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https://issues.apache.org/jira/browse/LUCENE-7020?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17390042#comment-17390042
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Michael McCandless commented on LUCENE-7020:
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+1 to remove.
My only worry is crazy abusive use cases that somehow have 100s of segments in
their index, try to forceMerge and maybe hit too many open file descriptors or
so. But such accidentally abusive use cases already have other ways to bang
their segments into more reasonable shape, and then the unlimited
{{forceMerge}} would work. So yeah +1 let's remove this limit!
> 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
> Priority: Major
> Attachments: LUCENE-7020.patch
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> 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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