OpenSearch/docs/reference/indices/forcemerge.asciidoc

[[indices-forcemerge]]
=== Force merge API
++++
<titleabbrev>Force merge</titleabbrev>
++++

Forces a <<index-modules-merge,merge>> on the shards of one or more indices.

[source,console]
----
POST /twitter/_forcemerge
----
// TEST[setup:twitter]


[[forcemerge-api-request]]
==== {api-request-title}

`POST /<index>/_forcemerge`

`POST /_forcemerge`


[[forcemerge-api-desc]]
==== {api-description-title}

Use the force merge API to force a <<index-modules-merge,merge>> on the
shards of one or more indices. Merging reduces the number of segments in each
shard by merging some of them together, and also frees up the space used by
deleted documents. Merging normally happens automatically, but sometimes it is
useful to trigger a merge manually.

WARNING: **Force merge should only be called against an index after you have
finished writing to it.** Force merge can cause very large (>5GB) segments to
be produced, and if you continue to write to such an index then the automatic
merge policy will never consider these segments for future merges until they
mostly consist of deleted documents. This can cause very large segments to
remain in the index which can result in increased disk usage and worse search
performance.


[[forcemerge-blocks]]
===== Blocks during a force merge

Calls to this API block until the merge is complete. If the client connection
is lost before completion then the force merge process will continue in the
background. Any new requests to force merge the same indices will also block
until the ongoing force merge is complete.


[[forcemerge-multi-index]]
===== Force merging multiple indices

The force merge API can be applied to more than one index with a single call, or
even on `_all` the indices. Multi index operations are executed one shard at a
time per node. Force merge makes the storage for the shard being merged
temporarily increase, up to double its size in case `max_num_segments` parameter
is set to `1`, as all segments need to be rewritten into a new one.


[[forcemerge-api-path-params]]
==== {api-path-parms-title}

include::{es-repo-dir}/rest-api/common-parms.asciidoc[tag=index]
+
To force merge all indices in the cluster,
omit this parameter
or use a value of `_all` or `*`.


[[forcemerge-api-query-params]]
==== {api-query-parms-title}

include::{es-repo-dir}/rest-api/common-parms.asciidoc[tag=allow-no-indices]
+
Defaults to `true`.

include::{es-repo-dir}/rest-api/common-parms.asciidoc[tag=expand-wildcards]
+
Defaults to `open`.

`flush`::
(Optional, boolean)
If `true`,
{es} performs a <<indices-flush,flush>> on the indices
after the force merge.
Defaults to `true`.

include::{es-repo-dir}/rest-api/common-parms.asciidoc[tag=index-ignore-unavailable]

`max_num_segments`::
+
--
(Optional, integer)
The number of segments to merge to.
To fully merge the index,
set it to `1`.

Defaults to checking if a merge needs to execute.
If so, executes it.
--

`only_expunge_deletes`::
+
--
(Optional, boolean)
If `true`,
only expunge segments containing document deletions.
Defaults to `false`.

In Lucene,
a document is not deleted from a segment;
just marked as deleted.
During a merge,
a new segment is created
that does not contain those document deletions.

NOTE: This parameter does *not* override the
`index.merge.policy.expunge_deletes_allowed` setting.
--


[[forcemerge-api-example]]
==== {api-examples-title}


[[forcemerge-api-specific-ex]]
===== Force merge a specific index

[source,console]
----
POST /twitter/_forcemerge
----
// TEST[continued]


[[forcemerge-api-multiple-ex]]
===== Force merge several indices

[source,console]
----
POST /kimchy,elasticsearch/_forcemerge
----
// TEST[s/^/PUT kimchy\nPUT elasticsearch\n/]


[[forcemerge-api-all-ex]]
===== Force merge all indices

[source,console]
----
POST /_forcemerge
----


[[forcemerge-api-time-based-index-ex]]
===== Time-based indices

Force-merging is useful for time-based indices,
particularly when using <<indices-rollover-index,rollover>>.
In these cases,
each index only receives indexing traffic for a certain period of time.
Once an index receive no more writes,
its shards can be force-merged to a single segment.

[source,console]
--------------------------------------------------
POST /logs-000001/_forcemerge?max_num_segments=1
--------------------------------------------------
// TEST[setup:twitter]
// TEST[s/logs-000001/twitter/]

This can be a good idea because single-segment shards can sometimes use simpler
and more efficient data structures to perform searches.