问题：
修改本地yum源，yum clean all 后尝试yum update 或者yum makecache 报错，内容为：

failure: repodata/repomd.xml from centos7: [Errno 256] No more mirrors to try.

解决：
备份/etc/yum.repos.d 下所有的镜像，新建一个CentOS-Base.repo，并且清空所有内容，添加以下内容：
备份源：

mkdir /opt/centos-yum.bak
mv /etc/yum.repos.d/* /opt/centos-yum.bak/

创建：CentOS-Base.repo

touch CentOS-Base.repo

粘贴内容：

# geographically close to the client.  You should use this for CentOS updates
# unless you are manually picking other mirrors.
# 
# If the mirrorlist= does not work for you, as a fall back you can try the 
# remarked out baseurl= line instead.
#
[base]
name=CentOS-$releasever - Base - 163.com
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os
baseurl=http://mirrors.163.com/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-7

#released updates
[updates]
name=CentOS-$releasever - Updates - 163.com
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=updates
baseurl=http://mirrors.163.com/centos/$releasever/updates/$basearch/
gpgcheck=1
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-7

#additional packages that may be useful
[extras]
name=CentOS-$releasever - Extras - 163.com
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=extras
baseurl=http://mirrors.163.com/centos/$releasever/extras/$basearch/
gpgcheck=1
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-7

#additional packages that extend functionality of existing packages
[centosplus]
name=CentOS-$releasever - Plus - 163.com
baseurl=http://mirrors.163.com/centos/$releasever/centosplus/$basearch/
gpgcheck=1
enabled=0
gpgkey=http://mirrors.163.com/centos/RPM-GPG-KEY-CentOS-7

保存退出: wq

执行：

yum clean all

yum makecache

yum update