今天想在阿里云CentOS8服务器上面安装mysql，执行yum命令报错：
Errors during downloading metadata for repository 'AppStream':
- Status code: 404 for http://mirrors.cloud.aliyuncs.com/centos/8/AppStream/x86_64/os/repodata/repomd.xml (IP: 100.100.2.148)
Error: Failed to download metadata for repo 'AppStream':
Cannot download repomd.xml: Cannot download repodata/repomd.xml: All mirrors were tried
字面意思看，可以理解为镜像地址已经失效了。搜索一番后，参考阿里云官网https://developer.aliyun.com/mirror/Centos，原因是22年元旦开始，官方停止对CentOS8的维护，需要更换yum镜像源地址。解决方案如下：
- 先备份
/etc/yum.repos.d/路径下所有.repo文件。
mv /etc/yum.repos.d/CentOS-Base.repo /etc/yum.repos.d/CentOS-Base.repo.backup
- centos8官方源已下线，建议切换centos-vault源：
wget -O /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-vault-8.5.2111.repo
若未安装wget，则使用curl:
curl -o /etc/yum.repos.d/CentOS-Base.repo https://mirrors.aliyun.com/repo/Centos-vault-8.5.2111.repo
- 执行
yum clean all&&yum makecache，成功后，可以正常执行yum命令了。
