title: Intermediate output when a reducer is specified
body: <p>I've written a Hadoop Map Reduce job. When I run it locally, I notice that if I don't specify any reduce tasks there are some temporary files written to the output directory.  If I specify reducers no temporary files are written.  Is this normal behavior?  I would expect to see the temporary files written otherwise it would mean that the mapper is trying to do everything in memory and then transfer to the reducer in memory.  This strikes me as implausible. </p>
  
  <p>Any insights into how/when/where the mapper writes intermediate output to the file system would be appreciated. </p>
  
  <p>Thanks</p>