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From "Shai Erera (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (LUCENE-5941) IndexWriter.forceMerge documentation error
Date Fri, 12 Sep 2014 06:13:33 GMT

     [ https://issues.apache.org/jira/browse/LUCENE-5941?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Shai Erera updated LUCENE-5941:
-------------------------------
    Attachment: LUCENE-5941.patch

Patch modifies the test to assert up to 3X disk usage. I beasted and it fails with this:

{noformat}
  [beaster] Started J0 PID(5216@SHAIE-TP).
  [beaster]   2> NOTE: reproduce with: ant test  -Dtestcase=TestIndexWriterForceMerge -Dtests.method=testForceMergeTempSpaceUsage
-Dtests.seed=AEA68CE694BB7732 -Dtests.slow=true -Dtests.locale=ru_RU -Dtests.timezone=US/Pacific-New
-Dtests.file.encoding=Cp1255
  [beaster] [09:05:02.935] FAILURE 1.52s | TestIndexWriterForceMerge.testForceMergeTempSpaceUsage
<<<
  [beaster]    > Throwable #1: java.lang.AssertionError: forceMerge used too much temporary
space: starting usage was 385138 bytes; max temp usage was 1216310 but should have been 1155414
(= 3X starting usage)
  [beaster]    >        at __randomizedtesting.SeedInfo.seed([AEA68CE694BB7732:B4644F15FAAB94F5]:0)
  [beaster]    >        at org.junit.Assert.fail(Assert.java:93)
  [beaster]    >        at org.junit.Assert.assertTrue(Assert.java:43)
  [beaster]    >        at org.apache.lucene.index.TestIndexWriterForceMerge.testForceMergeTempSpaceUsage(TestIndexWriterForceMerge.java:162)
{noformat}

I still haven't dug into this, will do so later. But if anyone has an explanation to why we
may consume up to 4X the starting disk usage, please post here.

> 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
>
>
> 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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