https://blog.wongcw.com/2019/05/28/centos7-%e6%b7%bb%e5%8a%a0%e3%80%81%e5%88%aa%e9%99%a4%e3%80%81%e8%a8%ad%e7%bd%aeswap%e4%ba%a4%e6%8f%9b%e5%88%86%e5%8d%80/