By Rob Horwitz [bio]
Posted: Sep. 15, 2008

The following is a sidebar accompanying an article published by Directions on Microsoft, an independent research firm focused exclusively on Microsoft strategy & technology. More samples of our content, as well as a list of upcoming articles and reports are also available.

Microsoft's Hyper-V virtualization technology was released as a built-in feature of x64-bit versions of Windows Server 2008 in June 2008. In Oct. 2008, Microsoft plans to release its hypervisor-based virtualization technology as a free, stand-alone product called Hyper-V Server that installs directly on hardware ("bare metal") and provides no functionality other than to host virtual machines (VMs). However, any Microsoft OS hosted in a VM on Hyper-V Server (or any other vendor's hardware virtualization technology, for that matter) must be licensed separately.

Hyper-V Server consists of a hypervisor—a small, highly specialized OS that hosts VMs—as well as a scaled-down version of Windows Server 2008 that serves as a "parent OS" whose main functions are to run device drivers and perform management tasks.

Hyper-V Server has scalability and availability limitations similar to those of Windows Server 2008 Standard Edition: support for systems with a maximum of four CPUs and 32GB of physical RAM and no support for server clustering. However, Hyper-V Server offers significant licensing advantages. Unlike the regular Windows Server 2008 editions that include Hyper-V, Hyper-V Server does not automatically trigger the need for clients who access VMs to have Windows Server 2008 Client Access Licenses (CALs). Instead, clients need only the CALs that correspond to whatever OS runs within the VM. For example, users accessing a VM running Windows Server 2003 R2 need only Windows Server 2003 CALs. This may make Hyper-V Server the preferred choice for customers who want to host VMs running legacy versions of Windows Server.