David Smiley commented on LUCENE-8962:

ok; makes sense.

> 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: 9.5h
>  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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