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From "Arun C Murthy (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-3586) keep combiner backward compatible with earlier versions of hadoop
Date Wed, 18 Jun 2008 23:51:45 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-3586?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12606149#action_12606149
] 

Arun C Murthy commented on HADOOP-3586:
---------------------------------------

+1

> 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
>          Components: mapred
>            Reporter: Olga Natkovich
>            Assignee: Chris Douglas
>            Priority: Blocker
>             Fix For: 0.18.0
>
>         Attachments: 3586-0.patch, 3586-1.patch, 3586-2.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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