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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