code: public ReadaheadRequest submitReadahead(
        String identifier, FileDescriptor fd, long off, long len) {
      ReadaheadRequestImpl req = new ReadaheadRequestImpl(
          identifier, fd, off, len);
      pool.execute(req);
      if (LOG.isTraceEnabled()) {
        LOG.trace("submit readahead: " + req);
      }
      return req;
    }
docs: Submit a request to readahead on the given file descriptor.
  @param identifier a textual identifier used in error messages, etc.
  @param fd the file descriptor to readahead
  @param off the offset at which to start the readahead
  @param len the number of bytes to read
  @return an object representing this pending request