Reducing Disk I/O Performance Sensitivity for Large Numbers of Sequential Streams

Retrieving sequential rich media content from modern commodity disks is a challenging task. As disk capacity increases, there is a need to increase the number of streams that are allocated to each disk. However, when multiple streams are accessing a single disk, throughput is dramatically reduced because of disk head seek overhead, resulting in requirements for more disks. Thus, there is a tradeoff between how many streams should be allowed to access a disk and the total throughput that can be achieved.  In this work we examine this tradeoff and provide an understanding of issues along with a practical solution. We use Disksim, a detailed architectural simulator, to examine several aspects of a modern I/O subsystem and we show the effect of various disk parameters on system performance under multiple sequential streams. Then, we propose a solution that dynamically adjusts I/O request streams, based on host and I/O subsystem parameters. We implement our approach in a real system and perform experiments with a small and a large disk configuration. Our approach improves disk throughput up to a factor of 4 with a workload of 100 sequential streams, without requiring large amounts of memory on the storage node. Moreover, it is able to adjust (statically) to different storage node configurations, essentially making the I/O subsystem insensitive to the number of I/O streams used.