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From Arun C Murthy <...@yahoo-inc.com>
Subject Re: What is the reason for putting the output of one mapper task into one file ?
Date Thu, 17 Jun 2010 14:57:07 GMT
Not performance, but stability.

We used to put the output of maps in r files (where r is number of  
reduces) and quickly found out that the local disk would run out of  
inodes after running a few mid-to-large sized jobs (in terms or m * r).



On Jun 16, 2010, at 7:53 PM, Jeff Zhang wrote:

> Hi all,
> I check the source code of Mapper Task, it seems that the output of
> one mapper task is one data file and one index file. And reducer task
> will fetch part of the output of mapper.
> I am wondering why not putting the output of mapper into n files (n is
> the reducer number), since mapper task knows the Partitioner. and the
> logic will be much easier. Is there any performance consideration for
> putting the output into one file ? Thanks.
> -- 
> Best Regards
> Jeff Zhang

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