The age old question of whether or not you should virtualize a Citrix server. It use to be a diffcult question because the virtualization platforms were not very optimized for a Terminal Server workload. Many things have changed in this regard, and now it’s not such a crazy decision anymore. There are certain use cases where it’s a no brainer (license servers, web interface, secure gateway, etc). However, for pure Presentation Servers it was always a bit of a toss up. On one hand you have a situation where you know you’re going to get less users than on physical hardware, versus the other side that says since 32-bit Windows is limited to 2 GB of kernel memory, then virtualizing some Citrix servers on a 16GB or 32GB server carved up into several Terminal Server VMs will scale more users than a single physical instance on th equivalent server hardware. But that’s comparing a physical install using 32-bit Windows, not 64-bit. Still, it does make a lot of sense to at least consider virtualizing your Terminal Servers on a VM platform. Now, which platform to choose? While VMware clearly has more experience (read: existence) in doing this, XenServer has an advantage which is that they have the developers of Citrix (and to some degree Microsoft) at their disposal. With some of the recent optimizations that Citrix has made in XenServer, they’ve been able to bring the overhead of virtualization down to 7% on 64-bit Citrix XenApp. The virtualized 64-bit XenApp was able to host up to 70% more users than the virtualized 32-bit instance. This isn’t that big of a surprise though since the 64-bit instance isn’t limited by kernel memory like the 32-bit instance is. I recommend checking out this blog entry over at Citrix for some more details on how they conducted their testing if you’re interested in this.