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https://issues.apache.org/jira/browse/LUCENE-8962?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17054163#comment-17054163
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Nhat Nguyen commented on LUCENE-8962:
-------------------------------------
I have translated some Elasticsearch tests (see [^failed-tests.patch]) to
Lucene. The test _testMergeOnCommitKeepFullyDeletedSegments_ consistently fails
because _mergeFinished_ was called twice. This happens because we forget to set
_success_ to _true_ when the new segment does not have any document (i.e.,
_SegmentMerger#shouldMerge_ is false).
Even with that fix, both _testRandomOperations_ and
_testRandomOperationsWithSoftDeletes_ still randomly fail after a few thousand
iterations. Since both are multi-threaded tests, we will need more time to
figure out the root cause.
> 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: 8.5
>
> 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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