Desktop virtualization at a glance: VDI

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Virtual desktop infrastructure, or VDI for short, is a method for taking virtual desktops -- virtual machines running the Microsoft Windows desktop operating system -- and hosting them in the data center, where they can be centrally managed. Virtual desktop images reside on networked storage in the data center, and running virtual desktops are hosted on Windows servers using software such as VMware View or Citrix’s XenDesktop.

To access these remote virtual desktops from home of office, users run special software on a thin client or personal computing device. The client software interacts with the virtual desktop by providing screen updates and sending mouse clicks and keystrokes to the virtual desktop. But everything – the Windows desktop, Windows applications and data – reside in the data center.

Most VDI implementations use either VMware’s VMware View or Citrix’s XenDesktop. With both each user can be assigned his own dedicated, “persistent,” virtual desktop –- one image for every user –- or groups of users may have “nonpersistent” virtual desktops spun up from a common, golden image.

Dedicated image VDI: The persistent desktop

A persistent virtual desktop spins up from a dedicated Windows virtual machine image file, which runs on a back-end host. Each user has his own dedicated virtual desktop image file, so any changes the user makes –- including Windows settings, user-installed applications or other personal data –- are preserved with the image: It persists after the user logs out and back in again. This arrangement works well for knowledge workers with constantly changing or compute-intensive needs, such as engineers or developers.

Shared image VDI: The nonpersistent desktop

With nonpersistent VDI virtual desktops for many users all spin up from a single, golden image. Any changes made to the virtual desktop during a work session aren’t saved: They simply disappear after the user logs out and the user’s session spins up from the same golden image.

This approach works well for task workers, such as call center staff, who require only a very limited application set and don’t need a personalized desktop. It also saves disk space, since many users can share the same virtual desktop image file. At 10-15 GB per desktop image, the savings in networked storage costs add up, says Raj Mallempati, director of product marketing at VMware.

Both nonpersistent and persistent virtual machines can be booted up on demand, or administrators leave them in a suspended state when the user logs out so that he can get up and running quickly the next time he logs in. Suspending virtual desktops also allows the user to log out with applications still open, go home, log in from another computer, and pick up exactly where he left off.

Suspended virtual desktops come back online more quickly and use fewer resources than just letting the session continue to run. But because suspended sessions still run some background services to maintain themselves, they still consume some resources. Not enough, however, to dissuade administrators from the practice. “Shutting down virtual machines is not done,” says Raj Mallempati, director of product marketing at VMware. “Most people leave desktops suspended.”

But Mick Slattery, global lead of Workplace Enablement Services for Accenture and Avanade Accenture, says perhaps they should shut them down. Because Windows and Windows applications are not virtualization aware, he recommends terminating virtual desktop sessions for better stability. (See Suspended virtual desktops: Approach with caution.)

Finally, because nonpersistent virtual desktops spin up from a common image, administrators can create a pool of available Windows desktop virtual machines that are already spun up and ready to be assigned.

Personalization: Persistence for the nonpersistent

Personalization technology adds a layer of customization on top of a nonpersistent virtual desktop. It allows for some customization of the cookie-cutter virtual desktop image while still allowing many users to have their virtual machines generated from a single, shared baseline image file.

WMware, for example, allows users to personalize nonpersistent virtual desktops by storing custom configuration data on what it calls the “User disk.” VMware’s View Composer software automatically applies Windows personalization data (such as printer settings and wallpaper) from the User Disk to the base image at run time to create a personalized virtual desktop. Additional customizations that the user makes during a session are saved back to the user disk when the user logs out. Citrix does something similar.

Application Virtualization: A Complementary Approach

Application virtualization tools, such as VMware’s ThinApp and Citrix’s XenApp, can stream applications on demand to a physical desktop PC, where those can execute locally, or to virtual Windows desktop for execution within a hosted virtual desktop.

In the latter scenario, users may access streamed applications by way of an icon installed on the virtual Windows desktop.

Application virtualization separates the Windows application from the Windows desktop environment on which the application normally runs. For example, the virtual application does not write to the registry of the Windows desktop on which is resides. This allows administrators to avoid potential conflicts with Windows or other applications, and they can maintain fewer golden images in a nonpersistent VDI environment by moving some applications outside of the base images.

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