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https://issues.apache.org/jira/browse/LUCENE-8004?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16215682#comment-16215682
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Michael McCandless commented on LUCENE-8004:
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Merges are usually compute bound, and a given merge is single threaded ... if
you look in IndexWriter's info stream log you'll see which parts take the most
time; it's usually postings in my experience.
Especially if you are merging away deleted docs, then we can't apply bulk copy
optos for stored fields and term vectors.
We don't have a bulk copy opto for postings.
> IndexUpgraderTool should rewrite segments rather than forceMerge
> ----------------------------------------------------------------
>
> Key: LUCENE-8004
> URL: https://issues.apache.org/jira/browse/LUCENE-8004
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Erick Erickson
>
> Spinoff from LUCENE-7976. We help users get themselves into a corner by using
> forceMerge on an index to rewrite all segments in the current Lucene format.
> We should rewrite each individual segment instead. This would also help with
> upgrading X-2->X-1, then X-1->X.
> Of course the preferred method is to re-index from scratch.
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