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Force merge

Introduced 1.0

The force merge API operation forces a merge on the shards of one or more indexes. For a data stream, the API forces a merge on the shards of the stream’s backing index.

The merge operation

In OpenSearch, a shard is a Lucene index, which consists of segments (or segment files). Segments store the indexed data. Periodically, smaller segments are merged into larger ones and the larger segments become immutable. Merging reduces the overall number of segments on each shard and frees up disk space.

OpenSearch performs background segment merges that produce segments no larger than index.merge.policy.max_merged_segment (the default is 5 GB).

Deleted documents

When a document is deleted from an OpenSearch index, it is not deleted from the Lucene segment but is rather only marked to be deleted. When the segment files are merged, deleted documents are removed (or expunged). Thus, merging also frees up space occupied by documents marked as deleted.

Force Merge API

In addition to periodic merging, you can force a segment merge using the Force Merge API.

Use the Force Merge API on an index only after all write requests sent to the index are completed. The force merge operation can produce very large segments. If write requests are still sent to the index, then the merge policy does not merge these segments until they primarily consist of deleted documents. This can increase disk space usage and lead to performance degradation.

When you call the Force Merge API, the call is blocked until merge completion. If during this time the connection is lost, the force merge operation continues in the background. New force merge requests sent to the same index will be blocked until the currently running merge operation is complete.

Force merging multiple indexes

To force merge multiple indexes, you can call the Force Merge API on the following index combinations:

  • Multiple indexes
  • One or more data streams containing multiple backing indexes
  • One or more index aliases pointing to multiple indexes
  • All data streams and indexes in a cluster

When you force merge multiple indexes, the merge operation is executed on each shard of a node sequentially. When the force merge operation is in progress, the storage for the shard temporarily increases so that all segments can be rewritten into a new segment. When max_num_segments is set to 1, the storage for the shard temporarily doubles.

Force merging data streams

It can be useful to force merge data streams in order to manage a data stream’s backing indexes, especially after a rollover operation. Time-based indexes receive indexing requests only during a specified time period. Once that time period has elapsed and the index receives no more write requests, you can force merge segments of all index shards into one segment. Searches on single-segment shards are more efficient because they use simpler data structures.

Path and HTTP methods

POST /_forcemerge
POST /<index>/_forcemerge/

Path parameters

The following table lists the available path parameters. All path parameters are optional.

Parameter Data type Description
<index> String A comma-separated list of indexes, data streams, or index aliases to which the operation is applied. Supports wildcard expressions (*). Use _all or * to specify all indexes and data streams in a cluster.

Query parameters

The following table lists the available query parameters. All query parameters are optional.

Parameter Data type Description
allow_no_indices Boolean If false, the request returns an error if any wildcard expression or index alias targets any closed or missing indexes. Default is true.
expand_wildcards String Specifies the types of indexes to which wildcard expressions can expand. Supports comma-separated values. Valid values are:
- all: Expand to all open and closed indexes, including hidden indexes.
- open: Expand to open indexes.
- closed: Expand to closed indexes.
- hidden: Include hidden indexes when expanding. Must be combined with open, closed, or both.
- none: Do not accept wildcard expressions.
Default is open.
flush Boolean Performs a flush on the indexes after the force merge. A flush ensures that the files are persisted to disk. Default is true.
ignore_unavailable Boolean If true, OpenSearch ignores missing or closed indexes. If false, OpenSearch returns an error if the force merge operation encounters missing or closed indexes. Default is false.
max_num_segments Integer The number of larger segments into which smaller segments are merged. Set this parameter to 1 to merge all segments into one segment. The default behavior is to perform the merge as necessary.
only_expunge_deletes Boolean If true, the merge operation only expunges segments containing a certain percentage of deleted documents. The percentage is 10% by default and is configurable in the index.merge.policy.expunge_deletes_allowed setting. Prior to OpenSearch 2.12, only_expunge_deletes ignored the index.merge.policy.max_merged_segment setting. Starting with OpenSearch 2.12, using only_expunge_deletes does not produce segments larger than index.merge.policy.max_merged_segment (by default, 5 GB). For more information, see Deleted documents. Default is false.
primary_only Boolean If set to true, then the merge operation is performed only on the primary shards of an index. This can be useful when you want to take a snapshot of the index after the merge is complete. Snapshots only copy segments from the primary shards. Merging the primary shards can reduce resource consumption. Default is false.

Example request: Force merge a specific index

POST /testindex1/_forcemerge

Example request: Force merge multiple indexes

POST /testindex1,testindex2/_forcemerge

Example request: Force merge all indexes

POST /_forcemerge

Example request: Force merge a data stream’s backing indexes into one segment

POST /.testindex-logs/_forcemerge?max_num_segments=1

Example request: Force merge primary shards

POST /.testindex-logs/_forcemerge?primary_only=true

Example response

  "_shards": {
    "total": 2,
    "successful": 1,
    "failed": 0

Response fields

The following table lists all response fields.

Field Data type Description
shards Object Contains information about the shards on which the request was executed. Integer The number of shards on which the operation was executed.
shards.successful Integer The number of shards on which the operation was successful.
shards.failed Integer The number of shards on which the operation failed.