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You can pass several additional parameters to the ForceMerge call, such as the following:

• max_num_segments: The default value is autodetect. For full optimization, set this value to 1.
• only_expunge_deletes: The default value is false. Lucene does not delete documents from segments, but it marks them as deleted. This flag only merges segments that have been deleted.
• flush: The default value is true. Elasticsearch performs a flush after a ForceMerge.
• wait_for_merge: The default value is true. If the request needs to wait, then the merge ends.