CentOS 6.1操作系統升級至Centos 7.7.1908操作過程記錄步驟如下：
1.查看下當前的系統版本
cat /etc/redhat-release
CentOS release 6.10 (Final)
2.配置更新源
vi /etc/yum.repos.d/upgradetool.repo
[upgrade]
name=upgrade
baseurl=http://vault.centos.org/centos/6/upg/x86_64/
enable=1
gpgcheck=0
安裝更新助手
由于centos 6.5自帶的助手之一版本較高，所以需要卸載了裝個更老的：
yum erase openscap -y
yum install redhat-upgrade-tool preupgrade-assistant-contents -y
如果提示以下錯誤信息：
Loaded plugins: fastestmirror, security
Setting up Install Process
Loading mirror speeds from cached hostfile
* base: mirrors.aliyun.com
* epel: my.mirrors.thegigabit.com
* extras: mirrors.aliyun.com
* updates: mirrors.aliyun.com
http://vault.centos.org/centos/6/upg/x86_64/repodata/repomd.xml: [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 404 Not Found"
Trying other mirror.
To address this issue please refer to the below wiki article
https://wiki.centos.org/yum-errors
If above article doesn't help to resolve this issue please use https://bugs.centos.org/.
No package redhat-upgrade-tool available.
No package preupgrade-assistant-contents available
可以下載rmp包進行本地安裝
https://buildlogs.centos.org/centos/6/upg/x86_64/Packages/preupgrade-assistant-1.0.2-33.0.3.el6.centos.x86_64.rpm
https://buildlogs.centos.org/centos/6/upg/x86_64/Packages/preupgrade-assistant-contents-0.5.13-1.0.5.el6.centos.noarch.rpm
https://buildlogs.centos.org/centos/6/upg/x86_64/Packages/preupgrade-assistant-ui-1.0.2-33.0.3.el6.centos.x86_64.rpm
https://buildlogs.centos.org/centos/6/upg/x86_64/Packages/redhat-upgrade-tool-0.7.22-3.el6.centos.noarch.rpm
使用yum localinstall redhat-upgrade-tool preupgrade-assistant-* -y
執行工具的預升級助手查看可用內容
preupg -s CentOS6_7
采用本地ISO更新升級系統
cd /home
wget http://mirrors.aliyun.com/centos/7.7.1908/isos/x86_64/CentOS-7-x86_64-DVD-1908.iso
centos-upgrade-tool-cli --iso=/home/CentOS-7-x86_64-DVD-1908.iso
(如果有問題可以加--force參數，但生產環境不建議)
設置好之后，執行重啟系統, 重啟系統會進入更新的操作
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