Hosted desktop virtualization
You virtualize the entire Windows desktop environment, including applications, and host them in the data center. The user then interacts remotely with the hosted virtual desktop by exchanging keystroke, mouse and video screen updates with it.
Vendors offer two approaches based on the VDI model. Under the first option, the persistent VDI design gives every user his own virtual desktop that runs within a virtual machine on a back-end server. Each user gets his own virtual desktop that spins up from a unique, dedicated virtual machine image file containing a full install of Windows. The user owns the image, and any changes that he makes to it will be saved.
The second option presents a "nonpersistent" virtual desktop, which gets spun up on demand from a common "golden" image file and serves multiple users. When a user logs out, any changes made to the virtual desktop disappear.
Citrix presents a third option: Its "hosted shared virtual desktop" follows its XenApp/Presentation Server (server-based computing) model by offering up a simulated Windows desktop in an RDP session on Windows Server.
In cases where organizations were already using XenApp for application delivery, some IT departments have decided it would be more cost-effective to roll out XenApp as a platform for hosted shared virtual desktops rather than build a new infrastructure for VDI, says INX's Kaplan. Technically, however, he doesn't consider it to be a virtual desktop technology, since users are really running a shared Windows Server operating system, not a native Windows XP or 7 desktop operating system hosted within a virtual machine.
"While it is possible to do almost anything with XenApp that one can do with VDI, it can become very complex and burdensome. That is why it never took off as a mainstream desktop replacement solution despite the overwhelmingly compelling economics," Kaplan says. "At the root of the problem, you have Windows Server being used in a way it was never designed for."
Most of the Citrix virtual desktop deployments he's seen to date have used Citrix's XenDesktop to host nonpersistent VDI desktops, he says.
User virtualization: The next tech level
User virtualization brings desktop virtualization to the next level: separating users and the unique attributes of their work environment from both the client hardware and the virtual desktop image. The idea is to allow access from any Internet-connected device, be it a laptop, a tablet PC or a smartphone, and regardless of whether it's running a native Windows, Mac or Android operating system. That ability to achieve seamless portability across many different device types is available now but still evolving.
It is possible (although not always practical on smaller screens) to deliver a full Windows desktop -- or individual applications -- to a smartphone or tablet. Citrix offers versions of its Receiver client software for accessing virtual Windows desktops from both the Android and iOS mobile operating systems, as well as from PCs running the Windows, OS X and Linux desktop operating systems. The VMware View Client is available for the Mac, Windows and the iPad. (A Linux version of VMware View Client is available to OEMs such as thin-terminal manufacturers.)
While the client software generally supports devices running all of those operating systems, not every personal or mobile computing device will work flawlessly, nor will all the peripherals you attach to a thin client or personal computing device running desktop virtualization client software. It's important to check each virtualization software vendor's hardware compatibility list.
And in a cross-platform environment, not everything works together. For example, Citrix Receiver will stream applications only to Windows-compatible client hardware.
A lack of offline access has long been an Achilles' heel for virtual desktop technologies. Both Citrix and VMware recently introduced support for offline mode (VMware calls it "local mode"), which moves the virtual desktop image to the user's laptop and keeps it synchronized with the host version, through manual or automated updates, whenever the user has a connection.
But the technology is still maturing. "Our customers have not really adopted [offline features] yet, but we are looking at some pilots," says Scott Mayers, a principal director at Align.
-- Robert L. Mitchell
Going with the approach of nonpersistent virtual desktops saves on back-end management and infrastructure costs, since the approach uses a few golden image files rather than one for each user, and that takes up less networked storage space.
When users log out, their virtual desktops can be shut down. But it's more typical to keep the virtual desktops in a suspended state so that users can get up and running more quickly when they log back in. In fact, for nonpersistent virtual desktops, administrators may keep a pool of virtual machine sessions running or in a suspended state all of the time so that new users can get up and running quickly after logging in.
Before rolling out VDI, slow boot-up times on older PCs were one of the biggest user complaints, says Kevin Summers, CIO at Whirlpool. Now, early users of VDI are finding that they're up and running more quickly. "Employees aren't as frustrated," he says. .]
You can virtualize individual applications -- using products such as VMware's ThinApp, Microsoft's App-V or Citrix XenApp -- and then deliver those into a virtual desktop or stream them down to a physical PC on demand and have them run locally. "Application virtualization is really software distribution done in a different way," Accenture's Slattery says.
The technique also promotes stability and eliminates application conflicts by isolating the application from other Windows apps as well as from the Windows operating system. No changes are made to the registry or other settings, so this mechanism can be used to, for example, run two versions of the same application side by side, or to avoid compatibility issues when running an old Windows XP application on top of the Windows 7 operating system.