Trading capacity for performance in disk arrays

During the last two decades, while disk capacity, disk bandwidth and CPU performance have been improving at a rate of about 60% per year following Moore's law, disk access latency has only been improving at a rate of about 10% per year. The performance gap between disk access latency and the rest of the computer system has been increasing exponentially. Moreover, Redundant Array of Independent Disks (RAID) has become a standard approach to improving the bandwidth and capacity of disk subsystems after over a decade of research and development. As a result, disk access latency becomes a performance bottleneck of many I/O intensive applications. Although several techniques, such as striping, mirroring and data replication within a disk track, have been proposed to reduce disk access latency by sacrificing disk capacity, it is not clear how to systematically configure disk arrays to reduce disk access latency and improve disk throughput for a variety of workloads and disk characteristics.; This dissertation proposes a novel way of designing disk arrays that can flexibly and systematically reduce disk access latency while improving disk throughput. We call this new disk array configuration family “SR-Array” because it considers reducing both seek time and rotational delay in a balanced manner. The dissertation shows, via theoretical and experimental studies, that the SR-Array approach can indeed improve disk access latency significantly compared with existing disk array configuration techniques.; The dissertation makes several contributions. First, we have developed analytical models for disk array configurations, and we show how to use the models to guide disk array designs towards optimal configurations by considering both disk and workload characteristics. Second, we have proposed a robust disk head position prediction mechanism without any hardware support and a new algorithm to reduce both seek time and rotational delay for the SR-Array disk configurations. Third, we have implemented a prototype disk array system together with an accurate simulator in a layered approach that incorporates the configuration models. Finally, we have demonstrated that the SR-Array approach can indeed improve disk I/O performance significantly for several I/O intensive workloads over existing disk array configuration techniques.