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

For a wide variety of computational tasks, disk I/O continues to be a serious obstacle to high performance. The focus of the present paper is on systems that use multiple disks per SCSI bus. We measured the performance of concurrent random I/Os, and observed bus-related phenomena that impair performance. We describe these phenomena, and present a new I/O performance model that accurately predicts the average bandwidth achieved by aheavy workload of random reads from disks on a SCSI bus. This model, although relatively simple, predicts performance on several platforms to within 12 % for I/O sizes in the range 16{128 KB. We describe a technique to improve the I/O bandwidth by 10{20 % for random-access workloads that have large I/Os and high concurrency. This technique increases the percentage of disk head positioning time that i