Virtualisation is great for companies looking for ways to consolidate servers and, in some cases, move user desktops out of the general office into more secure datacentres. But it’s not all good news, with a number of knock-on effects to take into consideration before going down the virtualisation route. Not least of which is the strain virtualisation can put on your network.

Most servers ship with one or more Gigabit Ethernet adapters as standard these days, which is more than adequate when it comes to a single server sharing out files or hosting web applications. However, as soon as you start to host multiple virtual servers on that hardware, perhaps linked to shared iSCSI storage, the network interface can very soon become a major bottleneck.

Host desktops on the server and it gets even worse, with some companies talking about hundreds on the same physical system, all competing to use the same set of physical networking hardware.

One way of tackling this issue is to throw in extra resources in the form of plug-in adapters. However, before you do that, you need to look long and hard at how you’re going to cable those ports up and integrate them into your switch network. Moreover, you need to decide on the best way of using the extra interfaces.

Should you, for example, map virtual to physical adapters on an exclusive basis and employ some kind of load balancer, or share the adapters across several or all of your virtual machines (VMs) and balance loads that way? Could some of the traffic between VMs be better handled by virtual networking? Does your virtualisation software offer this kind of functionality and what are the implications for server performance?

Another approach might be to upgrade to 10 Gigabit Ethernet (10GbE). Indeed, virtualisation could well be the problem this technology has been looking to solve and it would at least address some of the cabling issues. However, in order to use 10GbE you have to upgrade not just adapters but the backbone switches to which they’re attached. Plus, it’s important to look long and hard at whether your chosen virtualisation software can really take advantage of the extra bandwidth this technology can provide.

Interestingly, this is a problem that yet more virtualisation may be able to solve, in the form of I/O virtualisation (IOV) implemented on the network adapter. This enables VMs to make much more efficient use of physical network interfaces, and a number of firms offer adapters to do this already, including Neterion and Solarflare. A PCI standard is being developed to make it more generally available, and that could just provide the final part of the virtualisation jigsaw.