Performance Analysis of Disk Arrays under Failure

Disk arrays (RAID) have been proposed as a possible approach to solving the emerging I/O bottleneck problem. The performance of a RAID system when all disks are operational and the MTTF,,, (mean time to system failure) have been well studied. However, the performance of disk arrays in the presence of failed disks has not received much attention. The same techniques that provide the storage efficient redundancy of a RAID system can also result in a significant performance hit when a single disk fails. This is of importance since single disk failures are expected to be relatively frequent in a system with a large number of disks. In this paper we propose a new variation of the RAID organization that has significant advantages in both reducing the magnitude of the performance degradation when there is a single failure and can also reduce the MTTF,,,. We also discuss several strategies that can be implemented to speed the rebuild of the failed disk and thus increase the MTTF,,,. The efficacy of these strategies is shown to require the improved properties of the new RAID organization. An analysis is carried out to quantify the tradeoffs.