[ 
http://issues.apache.org/jira/browse/HADOOP-3?page=comments#action_12365934 ] 

Doug Cutting commented on HADOOP-3:
-----------------------------------

An even safer way to fix this would be to have JobClient throw an exception if 
the output directory already exists.  That way folks won't inadvertantly 
overwrite things.

> Output directories are not cleaned up before the reduces run
> ------------------------------------------------------------
>
>          Key: HADOOP-3
>          URL: http://issues.apache.org/jira/browse/HADOOP-3
>      Project: Hadoop
>         Type: Bug
>   Components: mapred
>     Reporter: Owen O'Malley
>     Priority: Minor
>  Attachments: clean-out-dir.patch
>
> The output directory for the reduces is not cleaned up and therefore if you 
> can see left overs from previous runs, if they had more reduces. For example, 
> if you run the application once with reduces=10 and then rerun with 
> reduces=8, your output directory will have frag00000 to frag00009 with the 
> first 8 fragments from the second run and the last 2 fragments from the first 
> run.

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