FAQ
Combiner that aggregates all the mappers from a machine
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Key: HADOOP-5340
URL: https://issues.apache.org/jira/browse/HADOOP-5340
Project: Hadoop Core
Issue Type: New Feature
Affects Versions: 0.19.1
Reporter: Nathan Marz

From what I can tell, the Combiner just aggregates data from a single map task. It would be useful, especially during map-only jobs, to have a combiner that aggregates data from all the map tasks on a given machine. My use case for this is to vertically partition a set of records which start out in the same files. By doing this in a map-only task, way too many files are created (About 50 files are created per input split). By pumping all the data through a reducer, a lot of unnecessary overhead occurs. With the proposed feature, I would get 50*number of machines files rather than 50*number of input splits files for this use case.
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groupcommon-dev @
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postedFeb 26, '09 at 7:07p
activeFeb 26, '09 at 7:07p
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