Hi Gianluca,
so generally speaking when host is in Maintenance status, engine doesn't communicate with this host, so you can do pretty much anything about it.
About the upgrade flow (host 4.0 -> 4.1), here's proper way(s) how to achieve that:
1. Put host to Maintenance
2. Add 4.1 repositories to the host
3. Upgrade the host - you have 2 options here:
a. UI
- Go to Hosts tab in webadmin
- Select host and click on Reinstall inside Installation menu
b. Command line
- Connect to the host using SSH and upgrade using yum
4. Activate the host
The main difference between UI and command line options is, that with UI option only necessary packages like VDSM and it's dependencies are upgraded.
Now regarding restart:
1. If host is in Maintenance, restart using SSH and shutdown/reboot execution is no problem. But if host is Up, then executing shutdown/reboot using SSH will make host NonResponsive and fencing will be executed
2. If you select Restart using SSH from Maintenance menu, it just connect using SSH and execute shutdown same way as it's done manually
3. If you select Restart using Power Management from Maintenance menu, it will execute restart using PM device and further actions are hardware dependent. On some servers it will send shutdown signal to OS similarly as if you execute reboot and afterwards it turns off power of the server (so the result may be the same as using shutdown from command line). But on different hardware it can just turn off power of the server.
So for planned restart using SSH restart either from UI or from command line is preferred way.
Martin