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Desktop virtualization is more than VDI

This week, Brian Madden lays out the categories of desktop virtualization and their differing qualities, including VDI, Terminal Server and OS streaming.

The name of this website is "SearchVirtualDesktop," which implies that it covers… well… virtual desktops. But what exactly is a virtual desktop? A lot of people immediately think "VDI." But that's only partially right.

At its most basic level, "virtualization" is separating the "physical" from the "logical," so therefore "desktop virtualization" is separating the physical desktop device (laptop, desktop, etc.) from the logical desktop software (Windows).

So it's true that VDI is a desktop virtualization -- but it's only one type of desktop virtualization. The entire desktop virtualization universe is much bigger than just VDI.

For example, Terminal Server (and the various Terminal Server-based solutions like Citrix XenApp) is desktop virtualization, since it provides a desktop to a user (from the remote server) to a user's device (like a thin client) where the desktop OS is not installed on the device.

In fact, from a user's standpoint, VDI and Terminal Server are just about identical. They are both server-based computing. They both use thin clients or remote connection software on fat clients. They both present a remote desktop to the user. The only difference between VDI and Terminal Server is on the back end, where VDI is a single-user remote user and Terminal Server is a multi-user remote host.

But, even though VDI and Terminal Server are both server-based computing forms of desktop virtualization, there are other types of desktop virtualization that don't use server-based computing at all.

For example, "OS streaming" describes a technology where a company of Windows runs locally on a client device (so "client-based computing" rather than "server-based computing"). But with OS streaming, the OS image is "streamed" from a central point down to the client rather than installed locally.

OS streaming works by performing a variant of the tried-and-true "network boot," where the client boots and mounts a desktop disk image from a server across the network. So this "counts" as desktop virtualization because even though the desktop software is physically running on the desktop device, it's all managed centrally as a disk image on a server. Since no software is installed on the client, it's virtualized.

OS streaming can be used "natively" on a client device -- where the only real change is that the copy of Windows is a shared copy coming across the network instead of installed locally -- or it can be used in combination with some kind of hardware virtualization running on the client device.

Combining hardware virtualization with client devices is a whole additional category of desktop virtualization. Citrix, VMware and startup companies Virtual Computer and Neocleus all intend to release "client hypervisor" products later this year. Client hypervisors are like VMware ESX or Microsoft Hyper-V for laptops, with the idea that a laptop could easily run one or more virtual machines, bringing the benefits of image portability, management and backup and control to laptops just like servers.

Finally, other companies, such as RingCube and Mokafive, have products which allow administrators to run centrally-controlled corporate virtual machines directly on client devices on top of existing client operating systems (even Linux or Mac), combining user freedom of choice and administrator control.

So there you have it. VDI, terminal server, OS streaming, client hypervisors, client-based virtual machines -- these are all forms of desktop virtualization, and these are all things we're going to cover on

Which technology will win? How do you decide which technology should be used in which case? Stay tuned!

Brian Madden is an independent industry analyst and blogger, known throughout the world as an opinionated, supertechnical desktop virtualization expert. He has written several books and more than 1,000 articles about desktop and application virtualization. Madden's blog,, receives millions of visitors per year and is a leading source for conversation, debate and discourse about the application and desktop virtualization industry. He is also the creator of BriForum, the premier independent application delivery technical conference.

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