Allowing higher-priority requests to preempt ongoing disk IOs is of particular benefit to delay-sensitive multimedia and real-time systems. In this paper we propose Semi-preemptible IO, which divides an IO request into small temporal units of disk commands to enable preemptible disk access. We present main design strategies to allow preemption of each component of a disk access - seek, rotation, and data transfer. We analyze the performance and describe implementation challenges. Our evaluation shows that Semi-preemptible IO can substantially reduce IO waiting time with little loss in disk throughput. For example, expected waiting time for disk IOs in a video streaming system is reduced 2.1 times with the throughput loss of less than 6 percent.