Modeling and optimizing I/O throughput of multiple disks on a bus

In modern I/O architectures, multiple disk drives are attached to each I/O controller. A study of the performance of such architectures under I/O-intensive workloads has revealed a performance impairment that results from a previously unknown form of convoy behavior in disk I/O. In this paper, we describe measurements of the read performance of multiple disks that share a SCSI bus under a heavy workload, and develop and validate formulas that accurately characterize the observed performance (to within 12% on several platforms for I/O sizes in the range 16-128 KB). Two terms in the formula clearly characterize the lost performance seen in our experiments. We describe techniques to deal with the performance impairment, via user-level work-arounds that achieve greater overlap of bus transfers with disk seeks, and that increase the percentage of transfers that occur at the full bus bandwidth rather than at the lower bandwidth of a disk head. Experiments show bandwidth improvements of 10-20% when using these user-level techniques, but only in the case of large I/Os.