Shawn Heisey created LUCENE-7020:
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Summary: TieredMergePolicy - cascade maxMergeAtOnce setting to
maxMergeAtOnceExplicit
Key: LUCENE-7020
URL: https://issues.apache.org/jira/browse/LUCENE-7020
Project: Lucene - Core
Issue Type: Improvement
Affects Versions: 5.4.1
Reporter: Shawn Heisey
Assignee: Shawn Heisey
SOLR-8621 covers improvements in configuring a merge policy in Solr.
Discussions on that issue brought up the fact that if large values are
configured for maxMergeAtOnce and segmentsPerTier, but maxMergeAtOnceExplicit
is not changed, then doing a forceMerge is likely to not work as expected.
When I first configured maxMergeAtOnce and segmentsPerTier to 35 in Solr, I saw
an optimize (forceMerge) fully rewrite most of the index *twice* in order to
achieve a single segment, because there were approximately 80 segments in the
index before the optimize, and maxMergeAtOnceExplicit defaults to 30. On
advice given via the solr-user mailing list, I configured
maxMergeAtOnceExplicit to 105 and have not had that problem since.
I propose that setting maxMergeAtOnce should also set maxMergeAtOnceExplicit to
three times the new value -- unless the setMaxMergeAtOnceExplicit method has
been invoked, indicating that the user wishes to set that value themselves.
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