I. Introduction
The past decade witnessed tremendous growth of online applications and services, including e-mail, blogging, social networking, entertainment, and e-commerce. Together with the recent trend of cloud computing, more and more data and computation are migrated to or hosted on a certain kind of physical infrastructure called Internet Data Center (IDC). IDC has many benefits including ease of management for end users and low amortized cost of ownership. For example, it uses geographical distribution and replication to improve data reliability [1]. Further, IDC operators such as Google, Microsoft, and Amazon.com, etc., can use economics of scale and statistical multiplexing to amortize the total cost of ownership over a large number of machines and users [2]–[5].