Modern virtualization systems must balance fair access to I/O resources while still maintaining high utilization of those resources. It is difficult to balance fairness and efficiency when scheduling disk accesses due to the non-uniform nature of disk I/O. Current open source virtualization systems, including Xen and KVM, utilize the stock Linux disk scheduler to provide access to storage. Although the Linux disk scheduler can provide good I/O performance for individual virtual machines, it does not necessarily provide equal access to disk I/O resources across competing virtual machines. This can result in unfair and unpredictable application I/O performance behavior among virtual machines. This thesis presents the Virtual Deadline I/O Scheduler, a new disk scheduler that improves the fairness of scheduling I/O resources across virtual machines. The virtual deadline scheduler makes the Linux deadline I/O scheduler virtualization-aware, enabling it to schedule I/O requests more adaptively and fairly. In particular, request deadlines are dynamically determined based upon the level of service that has been provided to the vir- tual machine from which the request originated. The virtual deadline scheduler increases fairness of I/O performance while minimizing aggregate performance degradation.