Analysis: Making sense of virtual desktops

The market for PC virtualisation software comprises products that enable users to run one or more virtual machines (VMs) on a physical PC. This type of virtualisation occurs between the guest OS running in the VM and the host hardware.

This approach is not new, and products targeting some technical uses have been available for years. Since 2006, the range of available products has expanded and new use models have emerged, doubling the number of virtualised PCs during 2007. Gartner estimates that the number of virtualised PCs increased from 4.6 million to 10.6 million during 2007.

Although licensing changes from Microsoft will drive new demand, they will also signal a point of inflection in growth for the PC virtualization software marketplace. In the longer term, the ability to run VMs will become a default part of the basic PC capabilities, embedded in motherboard and OS functions. Use of virtualisation will expand significantly, but the market for standalone PC virtualisation software will contract.

Technology definition

Each VM created with PC virtualisation software is a logical partition that has complete virtual PC hardware resources into which a PC OS can be installed. The layer of virtualisation software intercepts requests from VMs for virtual hardware resources and maps these to the physical hardware resources of the PC. This process can be multiplexed (and prioritised), so multiple VMs can be supported in parallel. In most cases, PC virtualisation software is used to run multiple operating systems simultaneously on the same physical PC.

The process of virtualisation decouples the OS in each VM from the configuration of the physical hardware, so VMs are portable between hardware configurations. There are two types of PC virtualisation software:

• Hosted, where the virtualisation layer runs on top of a host OS. The virtualisation layer is installed as an application, so almost any PC can be virtualised. The downside of this approach is that the host operating system duplicates some of the processing of guest operating systems, so the process of virtualisation adds significant performance overhead.
• Hypervisors, which run directly on hardware. Hypervisors are thin layers of virtualisation software that have much less impact on the performance of software running in the VM, but running the Microsoft Windows OS on a hypervisor requires hardware virtualisation support. This is available on some, but not all, newer PCs.

PC virtualisation products

As distinct use models have emerged, PC virtualisation products have evolved in different directions. Gartner recognises three categories of products, which differ in how they delineate a VM from the PC on which it runs. The differences among these categories are manifested in how the

PC virtualisation software is presented to the user:

1. User-configurable products, which provide the user with control of the virtualisation layer and VM configurations. The PC virtualisation software is installed and run as an application, and then used to open VMs.

2. Packaged products, which are designed to be centrally created and managed. These products enforce a firm boundary between the VM and the host OS. The PC virtualisation software layer is hidden from the user, who is presented with an icon to launch the VM in a single click.

3. Seamless products, which blend the applications installed in the VM into the host OS environment. The PC virtualisation software is transparent to the user.

The user-configurable products target the requirements of technical/academic users. Packaged products are targeted at enterprise-managed VM deployments, and the seamless products are primarily used by Mac users. Some products in the user-configurable and packaged categories also offer a seamless-interface option.

Once hypervisors move into the mainstream, all three product categories will be affected, although to different degrees. Most user-configurable implementations and many uses in the other two categories will eventually move toward hypervisors, probably switching their type of virtualisation software during the process of hardware refresh/replacement. In the longer term, use of hosted virtualisation will likely be restricted to Mac users and users who need to move VMs between different PCs.

- Brian Gammage is the vice-president and research fellow in Gartner's client computing research team.