code: public Path getInputFileForWrite(TaskID mapId, TaskAttemptID reduceTaskId, 
                                     long size)
      throws IOException {
      // TODO *oom* should use a format here
      return lDirAlloc.getLocalPathForWrite(TaskTracker.getIntermediateOutputDir(
                         jobId.toString(), reduceTaskId.toString())
                         + "/map_" + mapId.getId() + ".out", 
                         size, conf);
    }
docs: Create a local reduce input file name.
  @param mapTaskId a map task id
  @param reduceTaskId a reduce task id
  @param size the size of the file