[[indices-forcemerge]]
== Force Merge

The force merge API allows to force merging of one or more indices through an
API. The merge relates to the number of segments a Lucene index holds within
each shard. The force merge operation allows to reduce the number of segments by
merging them.

This call will block until the merge is complete. If the http connection is
lost, the request will continue in the background, and any new requests will
block until the previous force merge is complete.

WARNING: Force merge should only be called against *read-only indices*. Running 
force merge against a read-write index can cause very large segments to be produced 
(>5Gb per segment), and the merge policy will never consider it for merging again until 
it mostly consists of deleted docs. This can cause very large segments to remain in the shards.

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

[float]
[[forcemerge-parameters]]
=== Request Parameters

The force merge API accepts the following request parameters:

[horizontal]
`max_num_segments`:: The number of segments to merge to. To fully
merge the index, set it to `1`. Defaults to simply checking if a
merge needs to execute, and if so, executes it.

`only_expunge_deletes`:: Should the merge process only expunge segments with
deletes in it. In Lucene, a document is not deleted from a segment, just marked
as deleted. During a merge process of segments, a new segment is created that
does not have those deletes. This flag allows to only merge segments that have
deletes. Defaults to `false`.  Note that this won't override the
`index.merge.policy.expunge_deletes_allowed` threshold.

`flush`::  Should a flush be performed after the forced merge. Defaults to
`true`.

[source,js]
--------------------------------------------------
POST /kimchy/_forcemerge?only_expunge_deletes=false&max_num_segments=100&flush=true
--------------------------------------------------
// CONSOLE
// TEST[s/^/PUT kimchy\n/]

[float]
[[forcemerge-multi-index]]
=== Multi Index

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` is set
to `1`, as all segments need to be rewritten into a new one.


[source,js]
--------------------------------------------------
POST /kimchy,elasticsearch/_forcemerge

POST /_forcemerge
--------------------------------------------------
// CONSOLE
// TEST[s/^/PUT kimchy\nPUT elasticsearch\n/]