Server virtualization is rapidly becoming a common undertaking for IT departments. As a matter of fact, many organizations are now taking the next step: evaluating their virtualization investment to see whether it's helped them achieve the server consolidation goals they previously set. And along the way many IT departments have had some surprises such as a 'performance tax,' a reduction in server or application performance as a result of virtualizing applications using hardware virtualization technology. With the advent of operating system virtualization, this evaluation process is starting to reveal that some deployments and uses are perfectly suited to hardware virtualization, while others are more suited to OS virtualization.