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https://issues.apache.org/jira/browse/LUCENE-8962?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17142190#comment-17142190
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Erick Erickson commented on LUCENE-8962:
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Sounds like it may _not_ be related to SOLR-13268 then, it's a perfectly 
legitimate error if some run happens to produce more than 8K of output. If none 
of the messages look like they came from another test, it's probably legitimate 
and not a result of async logging.

Mostly I wanted you to be aware that it _might_ be related, but that's looking 
less likely.

> Can we merge small segments during refresh, for faster searching?
> -----------------------------------------------------------------
>
>                 Key: LUCENE-8962
>                 URL: https://issues.apache.org/jira/browse/LUCENE-8962
>             Project: Lucene - Core
>          Issue Type: Improvement
>          Components: core/index
>            Reporter: Michael McCandless
>            Priority: Major
>             Fix For: 8.6
>
>         Attachments: LUCENE-8962_demo.png, failed-tests.patch
>
>          Time Spent: 18h 50m
>  Remaining Estimate: 0h
>
> With near-real-time search we ask {{IndexWriter}} to write all in-memory 
> segments to disk and open an {{IndexReader}} to search them, and this is 
> typically a quick operation.
> However, when you use many threads for concurrent indexing, {{IndexWriter}} 
> will accumulate write many small segments during {{refresh}} and this then 
> adds search-time cost as searching must visit all of these tiny segments.
> The merge policy would normally quickly coalesce these small segments if 
> given a little time ... so, could we somehow improve {{IndexWriter'}}s 
> refresh to optionally kick off merge policy to merge segments below some 
> threshold before opening the near-real-time reader?  It'd be a bit tricky 
> because while we are waiting for merges, indexing may continue, and new 
> segments may be flushed, but those new segments shouldn't be included in the 
> point-in-time segments returned by refresh ...
> One could almost do this on top of Lucene today, with a custom merge policy, 
> and some hackity logic to have the merge policy target small segments just 
> written by refresh, but it's tricky to then open a near-real-time reader, 
> excluding newly flushed but including newly merged segments since the refresh 
> originally finished ...
> I'm not yet sure how best to solve this, so I wanted to open an issue for 
> discussion!



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