出错原因：
http://mirrors.aliyun.com/centos/6/os/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - “The requested URL returned error: 404 Not Found”
Trying other mirror.
http://mirrors.aliyuncs.com/centos/6/os/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 7 - “couldn’t connect to host”
Trying other mirror.
http://mirrors.cloud.aliyuncs.com/centos/6/os/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 6 - “Couldn’t resolve host ‘mirrors.cloud.aliyuncs.com’”
Trying other mirror.
Error: Cannot retrieve repository metadata (repomd.xml) for repository: base. Please verify its path and try again

原centos/6/ 下已经没有相关的文件，自然找不到报404，改成7即可

解决方案

1、cd /etc/yum.repos.d
2、vi CentOS-Base.repo
#命令行情况下输入,将文件中$releasever全部改成7
3、:%s/$releasever/7/g   

4、yum clean all # 清除
5、yum makecache # 缓存

$releasever 是获取你centos的版本号的，例如我的centos的版本号为6.，获取到的为6，但是已经找不到了，所以直接全局改成7即可