People ask me what I think of Virtualisation and whether I would virtualise SBS or Cougar or 10 desktops or more.  If we are talking about OS virtualisation, rather than application virtualisation then many people miss what virtualisation is - the running of more operating systems on one single piece of hardware.  This means that before you had, for example, 3 OSs to manage and now you have 4 (the OS running the virtualisation technology / layer and the 3 OSs that you now virtualise).  Now you do have to manage less hardware, but a single hardware failure now will stop 4 OSs rather than 1, so you need to plan around this.  Obviously virtualisation gives you benefits in portability and resources management as well as potentially increased hardware specs without the same increase in costs, but you also need to be aware of the issues.

I see two major issues outside of those people often think about (as above):

1. Not enough resources.  The process of virtualising an OS costs some tax in terms of performance of CPUs, disk and memory.  If you take an OS (such as SBS) that can take 4GB of RAM and 2 x dual core Xeons and eat them for breakfast, then trying to put this cat inside a box bound by virtual limits will have bad consequences.
2. Support.  Some Microsoft technology breaks / is not supported inside virtual machines (eg ISA), but don't just think about Microsoft.  This little snippet from Computerworld shows that many ISVs see trouble trying to support virtual - App support could be the party pooper that spoils the virtualization bash

December 18, 2007 (Computerworld) -- Server virtualization feels unstoppable these days. The worldwide market for virtualization platform software grew 69% last year to $1.1 billion, according to market research firm IDC, which predicts average annual growth of 27% through 2011, when global sales are expected to total $3.5 billion.

But several factors could still stall the technology's growth, according to analysts. For instance, software licensing terms often remain too restrictive or expensive for users that want to run their databases or applications on virtualized servers. In addition, finding IT workers who have virtualization experience can be a challenge for companies.

Another factor that has been less recognized thus far is the dearth of formal support for virtualization on the part of application developers and other independent software vendors.

When an application that hasn't been certified to run on virtual servers encounters technical issues that prevent it from working properly, a user can be left in a bind if none of the involved parties -- the virtualization or operating system vendors, or the software vendor -- is willing to step up and fix the problem.

Virtualisation is an amazing solution for many problems, but like any piece of technology you need to ensure that all layers to the solution are tested and work - a single link in the chain that is unsupported is not something you want to talk to a business owner 6 months after you "saved him some money" - downtime often exceeds the benefits gained by cutting a support corner!!

ttfn

David