Behavior of ForceMerge

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Let me ask about the behavior of _forcemerge API including parameter only_expunge_deletes=true.

I'm planning to execute _forcemerge?only_expunge_deletes=true to large index (more over 500GB/10 datanodes) to clean up a disk space.

So my question is, does it API use additional disk space to clean deleted documents, if yes how large does it consume?

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As all segments in Lucene are immutable new segments without the deleted documents will be created before the old ones are deleted. A forcemerge can therefore use a fair bit of additional disk space while it is running. I do not know how to predict exactly how much it needs as this depends on the parameters used and how the current segment distribution looks like.

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