To say that virtual desktop infrastructure (VDI) is making a comeback would be a bit of an overstatement. Of course, the technology has come a long way since the 1960s, when pretty much everyone using a computer worked in virtual sessions akin to terminal services such as Citrix Systems' XenApp and Microsoft Windows Server Remote Desktop Session Host (RDSH).Today, there are three main flavors of VDI, desktop virtualization or remote desktop: Terminal services, persistent desktops from VMware and Citrix \u2014 who collectively own 90 percent of the market \u2014 and non-persistent desktops that include a stored user profile. (The term "VDI" refers to all three flavors and their associated infrastructure.)Persistent desktops represent an image of the user's PC stored in the data center. This is the Cadillac of VDI rollouts, meaning it's the most expensive and infrastructure-intensive, but it's also the most desirable from a user point of view.Non-persistent desktop images and terminal services, meanwhile, are temporary desktop sessions that, once terminated, return to a shared pool for someone else to use. The big difference between the two is that terminal services are aimed at workers such as call center representatives who don't need personalized desktop images or applications.Non-persistent sessions, on the other hand, can be stored with an associated user profile. When a user logs back in, he starts up where he left off and can customize his settings and applications. It's like a persistent image, only without the associated storage- and server-side overhead. Even though it's not the same as a true one-to-one experience, it comes with some of its own storage overhead, as changes must be saved over time, this makes the most sense for some adopters in terms of cost and user experience.VDI Deployment Easier Now That Tech's Cheaper, FasterUntil recently, anything but terminal services was considered too infrastructure-intensive for everyday office and knowledge workers. But new technologies and falling prices have made VDI a viable and cost-effective alternative to putting workstations on every desktop, says Kevin Strohmeyer, director of product marketing for desktops and applications at Citrix."What we've seen over the last 12 to 18 months is massive in-line deduplication technologies, host-based cache technology \u2014 basically, the realization that, with so many desktop [virtual machines], you've got the same images running copy after copy after copy on the server," Strohmeyer says. "Why do I have to send all my reads and writes out to central storage to operate these big farms of VMs? Why can't I stash and operate those locally?"Analysis: Flash Storage Offers Fast Path to VDI, Better App PerformanceAs for pricing, Strohmeyer says Citrix partner NetApp offers "all-in" VDI storage setups for $35 per user. A few years ago, that number would have been as much as $500.Several advancements explain this dramatic price drop:Lower costs for archival and cache storage.\nImproved in-line data deduplication for desktop environments.\nCheap Teradici zero clients and thin clients.\nPCoIP (VMware\/Tersdici), HDX (Citrix) and Microsoft RemoteFX user datagram protocols for streaming graphics as bitmaps.\nSupport of Android and iOS mobile operating systems.\nThe Windows 7 desktop, which is more VDI-friendly than Windows XP.\nThin provisioning of tiered hybrid storage infrastructures, using SSD for caching and HDD for archival storage.\nFast caching appliances, from companies such as Alacritech, which also extend SSD life\nNew end-to-end application performance monitoring tools from startups such as Aternity and AppNeta.\nSoftware vendors allowing VDI licenses for their products.\nHigh-performance graphics cards (GPUs) that take over the heavy lifting from server and client CPUs and push out 3-D graphics in a VM environment.Taken together, these technologies bring numerous benefits to VDI: Centralized management, patching and support; improved data security; robust DR\/BC support, including shorter data recovery times; easier BYOD roll-out and management, and centralized document and data storage.But there's an additional cost to get started, says Gartner's Mark Margevicius, vice president and research director for client computing. "The way we see it, desktop virtualization is really a premium offering. While it has great applicability, it's something customers have to be willing to spend more money on."This premium, which can approach 40 percent, comes from having to build the infrastructure to support all those virtualized desktops and, potentially, buy virtual desktop access licenses from Microsoft for each device. (Unless a company has Microsoft Software Assurance as part of its licensing, it can cost up to $100 per device to extend Windows beyond the workstation.) Some costs can be offset with converged infrastructures from vendors such as Cisco Systems, Dell, Hewlett-Packard and VCE.Niche VDI Deployments Still DominateEven with renewed interest and increased adoption, the VDI market is puny, at no more than 4 percent of all workstation deployments, though Gartner projects this number to double by 2016.VDI is most often deployed in