centos7 yum slow download mirror the constantly changing optimization

Reference:  https://zhuanlan.zhihu.com/p/78471090

clear cache 

yum clean all

Metadata establish
 yum makecache

Update
 yum update

cd /etc/yum.repos.d/

备份Yum
 Mv CentOS-Base.Repo CentOS-Base.Repo.Backup

Download the new centos7-Base.repo of CentOS
 wget -O /etc/yum.repos.d/CentOS-Base.repo http://mirrors.aliyun.com/repo/Centos-7.repo  

Re-create the cache

 yum makecache