The effect of prefetch in scsi disk drive cache buffers

Locality of reference is known to exist in access to secondary memory (disk drive). As early as 1958 the designers of the first IBM disk drive were aware of the 80/20 rule, whereby 20% of the data accounts for 80% the requests. Currently, manufacturers of SCSI (Small Computer System Interface) disk drives use caching disk buffers to exploit this locality. However, an evaluation of this locality has been elusive in publication. The result has been a less than complete understanding of the benefit of disk cache in the memory hierarchy. This study combines a stack simulation model of cache activity, with a physical model of disk activity, to form a trace driven model of disk cache activity. The model is used to evaluate caching disk buffers in the DOS/Windows and UNIX environments, and provides cache hit rate and disk cache access time. The model is used to examine both spatial and temporal locality through the addition of cache memory and prefetch. A new prefetch technique, based on previous access patterns, is introduced and modeled as well.