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Date: Thu, 12 Mar 2009 01:52:50 -0700 (PDT)
From: "Devaraj Das (JIRA)" <jira@apache.org>
To: core-dev@hadoop.apache.org
Subject: [jira] Commented: (HADOOP-5474) All reduce tasks should be
 re-executed when tasktracker with a completed map task failed
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    [ https://issues.apache.org/jira/browse/HADOOP-5474?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12681219#action_12681219 ] 

Devaraj Das commented on HADOOP-5474:
-------------------------------------

bq. In this situation, if the outputs of multi map tasks on the same dataset are different, for example outputting a random number, the outputs of maptask and the re-executed maptask will probably are different. Then the re-executed reduce tasks will read the new output of the re-executed maptask, but reduce tasks that have read the data from the failed tasktracker have read the old output. This probably will cause correctness of the result.

I think your application should be tolerant to this happening and be written assuming that maps/reduces could fail or get killed, etc. We really don't want to do what you suggest.

> All reduce tasks should be re-executed when tasktracker with a completed map task failed
> ----------------------------------------------------------------------------------------
>
>                 Key: HADOOP-5474
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5474
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: mapred
>    Affects Versions: 0.19.0
>         Environment: CentOS 5,
> hadoop-0.19.0
>            Reporter: Leitao Guo
>            Priority: Critical
>   Original Estimate: 96h
>  Remaining Estimate: 96h
>
> When a tasktracker with a completed map task failed, the map task will be re-exectuted, and all reduce tasks that haven't read the data from that tasktracker should be re-executed. But the reduce task that have read the data from that tasktracker will not be re-executed. 
> In this situation, if the outputs of multi map tasks on the same dataset are different, for example outputting a random number, the outputs of maptask and the re-executed maptask will probably are different. Then the re-executed reduce tasks will read the new output of the re-executed maptask, but reduce tasks that have read the data from the failed tasktracker have read the old output. This probably will cause correctness of the result.
> A recommended solution is that all reduce tasks should be re-executed if one tasktracker with a completed map task failed.
> Any comments? thanks!

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