The imminent launch of Intel octal-core CPUs (codename Nehalem-EX) and servers with up to 48 cores (powered by AMD codename Magny-cours CPUs) will dramatically increase the virtualization hosts density but will highlight how the network layer is becoming one of the weakest point of high-capacity virtual infrastructures.
Anandtech just published a very interesting article on this topic, testing the performance of a couple of copper cable 10GBase-CX4 network interface cards against the popular quad-port gigabit NICs we use today in most virtualization hosts.
The benchmark measured dual-port Intel PRO/1000 PT Server adapter (82571EB) against a Supermicro AOC-STG-I2 dual-port 10Gbit/s Intel 82598EB and a Neterion Xframe-E 10Gbit/s.
Both NICs were tested with VMware vSphere 4.0 Update 1 and CentOS 5.4 guest OSes with appropriate drivers.
While NICs tested by Anandtech are not the lastest available on the market, the research still is a valuable reading for most virtualization administrators.