> Just one point of fact, the cost of running VMs is not "orders of magnitude" higher than running on a single system image, it's maybe 1-5% on the high side.
You mean in terms of CPU overhead, when a small number of VMs is running? I can agree. That's what I do here on my desktop all the time, because I want to run both stable versions and bleeding edge versions of different distros on the same machine.
But suppose I want to build a multi-user system. I could have e.g. 1000 accounts with some 50 of them logged in concurrently. Not an issue with a single kernel resource wise (depending on the HW of course). But running 50 VMs just to get stronger privacy??? Or even 1000??? (With the 50 VM variant I'd still need some kind of "session router", to make sure everybody logging in get a VM for his own. Doesn't sound very standard if there isn't some miracle package for this purpose out there I might have missed.) Don't see your 1-5% here, I would call it no way you do that with VMs for any reasonable price or HW.
Or maybe you can? I have seen at least one 64GB server. 1GB for every VM, you could already support more than 50 VMs without even sharing common pages or swapping. But I think the overhead on memory consumption is 10s of percents, not 1-5%. And I guess the price curve for server memory in that size is not linear. (Haven't bought anything over 4GB myself, so not sure)