We use an Internet data center (IDC) as a motivating example to discuss how performance and availability are interrelated. IDCs provide the means for geographically distributed Internet users to tap into other computers and applications. IDC users pay for the services they obtain, so they want them to be delivered according to established service-level agreements (SLAs). These agreements indicate service performance levels and availability. An IDC must provision enough capacity and redundant resources to ensure that it can meet its performance and availability SLAs.