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https://issues.apache.org/jira/browse/LUCENE-8962?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17051558#comment-17051558
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Michael Froh commented on LUCENE-8962:
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It's not immediately obvious to me how to fix the failure on
{{TestIndexWriterExceptions2}}.
A merge on commit fails (because it's using {{CrankyCodec}}), closing the merge
readers, which calls the custom {{mergeFinished}} override, which assumes the
merge completed (since it wasn't aborted), and tries to reference the files for
the merged segment (to increment their reference counts). That triggers an
{{IllegalStateException}} because the files weren't set (because we didn't get
that far in the merge).
Unfortunately, stepping through the debugger, I don't see a clear way of
telling in {{mergeFinished}} that a merge failed. Obviously, I could wrap the
call to {{SegmentCommitInfo.files()}} in a try-catch, and assume that the
{{IllegalStateException}} means that the merge failed, but that would fail to
catch an IOException when e.g. committing the merge.
I'm thinking of adding a {{boolean}} field to {{OneMerge}} that gets set once a
merge is successfully committed (e.g. just before the call to
{{closeMergeReaders}} in {{IndexWriter.commitMerge()}}), which the
{{mergeFinished}} override can use to determine if the merge completed
successfully or not.
> 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
>
> Time Spent: 6h 20m
> 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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