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.
翻译：The force merge API 允许强制将一个或者多个index做segment合并操作。这个操作可以减少segment数量。
The force merge API accepts the following request parameters:
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.
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.
Should a flush be performed after the forced merge. Defaults to true.
The force merge API can be applied to more than one index with a single call, or even on _all the indices.
POST /kimchy,elasticsearch/_forcemerge POST /_forcemerge