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https://issues.apache.org/jira/browse/LUCENE-8962?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17306249#comment-17306249
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Michael McCandless commented on LUCENE-8962:
--------------------------------------------
Hi Team! This issue was so exotic and such a great example of powerful
open-source collaboration, which I suspect many people didn't even notice, so I
wrote fun blog post explaining/uncovering all the juicy details:
[http://blog.mikemccandless.com/2021/03/open-source-collaboration-or-how-we.html]
> 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: main (9.0), 8.6, 8.7
>
> Attachments: LUCENE-8962_demo.png, failed-tests.patch,
> failure_log.txt, image-2021-03-22-10-36-32-201.png, test.diff
>
> Time Spent: 31h
> Remaining Estimate: 0h
>
> Two improvements were added: 8.6 has merge-on-commit (by Froh et. all), 8.7
> has merge-on-refresh (by Simon). See \{{MergePolicy.findFullFlushMerges}}
> The original description follows:
> ----
> 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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