Hadoop MapReduce: default number of mappers

Question

If I don\'t specify the number of mappers, how would the number be determined? Is there a default setting read from a configuration file (such as mapred-site.xml)?

Answer 1

It depends on a number of factors:

• Input format and particular configuration properties for the format
• for file based input formats (TextInputFormat, SequenceFileInputFormat etc):
  • Number of input files / paths
  • are the files splittable (typically compressed files are not, SequenceFiles are an exception to this)
  • block size of the files

There are probably more, but you hopefully get the idea