Evaluating Block-level Optimization Through the IO Path

This paper focuses on evaluation of the effectiveness of optimization at various layers of the IO path, such as the file system, the device driver scheduler, and the disk drive itself. IO performance is enhanced via effective block allocation at the file system, request merging and reordering at the device driver, and additional complex request reordering at the disk drive. Our measurements show that effective combination of these optimization forms yields superior performance under specific workloads. In particular, the impact on IO performance of technological advances in modern disk drives (i.e., reduction on head positioning times and deployment of complex request scheduling) is shown. For example, if the outstanding requests in the IO subsystem can all be accommodated by the disk queue buffer then disk level request scheduling is as effective as to close any gaps in the performance between IO request schedulers at the device driver level. Even more, for disk drives with write through caches, large queue depths improve overall IO throughput and when combined with the best performing disk scheduling algorithm at the device driver level, perform comparably with an IO subsystem where disks have write-back caches.