To answer my own question. I applied a non-repeatable random
number generator in the mapper. At mapper setup stage I generate
a pre-defined number of random numbers, then I use a counter
along the mapper. When the counter is contained in the random
number set, the Mapper executes and outputs data. The problem
now becomes how to know the ceiling of random number
[1...ceiling]. That ceiling number cannot be too small to make
sampling valid, it also cannot exceed the total number of data
records contained in each split. The problem is because my data
is not divided by line, sometimes a complete data record is
composed by multiple lines, so I am not sure how to estimate
that ceiling number ... Of course, if each line is a complete
record, that ceiling number is easy to obtain.
