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https://issues.apache.org/jira/browse/HADOOP-3226?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Robert Chansler updated HADOOP-3226:
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Release Note:
Changed policy for running combiner. The combiner may be run multiple times as
the map's output is sorted and merged. Additionally, it may be run on the
reduce side as data is merged. The old semantics are available in Hadoop 0.18
if the user calls:
job.setCombineOnlyOnce(true);
was:
Previously, the combiner was run exactly once on each record by the map task.
Now, the combiner may be run multiple times as the map's output is sorted and
merged. Additionally, it may be run on the reduce side as data is merged.
The old semantics are available in Hadoop 0.18 if the user calls:
job.setCombineOnlyOnce(true);
Hadoop Flags: [Incompatible change, Reviewed] (was: [Reviewed,
Incompatible change])
> Run combiner when merging spills from map output
> ------------------------------------------------
>
> Key: HADOOP-3226
> URL: https://issues.apache.org/jira/browse/HADOOP-3226
> Project: Hadoop Core
> Issue Type: Improvement
> Reporter: Chris Douglas
> Assignee: Chris Douglas
> Fix For: 0.18.0
>
> Attachments: 3226-0.patch, 3226-1.patch, 3226-2.patch, 3226-3.patch
>
>
> When merging spills from the map, running the combiner should further
> diminish the volume of data we send to the reduce.
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