The output of mappers are written on local disk rather than the HDFS Blocks. Because of the following reasons :
There are 2 levels of processing (Map and Reduce) involved to get the final desired outcome.
The result generated by mappers are just intermediate/temporary result which is intern result to the Reducers so writing this would be costly process and inefficient.
The final result (outcome of reducers) is stored on HDFS block.
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