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https://issues.apache.org/jira/browse/LUCENE-8264?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16448895#comment-16448895
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Erick Erickson commented on LUCENE-8264:
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So let me see if I have this straight. The guarantee is that Lucene N will read
indexes produced with N-1 if (and only if) the index was originally created
entirely by version N-1? IOW if Lucene N-2 was used at any point in history no
guarantees are made.
And any upgrade process that would overcome that issue would be on a
case-by-case, one-off basis that would stand no chance of making it into the
Lucene code base.
> Allow an option to rewrite all segments
> ---------------------------------------
>
> Key: LUCENE-8264
> URL: https://issues.apache.org/jira/browse/LUCENE-8264
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Erick Erickson
> Assignee: Erick Erickson
> Priority: Major
>
> For the background, see SOLR-12259.
> There are several use-cases that would be much easier, especially during
> upgrades, if we could specify that all segments get rewritten.
> One example: Upgrading 5x->6x->7x. When segments are merged, they're
> rewritten into the current format. However, there's no guarantee that a
> particular segment _ever_ gets merged so the 6x-7x upgrade won't necessarily
> be successful.
> How many merge policies support this is an open question. I propose to start
> with TMP and raise other JIRAs as necessary for other merge policies.
> So far the usual response has been "re-index from scratch", but that's
> increasingly difficult as systems get larger.
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