Quote on bottleneck on Virtual Machines:

Random Musings of Jeremy Jameson : Performance of Virtual Machines: "I have seen many VM environments where organizations use a high-end server with 4 or 8 state-of-the-art processors and 'gobs' of memory and subsequently assume that they can run 4 or more VMs on this one server without a problem. However, as noted earlier, CPU and memory represent only half of the resources that can ultimately become a bottleneck. In my experience, most VMs bottleneck first on disk I/O."