You could cluster different versions of VMS, or cluster Alpha VMS with Itanium VMS, or all 3 at once. Alphas and Itaniums can run VAX emulators for legacy compatibility, and the copy of VMS in the emulator can be clustered with the OS on the host machine it's running on.
VMS clusters had uptimes in decades.
Compaq bought DEC. HP bought Compaq. So HP inherited VMS.
When VMS later gained a POSIX mode and TCP/IP it was renamed OpenVMS. VMS Software is now porting VMS to x86-64.
You can port *nix apps to VMS and run them natively. The clustering is still there and still works.
There's a small chance OpenVMS might enjoy a small renaissance, bringing mainframe-like uptime and resilience to commodity x86 hardware, without all that pointless faffing around with VMs and one OS running another different OS inside a VM, with all the duplication and waste that this entails.
