code: DFSInputStream open(String src, int buffersize, boolean verifyChecksum,
        FileSystem.Statistics stats, boolean clearOsBuffer,
        ReadOptions options
        ) throws IOException {
      checkOpen();
  
      incFileReadToStats();
      // Get block info from namenode
      DFSInputStream stream = new DFSInputStream(this, src, buffersize,
          verifyChecksum, clearOsBuffer, options);
      if (blockLocationRenewal != null) {
        blockLocationRenewal.add(stream);
      }
      return stream;
    }
docs: Create an input stream that obtains a nodelist from the
  namenode, and then reads from all the right places.  Creates
  inner subclass of InputStream that does the right out-of-band
  work.