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https://issues.apache.org/jira/browse/LUCENE-8264?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16447707#comment-16447707
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Simon Willnauer commented on LUCENE-8264:
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[~dweiss] I think you are not aware of the fact that an index that was created
with N-2 won't be supported by N even if you rewrite all segments. The created
version is baked into the segments file and Lucene will not open it even if all
segments are on N or N-1. There are several reasons for this for instance to
reject broken offsets in term vectors in Lucene 7. We can never enforce limits
like this if we keep on upgrading stuff behind the scenes that didn't have
these protections.
> 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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