Gearing up for virtualisation

It is rare indeed for a set of technologies to see increased uptake during an economic downturn. Yet this is what has happened with virtualisation. Enterprise virtualisation software was immature and not widely used prior to the economic dip that started in 2008. Yet just a couple of years later, most businesses are either trialling virtualisation projects or rolling them out enterprise-wide, aware of the cost savings virtualisation can bring by increasing server utilisation and therefore reducing rackspace and energy costs.

Virtualisation comes in various flavours, including server, desktop, network and storage. Server virtualisation is the most widely adopted of the four, with interest from many firms in desktop virtualisation as well. Each form of virtualisation has a different impact on the network. We shall consider the first two.

Too often, businesses concentrate on the cost savings of server virtualisation and ignore the effect on the network. To do so can be disastrous. Though there are network advantages to server virtualisation such as fewer servers, power supplies and cabling, for example, there are many problem areas.

By concentrating resources onto fewer physical servers, the network requirements of those servers increase markedly. Each physical server will need to handle much greater volumes of traffic. For some businesses, they may need to upgrade the LAN from 10/100BaseT to gigabit speeds. New server NICs will then be required. Quality of service may need to be introduced to prioritise key applications over the now more heavily used links.

“With virtualisation, you have multiple VMs competing for the bandwidth, so that can slow down your applications,” says Martin Voelk, chief technology officer at ProNet Expert, a network consultancy. “The traffic volume per port changes significantly. It’s highly recommended [to use] gigabit Ethernet speeds.”

Unless the extra traffic on these new busier ports is addressed, latency can be expected to increase. Further latency is introduced as traffic passes through the software switches – softswitches – between applications. Vendors are attempting to develop offerings that allow softswitches to be bypassed to reduce latency, but these are yet to be released.

Another major networking risk from virtualisation results from the fact greater reliance is placed on a smaller number of servers. Should a server go down in a virtualised environment, the impact will be greater. It is therefore prudent for networking teams to provision appropriate redundancy in case of failure. Security requirements will change, too, in a virtualised environment.

Port density

Though the overall server footprint reduces with virtualisation, port density is likely to increase. This was the case at the Met Office, which has undertaken a large business-wide server virtualisation project. Before the project – which virtualised 200 physical servers onto just six hosts – the weather forecasting company required just 10 ports per cabinet, but after virtualising its datacentre, it needed 144 ports per cabinet. Provisioning these extra ports was initially problematic but was in the end solved by the purchase of additional top-of-rack switches. Extra bandwidth to the virtual machines was also required, according to the Met Office.

Desktop virtualisation has a very different impact on the network, and that impact is dependent on the flavour of desktop virtualisation chosen. For virtual desktop infrastructure (VDI), where the desktop is hosted in the datacentre and only the image is transmitted to the client, there are critical network dependencies. In this case, the link between user and datacentre must not fail.

Enterprises choosing a VDI approach must ensure the most stringent SLAs from their carrier and appropriate resilient links. However, VDI has the advantage of reducing network load because only images – and not the full files – are being sent.

For enterprises that choose to execute virtualisation on the client, there is far less impact on the network, and these machines can be treated more or less like non-virtualised machines.
With this last exception, the impact of virtualisation on the network is substantial, and networking considerations must be borne in mind alongside software constraints, licensing issues and cost savings when designing virtualisation projects.

“I’m not going to point my finger at the network and say: that’s where the problems are going to lie,” says Patrick Irwin, UK and Ireland product marketing manager at virtualisation vendor Citrix. “It’s not just a network problem. IT leaders need to think about performance from a user perspective. [Virtualisation] projects will live or die by how these devices perform for users.”