docs: add documentation for updating Ganeti RPM package from 3.0 to 3.1

Author: jfut

doc/update-rhel-3.0-to-3.1.md

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+# Update Ganeti RPM package from 3.0 to 3.1
+
+Ganeti RPM Packaging for RHEL/CentOS/others.
+
+## Update from a version earlier than 2.16
+
+If you are updating from a version earlier than 2.16, see the document for 3.0.
+
+- [Update Ganeti RPM package from 2.16 to 3.0](https://github.com/jfut/ganeti-rpm/blob/master/doc/update-rhel-2.16-to-3.0.rst)
+
+Official documentation:
+
+- [Ganeti 3.1.0 Release](https://github.com/ganeti/ganeti/releases/tag/v3.1.0)
+
+## Backup
+
+**Mandatory** on all nodes.
+
+Stop ganeti service and backup the configuration file.
+
+```bash
+systemctl stop ganeti-metad.service
+systemctl stop ganeti-kvmd.service
+systemctl stop ganeti.target
+tar czf /root/ganeti-$(date +%FT%H-%M-%S).tar.gz -C /var/lib ganeti
+chmod 600 /root/ganeti-$(date +%FT%H)*.tar.gz
+```
+
+## Update ganeti package
+
+**Mandatory** on all nodes.
+
+```bash
+yum --enablerepo=integ-ganeti update integ-ganeti-release
+yum --enablerepo=epel,integ-ganeti update ganeti
+```
+
+## Upgrade configuration files on master node
+
+**Mandatory** on **master** node only.
+
+Upgrade the configuration files(/var/lib/ganeti).
+
+```bash
+# dry run
+/usr/lib64/ganeti/tools/cfgupgrade --verbose --dry-run
+
+# upgrade
+/usr/lib64/ganeti/tools/cfgupgrade --verbose
+```
+
+## Start ganeti services
+
+**Mandatory** on **member** nodes.
+
+```bash
+systemctl start ganeti.target
+systemctl start ganeti-kvmd.service
+
+# Optional: ganeti-metad is the daemon providing the metadata service.
+systemctl start ganeti-metad.service
+```
+
+**Mandatory** on **master** node only.
+
+```bash
+systemctl start ganeti.target
+gnt-cluster redist-conf
+
+systemctl restart ganeti.target
+systemctl start ganeti-kvmd.service
+
+# Optional: ganeti-metad is the daemon providing the metadata service.
+systemctl start ganeti-metad.service
+
+gnt-cluster verify
+```
+
+**Troubleshooting**
+
+- 'Can't find node' messages
+
+You can ignore the 'Can't find node' messages and continue.
+
+```bash
+$ /usr/lib64/ganeti/tools/cfgupgrade --verbose --dry-run
+Please make sure you have read the upgrade notes for Ganeti 3.1.0
+(available in the UPGRADE file and included in other documentation
+formats). Continue with upgrading configuration?
+y/[n]/?: y
+2025-09-24 20:48:42,530: Found configuration version 3010000 (3.1.0)
+2025-09-24 20:48:42,531: No changes necessary
+2025-09-24 20:48:42,531: Writing configuration file to /var/lib/ganeti/config.data
+Configuration successfully upgraded to version 3.1.0.
+```
+
+- Restore an old configuration and downgrade Ganeti
+
+You can restore an old configuration from a backup file and downgrade Ganeti.
+
+```bash
+# on all nodes
+systemctl stop ganeti-metad.service
+systemctl stop ganeti-kvmd.service
+systemctl stop ganeti.target
+mv /var/lib/ganeti /var/lib/ganeti.failed
+tar zxf /root/ganeti-BACKUP_DATE.tar.gz -C /var/lib
+
+# el8
+yum --enablerepo=epel,integ-ganeti downgrade ganeti-x.y.z-n.el8
+# el9
+yum --enablerepo=epel,integ-ganeti downgrade ganeti-x.y.z-n.el9
+# el10
+yum --enablerepo=epel,integ-ganeti downgrade ganeti-x.y.z-n.el10
+
+systemctl start ganeti.target
+systemctl start ganeti-kvmd.service
+# Optional: ganeti-metad is the daemon providing the metadata service.
+systemctl start ganeti-metad.service
+```