centos7系统yum更新，centos7系统yum升级，centos下yum升级，centos7系统yum更新升级，yum优化
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backupcurl -o /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repoyum makecache分别执行...