Emerging video surveillance, environmental monitoring applications, and constantly evolving large scientific setups require large, high-performance, and reliable storage systems with guaranteed real-time data access. These systems are often implemented using redundant arrays of independent disks (RAID). In this paper we investigate the effectiveness of preemptible disk-scheduling algorithms to achieve better quality of service (QoS) in RAID systems. We present an architecture for QoS-aware RAID systems that use Semi-preemptible IO for servicing internal disk IOs. We show when and how to preempt IOs to improve the overall performance of the RAID system. We evaluate the benefits and estimate the overhead of our approach using a preemptible RAID simulator that we have implemented.