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https://issues.apache.org/jira/browse/LUCENE-8688?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16783034#comment-16783034
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Armin Braun edited comment on LUCENE-8688 at 3/4/19 6:56 AM:
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Thanks [~jpountz] you're right, I accidentally changed behaviour for the case
of an already running merge. I fixed that now I think by putting in that guard
for this breakout.
It's a little different than it was before in that not all merges will be of
the max-merge count before the final merge due to the size check, obviously.
Unfortunately I wasn't quite able to find the new utility methods to simulate
an ongoing merge, could you point me at what method(s) you had in mind? :)
In and of itself the logic seemed to work fine (I simulated the case by not
invoking `IndexWriter#forceMergeDeletes` before calling the force merge and
using a smaller buffered docs count to get larger (in number of segments)
merges, but obviously a proper test would be great :)
was (Author: original-brownbear):
Thanks [~jpountz] you're right, I accidentally changed behaviourfor the case of
an already running merge. I fixed that now I think by putting in that guard for
this breakout.
It's a little different than it was before in that not all merges will be of
the max-merge count before the final merge due to the size check, obviously.
Unfortunately I wasn't quite able to find the new utility methods to simulate
an ongoing merge, could you point me at what method(s) you had in mind? :)
In and of itself the logic seemed to work fine (I simulated the case by not
invoking `IndexWriter#forceMergeDeletes` before calling the force merge and
using a smaller buffered docs count to get larger (in number of segments)
merges, but obviously a proper test would be great :)
> 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
>
>
> 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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