The idea was to save 25% on the number of sessions the system would support. It cut its virtual desktop licenses from 4,100 static to 3,000 pooled. As it turned out, that grossly overestimated the need, Apsite says. Currently 4,200 Active Directory users use just 1,400 to 1,700 logged in sessions and just 500 to 700 of those simultaneously.

He uses the surplus sessions to give him a back-off option when deploying VDI upgrades. So if he deploys a new View version to a department and there is a problem with it, users can call up a virtual desktop from a pool of the previous version until the problem is fixed. It's as simple as logging off and logging back on, he says. He recommends having a third of the environment available in this way for migration planning.

Make sure the required applications are supported before any upgrades are made to the virtual desktop operating system, he says. The center is still migrating to Windows 7 from Windows XP, and that is being done with full support from all the required applications that must run on it.

Windows 8 is probably a long way off because all the medical applications will have to support it first. "The user interface for Windows 8 is slick," he says. "A lot of us like it but a lot of our core apps have to get their heads around it first."

(Tim Greene covers Microsoft for Network World and writes the Mostly Microsoft blog. Reach him at tgreene@nww.com and follow him on Twitter https://twitter.com/#!/Tim_Greene.)