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`.

`index_codec`::
(Optional, string)
Codec used to compress the document store. The only accepted value is
`best_compression`, which uses {wikipedia}/DEFLATE[DEFLATE] for a higher
compression ratio but slower stored fields performance. To use the default LZ4
codec, omit this argument.
+
WARNING: If using `best_compression`, {ilm-init} will <<indices-close,close>>
and then <<indices-open-close,re-open>> the index prior to the force merge.
While closed, the index will be unavailable for read or write operations.

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

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