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Elastic Docs ›Elasticsearch Guide [7.10] ›REST APIs ›Index APIs

Force merge API

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Force merge API

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Forces a 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.

POST /my-index-000001/_forcemerge

Request

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POST /<target>/_forcemerge

POST /_forcemerge

Prerequisites

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  • If the Elasticsearch security features are enabled, you must have the maintenance or manage index privilege for the target data stream, index, or index alias.

Description

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Use the force merge API to force a 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.

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.

Blocks during a force merge

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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.

Force merging multiple indices

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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 index aliases that point to multiple indices
  • All data streams and indices in a cluster

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.

Path parameters

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<target>

(Optional, string) Comma-separated list of data streams, indices, and index aliases used to limit the request. Wildcard expressions (*) are supported.

To target all data streams and indices in a cluster, omit this parameter or use _all or *.

Query parameters

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allow_no_indices

(Optional, Boolean) If false, the request returns an error if any wildcard expression, index alias, or _all value targets only missing or closed indices. This behavior applies even if the request targets other open indices. For example, a request targeting foo*,bar* returns an error if an index starts with foo but no index starts with bar.

Defaults to true.

expand_wildcards

(Optional, string) Controls what kind of indices that wildcard expressions can expand to. Multiple values are accepted when separated by a comma, as in open,hidden. Valid values are:

all
Expand to open and closed indices, including hidden indices.
open
Expand only to open indices.
closed
Expand only to closed indices.
hidden
Expansion of wildcards will include hidden indices. Must be combined with open, closed, or both.
none
Wildcard expressions are not accepted.

Defaults to open.

flush
(Optional, Boolean) If true, Elasticsearch performs a flush on the indices after the force merge. Defaults to true.
ignore_unavailable
(Optional, Boolean) If false, the request returns an error if it targets a missing or closed index. Defaults to false.
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.

This parameter does not override the index.merge.policy.expunge_deletes_allowed setting.

Examples

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Force merge a specific data stream or index

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POST /my-index-000001/_forcemerge

Force merge several data streams or indices

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POST /my-index-000001,my-index-000002/_forcemerge

Force merge all indices

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POST /_forcemerge

Data streams and time-based indices

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Force-merging is useful for managing a data stream’s older backing indices and other time-based indices, particularly after a 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.

POST /.ds-logs-000001/_forcemerge?max_num_segments=1

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

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