The real advantage of virtual infrastructures is flexibility, and to ensure it, IT has to be able to use VM mobility, detailed resource management, automated provisioning and change management, or the whole infrastructure won't keep working efficiently, he says.

The measure here should no longer be how high the utilization of a single server or group of VMs is in running one application, but how consistently high the utilization of the whole data center has become, Staten says.

That requires almost real-time awareness and management of data-center resources, which means being able to instrument, monitor and allocate resources to optimize performance for each workload, each server, each data center location and each physical server, according to Gartner's 2009 virtualization best-practices analysis.

Performance optimization is just one part of the equation, though, Chen says.

Costs rise dramatically with overuse or unsupervised use of licenses, not just the wasteful launch of VMs, he says.