The force merge API allows you 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.
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 can be useful with time-based indices and when using rollover. In these cases each index only receives indexing traffic for a certain period of time, and once an index will receive no more writes its shards can be force-merged down to a single segment:
This can be a good idea because single-segment shards can sometimes use simpler and more efficient data structures to perform searches.
The force merge API accepts the following request parameters:
The number of segments to merge to. To fully
merge the index, set it to
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
Should a flush be performed after the forced merge. Defaults to
The force merge API can be applied to more than one index with a single call, or
_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
1, as all segments need to be rewritten into a new one.
POST /kimchy,elasticsearch/_forcemerge POST /_forcemerge