问题现象：执行yum makecache、yum update等命令报如下错误：
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
One of the configured repositories failed (Unknown),
and yum doesn't have enough cached data to continue. At this point the only
safe thing yum can do is fail. There are a few ways to work "fix" this:
1. Contact the upstream for the repository and get them to fix the problem.
2. Reconfigure the baseurl/etc. for the repository, to point to a working
upstream. This is most often useful if you are using a newer
distribution release than is supported by the repository (and the
packages for the previous distribution release still work).
3. Run the command with the repository temporarily disabled
yum --disablerepo=<repoid> ...
4. Disable the repository permanently, so yum won't use it by default. Yum
will then just ignore the repository until you permanently enable it
again or use --enablerepo for temporary usage:
yum-config-manager --disable <repoid>
or
subscription-manager repos --disable=<repoid>
5. Configure the failing repository to be skipped, if it is unavailable.
Note that yum will try to contact the repo. when it runs most commands,
so will have to try and fail each time (and thus. yum will be be much
slower). If it is a very temporary problem though, this is often a nice
compromise:
yum-config-manager --save --setopt=<repoid>.skip_if_unavailable=true
Cannot find a valid baseurl for repo: centos-sclo-sclo
找了半天，终于拼凑出了一个较为靠谱的方法，方法如下：
① 依次执行如下命令
sed -i s/mirror.centos.org/vault.centos.org/g /etc/yum.repos.d/*.reposed -i s/^#.*baseurl=http/baseurl=http/g /etc/yum.repos.d/*.reposed -i s/^mirrorlist=http/#mirrorlist=http/g /etc/yum.repos.d/*.reposed -i 's/mirrorlist/#mirrorlist/g' /etc/yum.repos.d/CentOS-*sed -i 's|#baseurl=http://mirror.centos.org|baseurl=http://vault.centos.org|g' /etc/yum.repos.d/CentOS-*② 更换yum镜像源为阿里云
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.repo③ 清空yum缓存，重新执行yum makecache命令即可
yum clean allyum makecache