Virtualisation-enabled and hardware-assisted 8Gigabit Fibre Channel SAN adapters allow applications to run under Microsoft's Hyper-V at "near-native transaction performance", SAN specialist QLogic has claimed.

The company said that its benchmarks showed a range of enterprise applications running on virtual machines at between 90 and 97 percent of the I/O performance possible when running on a non-virtualised Windows Server 2008 system. The tests were done in co-operation with Microsoft, it added.

Adding a virtualisation layer (or hypervisor) to a server can allow multiple physical systems to be consolidated into one by converting each server into a virtual machine (VM), but there is an inevitable performance hit. That's partly because the virtual machines have to all share the server's physical network interfaces, and the hypervisor must manage that sharing process.

Hardware-assisted SAN adapters such as QLogic's offload the main processor by taking over much of that work - and by emulating multiple adapters, they give each VM its own virtual connection to the SAN.

QLogic said that application servers fitted with its 2500 Series 8Gig SAN adapters were tested first with just Windows Server 2008, and achieved 200,000 I/Os per second. They were then tested with the Hyper-V virtualisation layer as well, and yielded very similar performance.

That's thanks to Hyper-V features such as pass-through virtual machine disk access, which enable it to take advantage of this kind of I/O acceleration, claimed Mike Schutz, Microsoft's product management director for Windows Server.

So can we infer that, without hardware-assisted Fibre Channel adapters, SAN-attached virtual servers running on Hyper-V are rather less likely to achieve native performance?

It certainly looks that way: Schutz said that the QLogic benchmarks surpassed the existing benchmark results for Windows Server 2008 with Hyper-V.

To be fair, I/O is a bottleneck in many server virtualisation and consolidation projects, and applications may suffer a performance hit whichever virtualisation technology you use. It seems clear though that the likes of Hyper-V are not as performant as their developers would have us believe - at least, not without specialist hardware support.