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https://issues.apache.org/jira/browse/LUCENE-8264?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16447700#comment-16447700
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Adrien Grand commented on LUCENE-8264:
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The problem I described isn't related to merges but to the data that is stored
in segments. A 6x index won't be upgradeable to 8x, even if you merge all
segments so that they use the 7x file formats. This will require a reindex.
> 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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