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https://issues.apache.org/jira/browse/LUCENE-8264?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16449787#comment-16449787
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David Smiley commented on LUCENE-8264:
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Fascinating discussion.
Shawn said:
{quote}Now I'm hearing differently ... that any user who has successfully done
this has just gotten lucky, and that there's no guarantee for the future.
{quote}
I don't think it's quite that bleak. I believe each segment records metadata
of the Lucene version, so we could explicitly know wether or not the index
contains segments older than the current version. One could even write a tool
to spit out the IDs of those documents to facilitate a re-index of just those
documents.
> 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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