Virtualisation is revolutionising the data centre, mostly for the better. But no technology is without potential pitfalls. Problems related to management, security, ROI and power use can all trip up a virtualisation deployment that isn't planned properly.

"Virtualisation has the potential to deliver immense cost savings and technical benefits," through the consolidation of servers and reduction of space and power needs, notes Laura DiDio, lead analyst with Information Technology Intelligence. "However, these savings don't automatically happen."

Her are seven - oh, let's call them half-truths - to fully consider before your virtualisation project is implemented.

1. virtualisation will make my life easier
Virtualising servers will greatly reduce the time it takes to spin up new workloads. Some IT shops have reported being able to deploy new virtual machines (VM) in as little as 30 minutes, as opposed weeks for their physical counterparts. The promise that virtualisation simplifies IT is real in many respects.

But virtualisation simultaneously introduces management challenges that can't be ignored. IT shops need strict policies and perhaps third-party automation tools to prevent virtual server sprawl, the unchecked spread of VMs. Even if you end up with fewer physical servers, Burton Group analyst Chris Wolf says the overall number of managed objects can increase, because of the hypervisors and sheer number of VMs.

Many users assume administrative time will be lessened, but in reality the virtual infrastructure itself has to be managed and may require a new centralised storage system, says Martijn Lohmeijer, managing consultant with TriNext, an IT outsourcing and consulting firm.

Frustratingly, many software vendors don't offer the same levels of support for applications running in VMs as they do for applications running on bare metal. Microsoft eased up on some licensing restrictions last year, but analysts are still criticising Oracle and other vendors for restrictive policies related to support in virtualised environments. The calculation of software licensing fees can also be more complicated in a virtual data centre.

"Not all server virtualisation licenses are the same," DiDio says. "You have to really study the terms and conditions of your licensing contracts from the various vendors."

2. Consolidating onto fewer servers will be simple
The first goal of a server virtualisation project is often consolidation. If you can run the same number of workloads on 10 servers that you were running on 100, why not consolidate as fast as possible? Unfortunately, many IT shops that plan to consolidate end up doing so much slower than they expect, says George Pradel, director of strategic alliances for virtualisation management vendor Vizioncore. It's easy to say "every new workload has to go on a virtual machine," but moving old workloads from a physical box to a virtual one is not always a simple task, he says.

"Doing P-to-Vs, or physical to virtual conversions, there's a black art to it if you will," Pradel says. The conversions "don't happen in a vacuum. That happens in conjunction with different business units' schedules and the ability to withstand downtime."

3. Virtualisation automatically reduces power use
If you have consolidated onto fewer servers, it might be tempting to say "I've solved my power use problems." Not so fast. While you now have fewer servers using up watts, each server is running at a higher CPU capacity and has greater power needs. At Brandeis University in Massachusetts, a virtualisation project has actually increased overall power use, reports network and systems director John Turner. Although Brandeis dramatically reduced its number of servers, it is now offering more services to users because spinning up new VMs is so easy. Each new workload increases power use.

"If you walk behind the racks of virtualised servers, the heat is just pouring out of those guys," Turner says. "We're seeing heat dump into these rooms like never before."

Another issue to consider: If you're shutting off lots of servers, a data centre has to be reconfigured to prevent cooling from being directed to empty space, explains APC CTO Jim Simonelli.

"The need to consider power and cooling alongside virtualisation is becoming more and more important," he says. "If you just virtualise, but don't alter your infrastructure, you tend to be less efficient than you could be."

4. Virtualisation makes me safer
The ability to clone VMs and move them from one physical box to another opens up great possibilities for disaster recovery - and that in turn protects your business from data loss and downtime. But virtualisation, if not managed properly, also brings new security risks that could threaten the safety of data and continuity of business systems.

People and processes are often not ready for virtualisation and the security risks it introduces, IBM security expert Joshua Corman has argued.

Virtualisation brings new attack surfaces and various operational and availability risks. Consolidating many applications onto a single server "gives you a single point of failure," says DiDio.

If you're suffering from virtual server sprawl, it may be difficult to keep track of all your VMs, and it may thus be difficult to ensure that all of them are properly patched. Also, hypervisors do not perform encryption, leaving open the possibility of man-in-the-middle attacks such as Xensploit, which intercepts unencrypted data when VMs are migrated between physical servers. That doesn't mean you should avoid virtualisation altogether, but it's often best to start with minor systems and work your way up to mission-critical applications.

5. Desktop virtualisation will save me money right away
Virtualisation should make it easier to deploy new desktops to users, apply patches and perform other management tasks. Desktop virtualisation can also save money in the long run. But IT shops have to remember desktop virtualisation requires significant upfront costs, from purchasing user devices such as thin clients to back-end infrastructure such as servers, PC blades and networked storage to support VMs.

Anecdotally, Forrester Research analysts have found that enterprises spend about $860 (£520) per user, plus network upgrades, to get a desktop virtualisation project up and running in the first year. A well-done desktop virtualisation can certainly cut long-term costs. It just might take a few years to achieve ROI.

6. Virtualisation is the same as cloud computing
Virtualisation is a key enabler of cloud computing. But installing VMware on a few servers doesn't mean you've built a private cloud. In addition to virtualisation, a private cloud requires service automation technologies and a self-service interface for provisioning new resources, says IBM cloud software chief Kristof Kloeckner.

In a blog post titled Virtualisation isn't cloud computing, cloud start-up Enomaly's founder Reuven Cohen says virtualisation is a building block for cloud computing, but the real key is abstraction at every level of the IT stack.

"The key to cloud infrastructure is abstraction to the point that it 'just doesn't matter,'" Cohen wrote. "Your infrastructure is always available and completely fault tolerant. Think more along the lines of the iPhone application delivery model (App Store), and less like the desktop application models of the past. The companies that will succeed are the ones who embrace this new hybrid internet centric model. More simply, the cloud is the computer."

A cloud doesn't necessarily even need virtualisation. This has been proven by none other than Google officials, who have said they do not virtualise production hardware and instead use a job scheduling system of Google's own design to manage its many thousands of servers.

7. Virtualisation is all about technology
This one won't surprise any longtime IT veteran: Sometimes, it's the people and not the technology that gets in the way. As Corman noted, people and processes are often not ready for the new challenges raised by virtualisation.

Even if your virtualisation project is a hit, you might become a victim of your own success. Once users might realise how easy it is to spin up a VM, they may become more demanding, making it harder for IT to focus on other tasks. Conversely, there may be resistance from users who prefer to stick with physical servers.

Pradel likes to call politics the "eighth layer" of the network stack.

"The political layer is the most difficult thing you have to deal with as far as virtualisation goes," Pradel says. "You have members of your business community who I like to call serer huggers. They do not want to get rid of their physical machines, even though they will benefit from going to virtual."