code: public OpenFileInfo[] iterativeGetOpenFiles(
      Path prefix, int millis, String start) throws IOException {
      checkOpen();
      try {
        return namenode.iterativeGetOpenFiles(prefix.toString(), millis, start);
      } catch (RemoteException re) {
        throw re.unwrapRemoteException(AccessControlException.class,
                                       NSQuotaExceededException.class,
                                       DSQuotaExceededException.class);
      }
    }
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 where to start searching when there are large numbers of
  files returned. pass null the first time, then pass the last
  value returned by the previous call for subsequent calls.
  @return array of OpenFileInfo objects
  @throw IOException