Performance analysis of disk cache write policies

Abstract

Many different techniques for bridging the performance gap between CPUs and I O devices have been studied. Among these is data caching at the disk controller. Of interest is the effectiveness of a disk cache under the presence of a higher level cache, such as the UNIX file buffer cache. Disk activity logs from a UNIX workstation file server have been collected and analysed. Results have been used to guide the design of disk cache architectures, which are analysed using simulations driven by the activity logs. The primary focus is on disk cache write strategies. It is shown that write-through policies achieve the best read response time, with a 4MB cache performing approximately 30% better than no cache. Write-back policies do not achieve the performance of write-through policies (nor even the performance of a system with no disk cache) due to a phenomenon referred to as ‘head thrashing’. Scheduled write-back, a new write policy that utilizes disk idle time to write dirty lines from the cache efficiently, is examined as a solution to the problem and is shown to achieve performance equivalent to that of write-through under light to moderate loads. A variation of the pure LRU replacement algorithm, LRU over clean lines, is proposed for use with scheduled write-back, and is shown to achieve read response times significantly less than pure write-through under simulated heavier loads.