At work we jumped on the virtualization wagon some time ago first when User Mode Linux and then later with Xen. UML was pretty good but Xen has been great. We had a few reasons for moving from physical machines to virtual ones:
- Rack space is a recurring cost so maximizing the use of space is important. I have a philosophy of breaking up a lot of functions into their own servers. That is web servers shouldn't be also doing email for 1000 people.
- Lots of people don't need the full power of a physical machine.
- You can get a console for a virtual machine without having to visit the datacenter.
- There is hope that at some point you may never need downtime since Xen and VMWare have the ability to do live migrations from one set of hardware to another. Upgrading a machine couldn't be easier.
Now it seems that more people are starting to move towards virtualization and Xen seems to have pushed things over the edge. I think a lot of that was interest generated because Xen was free and people could get their feet wet. That is probably what also drove VMWare to make part of their suite free. Xen is still not VMWare but it is gaining ground. There is support coming for Windows and Solaris with support for NetBSD already. The main gap I see in Xen's support is in the storage area. Before they can compete with VMWare at the highest levels they need to add better support for SANs or other distributed disk setups.