Improvements in disk speeds have not kept up with improvements in processor and memory speeds. One way to correct the resulting speed mismatch is by striping data across many disks. In this paper, we address how to stripe data to get maximum performance from the disks. Specifically, we examine how to choose the striping unit, i.e. the amount of logically contiguous data on each disk. We synthesize rules for determining the best striping unit for a given range of workloads. We show how the choice of striping unit depends on only two parameters: 1) the number of outstanding requests in the disk system at any given time, and 2) the average positioning time * data transfer rate of the disks. We derive an equation for the optimal striping unit as a function of these two parameters; we also show how to choose the striping unit without prior knowledge about the workload.