Adrien Grand created LUCENE-8688:
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Summary: Forced merges merge more than necessary
Key: LUCENE-8688
URL: https://issues.apache.org/jira/browse/LUCENE-8688
Project: Lucene - Core
Issue Type: Bug
Reporter: Adrien Grand
A user reported some surprise after the upgrade to Lucene 7.5 due to changes to
how forced merges are selected when maxSegmentCount is greater than 1.
Before 7.5 forceMerge used to pick up the least amount of merging that would
result in an index that has maxSegmentCount segments at most. Now that we share
the same logic as regular merges, we are almost sure to pick a
maxMergeAtOnceExplicit-segments merge (30 segments) given that merges that have
more segments usually score better. This is due to the fact that natural merges
assume that merges that run now save work for later, so the more segments get
merged, the better. This assumption doesn't hold for forced merges that should
run on read-only indices, so there won't be any future merging.
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