ANALYSIS OF THE PERIODIC UPDATE WRITE POLICY FOR DISK CACHE

A disk cache is typically used in file systems to reduce average access time for data storage and retrieval. The "periodic update" write policy, widely used in existing computer systems, is one in which dirty cache blocks are written to a disk on a periodic basis. In this paper we determine the average response time for disk read requests when the periodic update write policy is used. Read and write load, cache-hit ratio, and the disk scheduler's ability to reduce service time under load are incorporated in the analysis, leading to design criteria that can be used to decide among competing cache write policies. The main conclusion of this paper is that the bulk arrivals generated by the periodic update policy cause a "traffic jam" effect which results in severely degraded service. Effective use of the disk cache and disk scheduling can alleviate this problem, but only under a narrow range of operating conditions. Based on this conclusion, alternate write policies that retain the periodic update policy's advantages and provide uniformly better service are proposed.