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From "Shawn Heisey (JIRA)" <>
Subject [jira] [Commented] (LUCENE-7020) TieredMergePolicy - cascade maxMergeAtOnce setting to maxMergeAtOnceExplicit
Date Wed, 10 Feb 2016 16:15:18 GMT


Shawn Heisey commented on LUCENE-7020:

On the issue of file descriptors ... does Lucene open segment files a second time for a merge?
 The files should already be opened so that queries will work.  I would hope there would not
be any additional file descriptors required beyond the files for the new segment that is being

> TieredMergePolicy - cascade maxMergeAtOnce setting to maxMergeAtOnceExplicit
> ----------------------------------------------------------------------------
>                 Key: LUCENE-7020
>                 URL:
>             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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