[jjuhl]

I've upgraded both RAM and CPUs on IBM RS/6000 machines (pSeries these days I believe) while the system was running. Very nice capability to minimize downtime. The procedure is 100% supported. You start by telling the OS (AiX in the cases where I did this) that you are going to remove these RAM modules or these CPUs and the OS let's you know when it has moved everything off of those. Then you remove them and plug in the replacement, then tell the OS that the new hardware can be used. Very smooth process.

[iforgotpassword]

Though I think on Linux it was added for VMs, so you can scale those up and down without rebooting.