An AIX admin claimed it could do a live update of even kernel code without a reboot. The solution for most others is clustering, needed to deal with hardware risk anyway. OpenVMS clusters have reportedly gone 20+ years without system downtime. Individual instances needed reboots so little that admins occasionally forgot how to do that. So, yes, there's precedents in other operating systems according to their users.
Note: And as far as OpenVMS, I believe it because those designers built it like their job depended on it not failing. A cluster of that OS shouldn't experience any significant downtime given about everything I've ever read on it.
That's funny. I swear I've considered just buying up a boatload of used Alpha and Itanium machines to keep a VMS cluster going another decade. Put a guard in front of it to block any attacks due to its age or protocols. People might laugh but my stuff would stay running no matter what. Example below:
Notice how the Intel hardware all failed when things heated up a bit. The AlphaServers running VMS just kept chugging along. The eventual fail-over didn't loose a single transaction. Aggravates me that I can't easily obtain such reliable IT hardware/software anymore outside eBay. I mean, HP NonStop sure as hell doesn't have a hobbyist program with used servers for $130. ;)
The mid-range HP stuff is pretty reliable. We had a DL380p Gen7 survive the switch underneath it catching fire. Had zero chassis failures on about 500 nodes in the last 12 years as well. Lose disks and power supplies all the time and the odd Ethernet interface but nothing else.
Agree with ebay. I still look around for Sun Ultra kit now and then but the wife has other ideas because it's noisy and expensive to run.
Note: And as far as OpenVMS, I believe it because those designers built it like their job depended on it not failing. A cluster of that OS shouldn't experience any significant downtime given about everything I've ever read on it.