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Date: Tue, 30 Aug 2011 03:48:38 +0000 (UTC)
From: "Binglin Chang (JIRA)" <jira@apache.org>
To: mapreduce-dev@hadoop.apache.org
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Subject: [jira] [Created] (MAPREDUCE-2910) Allow empty MapOutputFile
 segments
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Allow empty MapOutputFile segments
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                 Key: MAPREDUCE-2910
                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-2910
             Project: Hadoop Map/Reduce
          Issue Type: Improvement
          Components: task, tasktracker
    Affects Versions: 0.20.2, 0.23.0
            Reporter: Binglin Chang
            Priority: Minor
             Fix For: 0.23.0


As the scale of cluster and job get larger, we see a lot of empty partitions in MapOutputFile due to large reduce numbers or partition skew. When map output compression is enabled, empty map output partitions gets larger & has additional compressor/decompressor initialization overhead. 
This can be optimized by allowing empty MapOutputFile segments, where the rawLength & partLength of IndexRecord all equal to 0. Corresponding support need to be added to IFile reader, writer, and reduce shuffle copier.


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