Virtual server sprawl could eliminate the very cost savings that have driven organisations to adopt virtualisation, warns a Boeing IT professional who is leading a virtualisation initiative at the company.

"We're under the gun to try to find any kind of cost savings we can," said Jett Thompson, a Boeing computing infrastructure architect based. "Virtualisation has been one of the hot ones because there's readily identified savings there."

Boeing developed a cost model to determine the savings that can be gained through virtualisation, taking into account the cost of server and SAN hardware, operating system licences, virtualisation software and management tools, power and cooling, and labour costs.

Thompson, giving a presentation at Gartner's annual data center conference, could not reveal any numbers specific to Boeing. Instead, he detailed a hypothetical model based on the same methods he used to determine Boeing's potential cost savings.

In the model, a business with 6,000 physical servers goes from 15 percent to 75 percent virtualisation, using ESX Server. The business places about 20 virtual machines on each host and is able to get rid of several thousand physical servers. The hypothetical business saves US$28 million over five years by going virtual.

However, all of those savings can be eliminated if sprawl isn't controlled. With virtual servers easy to spin up, users may ask for large numbers of new virtual machines and it's up to IT to hold the line, Thompson said.

"If you don't have demand management and good governance in place you're actually going to cost your company money," he says. "Virtual server sprawl can wipe out any savings."

Gartner analyst Thomas Bittman also says virtual server sprawl can be tough to control and is harder to measure than physical server sprawl. "Fundamentally, we believe virtualisation sprawl can be a much bigger problem than physical sprawl," Bittman said.

In Thompson's hypothetical cost model, a 50 percent growth in server demand causes the virtualisation project to be unprofitable. The extra demand increases labour costs, storage needs, and means you have to keep extra physical servers.

The benefits to end users might make this an acceptable cost, depending on your business's needs, Thompson says. But it's important to take into account all the effects virtualisation has on the data centre.

virtualisation should bring savings in system administration labour, hardware maintenance, and power and cooling, Thompson said. Increased costs might be incurred in SAN storage and labour for physical-to-virtual migrations. But assuming you control sprawl, virtualisation is typically worth it from an ROI perspective, Thompson said."Virtualisation from a cost perspective is easy to justify," he added.