Memory is another big performance-buster, Wolf says, especially with virtualizing multithreaded applications. When separate threads within an operating system continually try to refresh memory, the hypervisor's shadow page tables get backed up. The result: latency. For applications that rely heavily on memory, latency spikes and application responsiveness deteriorates. Users may start seeing connection timeouts.

"Hardware-assisted memory is one solution, but it's also a crap shoot," Wolf says. "Some applications run better with hardware-assisted memory virtualization while others run better with shadow page tables."

The fallout of lackluster performance can be huge. A company might have to fork out more cash for servers. Business execs may demand that applications be given their own servers again. "Restoring trust in virtualization technology, it may take a couple of years before a company attempts to virtualize again," Wolf says.

And There Are Even More Gotchas to Avoid

Poor performance, unprepared staff, and hidden costs are only a sampling of the pitfalls in server-virtualization adoption. Managing the whereabouts of virtual machines can be a nightmare, given that they can be moved from one physical server to another, or even walk out the door on a portable hard disk. Security risks abound, too. Audit failures due to the lack of full separation of security zones can happen more easily in a virtual environment.

And then there's the threat of virtual server sprawl; new applications are easy to get up and running in a virtual world. "Virtualization increases your appetite for software," IDC's Humphreys says. "One company went from 1,000 applications to almost 1,300." Not only are there potential software licensing costs involved but also the task of tagging and tracking the applications.

Of course, server virtualization's plethora of pitfalls won't stop people from adopting the technology. After all, the benefits in a good implementation can be tremendous. But knowing how to identify and avoid those gotchas can make the journey more pleasant and the reward that much sweeter. "There's no slowing virtualization down; the benefits are too great," Dineley says. "But you will get buried if you don't steer clear of the gotchas."

© IDG 2009