One poorly written application can ruin the performance of all the others. It's known as the Noisy Neighbor.
In a highly virtualized environment, the Noisy Neighbor problem can be amplified — forcing application migrations and reconfigurations that result in unhappy users. More time, more cost, more headaches.
Learn how storage performance affects virtualization, and how to reduce risk within your virtualized infrastructure in this article: You're Risking Your VMware Investment, and Here's Why.