https://ymgd.github.io/codereader/2018/03/12/NameNode%E6%BA%90%E7%A0%81%E5%88%86%E6%9E%90-NameNode%E7%BB%93%E6%9E%84%E5%92%8C%E5%8A%9F%E8%83%BD/#