niche environments for a small subset of users or in places such as call centers. In addition, industries that put a premium on security and ease of use, including financial services and healthcare, are warming to VDI. So is manufacturing, Strohmeyer says, which sees VDI as a way to enable secure collaboration and version control of globally distributed design-side processes and intellectual property.Related: VDI Brings Need for Endpoint Protection for VirtualizationThen again, companies that put a high value on flexibility have rolled out VDI to almost everyone. After piloting VDI in 2009, the health insurer Aetna has since rolled it out to 27,000 employees and business partners."It wasn't about saving money," says Alan Pawlack, Aetna's head of network and distribution engineering. "It was about flexibility to the business and reacting to the business shifts \u2014 where they wanted to do business, starting up new offices or new extensions to the company."The recent advances detailed above let Aetna realize the benefits of its early investments, Pawlack says. So did negotiating better terms with Microsoft \u2014 a process that gives many companies pause. "The licensing stack is still artificial, and we'd love to see that simplified," he says. "The overhead of the bureaucratic process is not needed."Meanwhile, for American Electric Power, cost remains a secondary consideration in a post-Stuxnet world. The primary driver, then, is the capability to manage and secure data and systems centrally."This seemed to be a sound practice to provide us with the opportunity to manage our administrative costs and provide a solution that is compatible with the business needs," says Derek Myers, senior manager of infrastructure and complex services.Case Study: City of Barcelona Consolidates VDI with Windows Server 2012However, a long-overdue desktop refresh and Windows 7 migration in early 2012 really drove the decision to adopt. Even then, the rollout was limited; to date, about 13 percent of the company's 23,000 employees use desktop virtualization.As that number increases over time, the utility should begin to see some return on their investment, Meyers says. "When we looked at the number of units we realized we were not going to have an immediate cost savings on the first push. It's an up-front investment."VDI Users Can Live on Cutting Edge \u2014 For a PriceFor Florida Atlantic University's College of Engineering and Computer Science, going the VDI route was part leading-edge thinking \u2014 leaders wanted to complement a newly open LEED-certified building with green technology \u2014 and part curiosity about what they could do with the latest round of technology advancements.According to Mahesh Neelakanta, director of IT for FAU's engineering department, the thinking was, "if we can do [VDI] for our office staff and our researchers, we can pretty much handle any type of staff in the university."Not only do FAU's 2,300 engineering student remotely access high-end engineering applications such as MathLab and AutoCAD remotely, Neelakanta also supports FAU branches in other parts of the state.Normally, such graphic-intensive programs are poor candidates for VDI. Aetna and American Electric Power roll out VDI for office workers and outsourcers who use the same set of applications with little user-level customization.FAU, on the other hand, can push 3D graphics to remote users by using PCoIP and zero-clients from Teradici,as well as server-side K1 and K2 graphics cards from nVidia, which virtualize and share GPU processing with all running VMs. For 3-D graphics, meanwhile, FAU relies on 18 10Gbps Lenovo blade servers, 1Gbps nVidia K1 GPUs and ATI 3-D accelerator cards. Each server will eventually be able to support up to 16 concurrent users \u2014 but at $16,000 per setup, they are costly.FAU's VDI implementation is just three months along, and Neelakanta says it's about as close to cutting edge as you can get today. In fact, FAU presented its case study alongside Teradici at VMworld 2013.Storage, Licensing, Connectivity Still Stifle VDI ImplementationEven with those technological advancements, storage \u2014 specifically, the gigabits of IOPS allocated to each user \u2014 still rates as the aspect of VDI implementation area to figure out first, Neelakanta says. Without the right storage architecture to provides users with so much IOPS that they don't know their desktop is being hosted in the cloud, VDI suffers or fails altogether.Microsoft licensing will probably cause fits for most folks, too. Finally, complexity and connectivity can be inhibitors. If you don't have a highly automated IT infrastructure with a CMMI score of 2 or 3, 10Gbps switches over fast LANs and broadband for remote workers, then attempting VDI is probably a bad idea."Even though it's really good stuff, until those [issues] get solved, this remains a niche market," Gartner's Margevicius says.Allen Bernard is a Columbus, Ohio-based writer who covers IT management and the integration of technology into the enterprise. You can reach him via email or follow him on Twitter @allen_bernard1. Follow everything from CIO.com on Twitter @CIOonline, Facebook, Google + and LinkedIn.