Competitive prefetching for concurrent sequential I/O

Abstract

During concurrent I/O workloads, sequential access to one I/O stream can be interrupted by accesses to other streams in the system. Frequent switching between multiple sequential I/O streams may severely affect I/O efficiency due to long disk seek and rotational delays of disk-based storage devices. Aggressive prefetching can improve the granularity of sequential data access in such cases, but it comes with a higher risk of retrieving unneeded data. This paper proposes a competitive prefetching strategy that controls the prefetching depth so that the overhead of disk I/O switch and unnecessary prefetching are balanced. The proposed strategy does not require a-priori information on the data access pattern, and achieves at least half the performance (in terms of I/O throughput) of the optimal offline policy. We also provide analysis on the optimality of our competitiveness result and extend the competitiveness result to capture prefetching in the case of random-access workloads. We have implemented the proposed competitive prefetching policy in Linux 2.6.10 and evaluated its performance on both standalone disks and a disk array using a variety of workloads (including two common file utilities, Linux kernel compilation, the TPC-H benchmark, the Apache web server, and index searching). Compared to the original Linux kernel, our competitive prefetching system improves performance by up to 53%. At the same time, it trails the performance of an oracle prefetching strategy by no more than 42%.