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https://issues.apache.org/jira/browse/LUCENE-7020?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15140689#comment-15140689
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Christine Poerschke commented on LUCENE-7020:
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bq. ... keep these two settings clearly separated.
Just wondering if there is a 'recommended' ratio between the two, and assuming
there is a recommended ratio, then could it be surfaced somehow when users
choose to configure an 'unusual' ratio?
Or in other words, the user still gets what they configure but if say the
maxMergeAtOnceExplicit is unusually low relative to maxMergeAtOnce then some
warning would be logged? Having said that, currently TieredMergePolicy logs
nothing and if it did then a warning would be logged every time a new index
writer/merge policy is created, which would be too much maybe.
> 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
> Attachments: LUCENE-7020.patch
>
>
> 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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