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https://issues.apache.org/jira/browse/LUCENE-7020?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15140510#comment-15140510
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Michael McCandless commented on LUCENE-7020:
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I'm nervous about this change: three times the {{maxMergeAtOnce}} value is
dangerous since this implies opening 3X the number of file descriptors on force
merge, spreading the OS's readahead efforts by 1/3rd, etc.
Also the new boolean {{explicitValueManuallySet}} is sort of awkward ... how do
I unset it once I've set it? It's spooky to have such unexpected
state/behavior what at first blush looks like an ordinary setter.
I'd rather keep these two settings clearly separated.
> 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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