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https://issues.apache.org/jira/browse/LUCENE-8264?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16447257#comment-16447257
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Shawn Heisey commented on LUCENE-8264:
--------------------------------------

On the dev list, [~yriveiro] replied to this issue.  His indexes are up to 15 
terabytes.  (yowza!)

Reindexing from scratch on an index that big is something you can't just decide 
to do one day.

I really like the idea of rewriting all segments without merging them.  The way 
that IndexUpgrader currently works can cause the LUCENE-7976 problems.

> 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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