code: @Override
    public OpenFileInfo[] iterativeGetOpenFiles(
      Path prefix, int millis, String start)
      throws IOException{
      return dfs.iterativeGetOpenFiles(prefix, millis, start);
    }
docs: Fetch the list of files that have been open longer than a
  specified amount of time.
  @param prefix path prefix specifying subset of files to examine
  @param millis select files that have been open longer that this
  @param start where to start searching in the case of large number
  of files returns, or null
  @return array of OpenFileInfo objects
  @throw IOException