Some virtualization software (like VMware's server products)
replace the host OS completely.
No, they run a hypervisor or virtual machine manager. They do not
replace the OS, the OS runs on a "Virtual Machine", hence the name
for the technology.
Well, I was trying to explain in terms that the OP was using. The
VMM (I called it virtualization software -- my bad) is a software
layer between the guest OSes and the hardware, and that's what I was
trying to convey.
Virtualization for server environments is handy because you can
run > 1 isolated OS environments on one physical (high
performance) computer, which saves rack space and power. There
are other reasons to run services on a virtual machine:
- Sometimes it's not good to run all your services on one machine
- Botched updates can be rolled back to a known good state since a
lot of virtualization systems allow the creation of VM snapshots
- Trying new things out doesn't require a separate test machine
- Better machine utilization, a lot of times our systems are
twiddling their thumbs
...stuff like that.
The trend, especially in Windows server environments, was to place
individual service on individual "boxes" since, among other
reasons, the OS sucked so bad and crashes are so common. This way
if your Exchange server crashed your Firewall box and web box
didn't die too. So there soon became rooms full of servers.
On top of this server hw became more powerful and power hungry. The
servers were no longer utilizing their full resources, but
continued to chew the same power. By consolidating them back onto
less hardware the resources can be utilized and the power savings
and space savings are huge.
As for "testing" things on production VM boxes -- well that's just
stupid.
What happens on a VM, stays on a VM. So what's wrong with creating a
VM to f-around with a piece of software. Plus, I didn't mention
"production".
If it was just about running all your services on one machine, well
you can do that already w/o VMs.
Of course. That's why I listed other reasons for VMs.
--
- Peter Schwenk
- CITA-3, Systems Administrator
- Mathematical Sciences
- University of Delaware
- (302) 831-0437
- schwenk _at_ math _dot_ udel _dot_ edu
- http://www.math.udel.edu/~schwenk
