由于之前买的云服务器是Centos6.9 ，安装Mysql时出现：

http://mirrors.cloud.aliyuncs.com/epel/6/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 404 Not Found"
Trying other mirror.
To address this issue please refer to the below knowledge base article 

https://access.redhat.com/articles/1320623

If above article doesn't help to resolve this issue please open a ticket with Red Hat Support.

Error: Cannot retrieve repository metadata (repomd.xml) for repository: epel. Please verify its path and try again

然后查询网上资料发现是因为阿里云已经没有维护Centos6这个版本，尝试了一个晚上之后还是决定直接更换系统。
更换方法：
1、点击ID。
2、将系统停止。

3、更换系统。

4、在运行yum -y install xz。已经可以使用。