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