private int readHeaderInt(final int offset) throws IOException {
    assert (0 <= offset && offset <= HeaderLayout.HEADER_SIZE - Integer.BYTES)
      : "header offset(=" + offset + ") must be in [0," + (HeaderLayout.HEADER_SIZE - Integer.BYTES) + "]";
    return pagedStorage.getInt(offset);
  }
private int readHeaderInt(final int offset)