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https://issues.apache.org/jira/browse/LUCENE-5941?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Shai Erera updated LUCENE-5941:
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Attachment: LUCENE-5941.patch
Clarify IW.forceMerge jdocs a bit. So far beasting seems happy. I think it's
ready.
> IndexWriter.forceMerge documentation error
> ------------------------------------------
>
> Key: LUCENE-5941
> URL: https://issues.apache.org/jira/browse/LUCENE-5941
> Project: Lucene - Core
> Issue Type: Improvement
> Components: core/index
> Reporter: Shai Erera
> Assignee: Shai Erera
> Attachments: LUCENE-5941.patch, LUCENE-5941.patch, LUCENE-5941.patch
>
>
> IndexWriter.forceMerge documents that it requires up to 3X *FREE* space in
> order to run successfully. We even go further with it and test it in
> TestIWForceMerge.testForceMergeTempSpaceUsage(). But I think that's wrong. I
> cannot think of a situation where we consume 3X *additional* space during
> merge:
> * 1X - that's the source segments to be merged
> * 2X - that's the result non-CFS merged segment
> * 3X - that's the CFS creation
> At no point do we publish the non-CFS merged segment, therefore the merge, as
> I understand it, only consumes up to 2X additional space during that merge.
> And anyway, we only require 2X of additional space of the *largest* merge (or
> total batch of running merges, depends on your MergeScheduler), not the whole
> index size. This is an important observation, since if you e.g. have a 500GB
> index, users shouldn't think they need to reserve an additional 1TB for
> merging, since most of their big segments won't be merged by default anyway
> (TieredMP defaults to 5GB largest segment).
> I'll post a patch which fixes the documentation and the test. If anyone can
> think of a scenario where we consume up to 3X *additional* space, please
> chime, and I'll only modify IW.forceMerge documentation to explain that.
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