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Why is there option to set number of mappers in map reduce

 
Monica Shiralkar
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In hadoop map reduce there is a mapper for each block. Why is there option to set number of mappers in map reduce when it is simply the number of 64 bit blocks.(or 124 bit blocks).

thanks
 
Jp bordi
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mainly for parallelism and charge, depend of the resources and context of your cluster, and size of input data for get compromise performance

http://wiki.apache.org/hadoop/HowManyMapsAndReduces
 
Monica Shiralkar
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mainly for parallelism and charge, depend of the resources and context of your cluster, and size of input data for get compromise performance



Suppose there is data of 256 MB. There will be 4 blocks of 64 MB. So the number of mappers will be 4. This is automatically decided . So what will the option to set number of mappers do?If I set it to 5 does it make sense when it should be 256/64=4 and not 5?

Thanks
 
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