Failed to download metadata for repo ‘AppStream’: Cannot download repomd.xml: Cannot download repodata/repomd.xml: All mirrors were tried
由于centos8官方已经停止提供服务，相应的yum源也已经移到归档源，请您在当前云服务器执行以下命令，将默认yum源修改为归档源的路径:
sed -i "s@https://repo.huaweicloud.com@https://repo.huaweicloud.com/centos-vault@g" /etc/yum.repos.d/CentOS*repo
然后
yum makecache
ok!