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Date: Sat, 12 Jul 2008 17:47:31 -0700 (PDT)
From: "Runping Qi (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Commented: (HADOOP-3702) add support for chaining Maps in a
 single Map and after a Reduce [M*/RM*]
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    [ https://issues.apache.org/jira/browse/HADOOP-3702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12613141#action_12613141 ] 

Runping Qi commented on HADOOP-3702:
------------------------------------


I don't understand why the map chaining is necessary.
Why not just compoite all the function into one and use the composited one as the mapper or reducer.


> add support for chaining Maps in a single Map and after a Reduce [M*/RM*]
> -------------------------------------------------------------------------
>
>                 Key: HADOOP-3702
>                 URL: https://issues.apache.org/jira/browse/HADOOP-3702
>             Project: Hadoop Core
>          Issue Type: New Feature
>          Components: mapred
>         Environment: all
>            Reporter: Alejandro Abdelnur
>            Assignee: Alejandro Abdelnur
>            Priority: Minor
>         Attachments: patch3702.txt
>
>
> On the same input, we usually need to run multiple Maps one after the other without no Reduce. We also have to run multiple Maps after the Reduce.
> If all pre-Reduce Maps are chained together and run as a single Map a significant amount of Disk I/O will be avoided. 
> Similarly all post-Reduce Maps can be chained together and run in the Reduce phase after the Reduce.

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