A Performance Evaluation of RAID Architectures

In today's computer systems, the disk I/O subsystem is often identiied as the major bottleneck to system performance. One proposed solution is the so-called redundant array of inexpensive disks (RAID). In this paper, we examine the performance of two of the most promising RAID architectures, the mirrored array and the rotated parity array. First, we propose several scheduling policies for the mirrored array and a new data layout, group-rotate declustering, and compare their performance with each other and in combination with other data layout schemes. We observe that a policy that routes reads to the disk with the smallest number of requests provides the best performance, especially when the load on the I/O system is high. Second, through a combination of simulation and analysis, we compare the performance of this mirrored array architecture to the rotated parity array architecture. This latter study shows that, i) given the same storage capacity (approximately double the number of disks), the mirrored array considerably outperforms the rotated parity array and, ii) given the same number of disks, the mirrored array still outperforms the rotated parity array in most cases, even for applications where I/O requests are for large amounts of data. The only exception occurs when the I/O size is very large; most of the requests are writes, and most of these writes perform full stripe write operations.