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https://issues.apache.org/jira/browse/LUCENE-8688?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16791972#comment-16791972
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Armin Braun commented on LUCENE-8688:
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[~jpountz] thanks for taking a look. You are right those two points imo.
I fixed the first one, by switching the order of the force merge in progress
check around to address this point. I left the special case handling though
because otherwise the case of merging from a single segments with deletes to a
single segment becomes a NOOP when before we'd merge here in any case and
expunge the deletes which I guess we want to keep?
The second point makes perfect sense to me too, I adjusted the logic to only
allow for a max count sized merge or a merge that has 70% of the max size or
more for the result (since we jack up the max size by 25% in the beginning of
the method, that seemed appropriate but tbh. this number was just a guess).
=> This should be good for another review now :)
> 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
> Priority: Minor
> Attachments: LUCENE-8688.patch, LUCENE-8688.patch, LUCENE-8688.patch,
> LUCENE-8688.patch
>
>
> 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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