设置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
安装工具
yum install redhat-upgrade-tool preupgrade-assistant-contents
分析系统
preupg -l
preupg -s CentOS6_7
添加GPG密钥
rpm --import http://vault.centos.org/centos/RPM-GPG-KEY-CentOS-7
执行更新
centos-upgrade-tool-cli --force --cleanup-post --network 7 --instrepo=http://vault.centos.org/centos/7.2.1511/os/x86_64/
重启
reboot
启用ELRepo
rpm --import https://www.elrepo.org/RPM-GPG-KEY-elrepo.org
rpm -Uvh http://www.elrepo.org/elrepo-release-6-6.el6.elrepo.noarch.rpm
关闭 grub.conf 锁
chattr -a /boot/grub/grub.conf
安装内核（lt=long-term,ml=mainline）
yum --enablerepo=elrepo-kernel install kernel-lt
修改grub.conf,default改成新内核指向的位置
default=0
timeout=5
serial --unit=0 --speed=9600
terminal --timeout=5 serial console
title CentOS (3.10.107-1.el6.elrepo.x86_64)
root (hd0,0)
kernel /vmlinuz-3.10.107-1.el6.elrepo.x86_64 ro root=UUID=80f436e4-8c35-46c8-a5ad-0f5dd183c68a rd_NO_LUKS rd_NO_LVM LANG=en_US.UTF-8 rd_NO_MD SYSFONT=latarcyrheb-sun16 console=ttyS0 crashkernel=auto KEYBOARDTYPE=pc KEYTABLE=us rd_NO_DM
initrd /initramfs-3.10.107-1.el6.elrepo.x86_64.img
title CentOS 6 (2.6.32-504.el6.x86_64)
root (hd0,0)
kernel /vmlinuz-2.6.32-504.el6.x86_64 ro root=UUID=80f436e4-8c35-46c8-a5ad-0f5dd183c68a rd_NO_LUKS rd_NO_LVM LANG=en_US.UTF-8 rd_NO_MD SYSFONT=latarcyrheb-sun16 console=ttyS0 crashkernel=auto KEYBOARDTYPE=pc KEYTABLE=us rd_NO_DM
initrd /initramfs-2.6.32-504.el6.x86_64.img
锁定内核
chattr +a /boot/grub/grub.conf
重启
reboot
Usage: preupg [options] preupg: error: [Errno 2] No such file or directory: '/root/preupgrade/result.html'
问题原因：
openscap 版本过高
解决方法：
降低 openscap 版本
yum downgrade openscap
Downloading failed: invalid data in .treeinfo: No option 'upgrade' in section: 'images-x86_64'
使用http://vault.centos.org/的源