How to set number of Maps with Hadoop

Setting number of map tasks is not simple like the reduce tasks. User can not explicitly give a fixed number, FileInputFormat decides how to split input files using various parameters.

First one is isSplitable, determines whether file is splittable or not.
Next three variables, mapred.min.split.size, mapred.max.split.size, dfs.block.size determine the actual split size used if input is splittable. By default, min split size is 0 and max split size is Long.MAX and block size 64MB. For actual split size; minSplitSize&blockSize set the lower bound and blockSize&maxSplitSize together sets the upper bound. Here is the function to calculate:

max(minsplitsize, min(maxsplitsize, blocksize))

Note: compressed input files (eg. gzip) are not splittable, there are patches * * available.