备份原有的Yum仓库配置：
$ mkdir /etc/yum.repos.d/backup
$ mv /etc/yum.repos.d/*.repo /etc/yum.repos.d/backup/
下载阿里云CentOS镜像的repo文件：
$ curl -o /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-7.repo
清除缓存、重建仓库数据库
$ yum clean all
$ yum makecache
