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https://issues.apache.org/jira/browse/MAPREDUCE-3252?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13134591#comment-13134591
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Hudson commented on MAPREDUCE-3252:
-----------------------------------

Integrated in Hadoop-Common-trunk-Commit #1148 (See 
[https://builds.apache.org/job/Hadoop-Common-trunk-Commit/1148/])
    MAPREDUCE-3252. Fix map tasks to not rewrite data an extra time when map 
output fits in spill buffer. Contributed by Todd Lipcon.

todd : http://svn.apache.org/viewcvs.cgi/?root=Apache-SVN&view=rev&rev=1188424
Files : 
* /hadoop/common/trunk/hadoop-mapreduce-project/CHANGES.txt
* 
/hadoop/common/trunk/hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapred/MapTask.java

                
> MR2: Map tasks rewrite data once even if output fits in sort buffer
> -------------------------------------------------------------------
>
>                 Key: MAPREDUCE-3252
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-3252
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: mrv2, task
>    Affects Versions: 0.23.0
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>            Priority: Critical
>             Fix For: 0.23.0
>
>         Attachments: mr-3252.txt, mr-3252.txt
>
>
> I found that, even if the output of a map task fits entirely in its sort 
> buffer, it was rewriting the output entirely rather than just renaming the 
> first spill into place. This is due to RawLocalFileSystem.rename() falling 
> back to a copy if renameTo() fails. The first rename attempt was failing 
> because no one has called mkdir for the output directory yet.

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