Quality of service is the ability to provide different priority to different applications, users, or data flows, or to guarantee a certain level of performance to a data flow. Up until we started virtualizing servers, you generally only needed this at the network switch level. Now with the multitenant nature of virtualization hosts, we need QoS at the network interface.As I have alluded to in my last couple of blog entries, companies like Neterion already are doing this, and I expect the Fibre Channel over Ethernet companies like Emulex to follow suit soon. Why the sudden need? First, as I mentioned above, because in a virtual environment you can have multiple virtual servers all accessing the same I/O path, the network I/O queue that the hypervisor creates becomes the bottleneck. The second issue is that the amount of bandwidth that is provided by 10 GB, no matter what the protocol (FCoE, Fibre Channel, standard IP, or iSCSI), is more than enough I/O bandwidth for most virtual machines.

With I/O quality of service you can assign multiple virtual queues to the 10 GB of bandwidth and then assign those queues to specific virtual machines. A simple example is having a 10 GB card act like 10 1-GB virtual cards. Each of these virtual cards can be assigned to 10 virtual machines, guaranteeing 10 GB of bandwidth for each.

In virtualized environments, this becomes critical and allows for deeper virtualization by virtualizing even high I/O servers, since these servers can be given their own I/O queue. This assures them the needed bandwidth while not interfering with the other workloads on the host.

Virtualization continues to change the way you solve problems, but also changes the problems themselves. In the old days of one server and one application, there was little need for I/O QoS, but with multiple virtual machines demanding I/O bandwidth, it quickly becomes a requirement.

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George Crump is founder of Storage Switzerland, an analyst firm focused on the virtualization and storage marketplaces. It provides strategic consulting and analysis to storage users, suppliers, and integrators. An industry veteran of more than 25 years, Crump has held engineering and sales positions at various IT industry manufacturers and integrators. Prior to Storage Switzerland, he was CTO at one of the nation's largest integrators.