/my_policy
{
  "policy": {
    "phases": {
      "warm": {
        "actions": {
          "forcemerge" : {
            "max_num_segments": 1
          }
        }
      }
    }
  }
}
--------------------------------------------------