Desktop virtualisation, with its promises of improved security, manageability and flexibility, may be on the verge of huge adoption, some experts are predicting.

But as with many new technologies, there is a catch. ROI is one of the main selling points, but desktop virtualisation requires significant upfront costs and it can easily take three or four years to realise financial rewards.

"I see huge interest right now, for many reasons," says Forrester analyst Natalie Lambert. "But the challenge is that desktop virtualisation is a very costly endeavour. I don't care what people tell you otherwise, they're wrong."

Gartner's latest numbers released this month predict that hosted virtual desktop revenue will quadruple this year, going from US$74.1 million worldwide in 2008 to nearly $300 million in 2009.

Meanwhile, a survey of 340 IT managers found that 41 percent are already investing in desktop virtualisation, and that the technology is a "critical priority" for 22 percent, according to IDG Research Services Group.

Respondents were virtualising 6 percent of desktops at the time of the survey, and expected to virtualise one-third by 2010. But the survey was conducted in April 2008, so recent economic changes could affect those numbers.

"Is [desktop virtualisation] going to break out in 2009? I don't see any reason it would," IDC analyst Michael Rose says. "Frankly, the current economic environment is going to be a significant barrier for adoption of virtual desktops in the data center."

True ubiquity could take another five years, given current financial problems and the nature of PC refresh cycles, he says.

Nonetheless some early adopters are reporting success, with users embracing the notion of being able to access desktops from multiple locations and multiple devices.

"[Virtualising desktops] is going to save us $250,000 per year that we were spending on desktop refreshes. There were some upfront costs, but we figure there will be a two-year ROI," says Dustin Fennell, CIO of Scottsdale Community College in Arizona.

Additionally, vendors such as VMware and Citrix are working on new ways of providing virtual desktops, which they believe will spur greater adoption.

How it works

In virtualised desktop environments, the operating system, applications and associated data are abstracted from the user's PC. Broadly speaking, there are two types of desktop virtualisation. Local desktop virtualisation runs the entire desktop environment in a protected "bubble" on the user's PC. Hosted desktop virtualisation stores the users' desktops in the data centre, requiring users to access their desktop images through a network connection.

Within these categories are several sub-types.

In the hosted desktop virtualisation realm, enterprises can store virtual desktops on a standard server accessed by multiple users simultaneously, or a PC blade architecture in which each blade typically serves just one user at a time. Users can connect to their desktops using thin clients, laptops or regular desktops, but hosted desktops usually preclude any sort of offline access.

Local desktop virtualisation is achieved either with a bare-metal, or Type 1, hypervisor, or a Type 2 hypervisor that is installed on top of the PC's operating system. Bare-metal hypervisors are not yet widely available, but vendors say they will provide better security than Type 2 hypervisors, because the bare-metal type runs independently of the client operating system. They also deliver better performance than hosted desktops, because applications run on the local client instead of a remote server. Bare-metal hypervisors are being developed by VMware and Citrix as well as start-ups Neocleus and Virtual Computer. Citrix and VMware plan to release their bare-metal hypervisors in the second half of this year, while Virtual Computer is in beta and Neocleus has released a limited version of its hypervisor.