Letting the number of virtual machines grow exponentially could cost an organisation thousands of dollars in redundant servers and underutilised resources, is the predictable warning from virtual server management start-up Embotics.

Embotics this week released a white paper report "Estimating the Real Cost of Virtual Sprawl," examining the cost behind the sprawl. The ease with which administrators can provision virtual servers could belie the fact that the resources needed to support virtual machines can get expensive, the vendor study states.

The definition of virtual server sprawl is the proliferation of virtual machines without adequate IT control in place. Once an IT organisation pays for the basic virtualisation infrastructure, it might seem as though rolling out additional virtual machines is free.

"Applications need processing, memory, storage and networking, whether they are contained in a [virtual machine] or not," reads the report by Anthony Mar, product marketing manager at Embotics. "The more [virtual machines] you have the more of these resources you will need and the more it costs."

The company devised a set of criteria to determine the cost of under-utilised virtual machines in an organisation. Embotics interviewed customers to determine classes of costs for virtual machines. The vendor determined costs could be derived from infrastructure, management systems, server software and administration resources and expenses.

"On average, an environment of 150 [virtual machines] will have anywhere from $50,000 to $150,000 locked up in redundant [virtual machines]," Embotics estimates based on its formula.

The vendor says some of its customers have found after doing a physical audit that more than 50 percent of the virtual machines in their environment were redundant. Embotics scaled the number back to a more conservative 30 percent and estimated that the average licensing costs per generic Windows or Linux server fell between $1,000 to $3,000. One customer reported to Embotics that $50,000 of disk and licence costs were tied up in 42 offline virtual machines that had been inactive for greater than 90 days.

Embotics argues the flexible nature of a virtual server environment presents a challenge in controlling sprawl and containing costs not normally an issue within the confines of physical servers.

"Physical server costs in datacentres have a built-in limit. As they proliferate, eventually you simply run out of space or budget for power and cooling," the report reads. "If left unchecked, the costs due to virtual server sprawl will eat up your entire software licence budget, more and more administrator time, and eventually require more physical servers. It's a situation that worsens as IT budgets grow."