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

Forces a <<index-modules-merge,merge>> on the shards of one or more indices.
For data streams, the API forces a merge on the shards of the stream's backing
indices.

[source,console]
----
POST /my-index-000001/_forcemerge
----
// TEST[setup:my_index]


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

`POST /<target>/_forcemerge`

`POST /_forcemerge`

[[forcemerge-api-prereqs]]
==== {api-prereq-title}

* If the {es} {security-features} are enabled, you must have the `maintenance`
or `manage` <<privileges-list-indices,index privilege>> for the target data
stream, index, or alias.

[[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.

// tag::force-merge-read-only-warn[]
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.
// end::force-merge-read-only-warn[]


[[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

You can force merge multiple indices with a single request by targeting:

* One or more data streams that contain multiple backing indices
* Multiple indices
* One or more aliases
* All data streams and indices in a cluster

Each targeted shard is force-merged separately using <<modules-threadpool,the
`force_merge` threadpool>>. By default each node only has a single
`force_merge` thread which means that the shards on that node are force-merged
one at a time. If you expand the `force_merge` threadpool on a node then it
will force merge its shards in parallel.

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}

`<target>`::
(Optional, string) Comma-separated list of data streams, indices, and aliases
used to limit the request. Supports wildcards (`*`). To target all data streams
and indices, omit this parameter or use `*` or `_all`.

[[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 indices,
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 data stream or index

[source,console]
----
POST /my-index-000001/_forcemerge
----
// TEST[continued]


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

[source,console]
----
POST /my-index-000001,my-index-000002/_forcemerge
----
// TEST[s/^/PUT my-index-000001\nPUT my-index-000002\n/]


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

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


[[forcemerge-api-time-based-index-ex]]
===== Data streams and time-based indices

Force-merging is useful for managing a data stream's older backing indices and
other time-based indices, particularly after a
<<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 /.ds-my-data-stream-2099.03.07-000001/_forcemerge?max_num_segments=1
--------------------------------------------------
// TEST[setup:my_index]
// TEST[s/.ds-my-data-stream-2099.03.07-000001/my-index-000001/]

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