入了一个HK的kvm，但是预置镜像都是老版本：centos6，debian7。因为只是为了做个DOCKER REGESTRY和扶墙，想升级下系统版本和内核，折腾了debian7无果，折腾下centos升级版本，再换内核。
网上找了一下，按照说明升级，发现已经不支持从6.x升级到最新的7.3了。
WARNING! Do not use this method to upgrade from CentOS 6.7 or higher as several system critical packages are of a higher version number than they are in centos 7 so those do not get upgraded correctly. This renders the system broken and several system tools non-functional. You will not be able to login in or use networking.
Also CentOS 6.6 fails now with the upgrade with error “Downloading failed: invalid data in .treeinfo: No section: 'checksums' ” so this is also broken on CentOS 6.6 now and can ONLY be done on CentOS 6.5.
那就先升级到7.2吧。参照官方指导
添加源
vi /etc/yum.repos.d/upgradetool.repo
[upg]
name=CentOS-$releasever - Upgrade Tool
baseurl=http://dev.centos.org/centos/6/upg/x86_64/
gpgcheck=1
enabled=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-6
Install Old Version OpenSCP
因为如果不重新装旧版本的openscp就会提示错误：preupg: error: [Errno 2] No such file or directory: ‘/root/preupgrade/result.html’
yum erase openscap
yum install http://dev.centos.org/centos/6/upg/x86_64/Packages/openscap-1.0.8-1.0.1.el6.centos.x86_64.rpm
yum install redhat-upgrade-tool preupgrade-assistant-contents
导入RPM KEY升级
这里不可以导入最新的，因为最新版的.treeinfo中没有upgrade项了，所以这里导入7.2的。
rpm --import http://vault.centos.org/centos/7.2.1511/os/x86_64/RPM-GPG-KEY-CentOS-7
centos-upgrade-tool-cli --network 7 --instrepo=http://vault.centos.org/centos/7.2.1511/os/x86_64/
Finally
清理el6的rpm包 rpm -qa | grep -i el6 ，具体删什么升级什么，视个人情况。
el6依赖包版本降级 yum downgrade grep