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https://issues.apache.org/jira/browse/HADOOP-3586?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Chris Douglas updated HADOOP-3586:
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Attachment: (was: 3586-0.patch)
> keep combiner backward compatible with earlier versions of hadoop
> -----------------------------------------------------------------
>
> Key: HADOOP-3586
> URL: https://issues.apache.org/jira/browse/HADOOP-3586
> Project: Hadoop Core
> Issue Type: Bug
> Reporter: Olga Natkovich
> Assignee: Chris Douglas
> Priority: Blocker
> Fix For: 0.18.0
>
> Attachments: 3586-0.patch
>
>
> In hadoop 16 and earlier, the combiner was guaranteed to run once and only
> once for each map. In 17 this compatibility was slightly broken: the combiner
> does not run if a single <K,V> occupies the entire sort buffer. In 18, this
> is further changed to where the combiner can be called multiple times on both
> map and reduce sides.
> This breaks Pig's current implementation of the combiner and it is not easy
> to fix in a short period of time.
> We would like to ask that for a way for an application to ask for a backward
> compatible behavior for some period of time until it can adjust to the new
> behavior.
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