Disk built-in caches: evaluation on system performance

Disk drive manufacturers are putting increasingly larger built-in caches into disk drives. Today, 2 MB buffers are common on low-end retail IDE/ATA drives, and some SCSI drives are now available with 16 MB. However, few published data are available to demonstrate that such large built-in caches can noticeably improve overall system performance. In this paper, we investigated the impact of the disk built-in cache on file system response time when the file system buffer cache becomes larger. Via detailed file system and disk system simulation, we arrive at three main conclusions: (1) With a reasonably-sized file system buffer cache (16 MB or more), there is very little performance benefit of using a built-in cache larger than 512 KB. (2) As a readahead buffer, the disk built-in cache provides noticeable performance improvements for workloads with read sequentiality, but has little positive effect on performance if there are more concurrent sequential workloads than cache segments. (3) As a writing cache, it also has some positive effects on some workloads, at the cost of reducing reliability. The disk drive industry is very cost-sensitive. Our research indicates that the current trend of using large built-in caches is unnecessary and a waste of money and power for most users. Disk manufacturers could use much smaller built-in caches to reduce the cost as well as power-consumption, without affecting performance.