Xen is a linux virtualization technology written in python twisted. Its main features are ease of use, speed and the ability to pause virtual machines or live migrate across hosts.
At first, I thought Xen was just another product like User-Mode linux or VMware, cool in theory but probably just ok in practice. Luckily, I was wrong. It is incredibly fast, in our experience at work running a very basic desktop we were running up 7 or 8 virtual machines. The main overhead is RAM, the RAM is more or less statically assigned. The rest is dynamically load balanced using custom patched kernels (unfortunately you have to build these yourself at the moment and its a bit tricky).
We are using Xen at work for security, backup and management. No more do we have to wonder how complex services are interfering with one another, we can just run monitoring software over the entire virtual machine. I envisage at some point there may be tools specifically for the purpose of monitoring Xen VM’s. No more do we have to wodner about the implications of a service being cracked and what else that will give the attacker access to, they will be trapped in the VM and have gained no wider access. Just blow the VM away and restore from backup (patching the problem hopefully!). No more do we have to wonder about complex backup procedures – just copy the Xen file or disk somewhere safe.
Of course it has extra complications, having to build cusotm kernels is a bit of a pain. I don’t think it will be too long before Xen 3.0 is integrated in the Debian tree (there may be licensing issues here, I’m not sure). Also you have to dedicate your hosts to be Xen hosts right from the start, and for critical machines this is a huge leap. Finally, it is a shame Xen is currently linux only. They plan to include BSD and Windows very soon which will be so tasty.
In other news, VMware has released a free version of their software, obviously because they are feeling the squeeze. In my opinion, Xen is the hottest thing in Linux right now and could invigorate its use once management and IT professionals get around what it means. Unfortunately, so many clowns out there calling themselves administrators know nothing about Linux and are really just Microsoft fan-boys.