A performance evaluation tool for RAID disk arrays

While the performance of individual disks is well understood, the performance of disk arrays configured as redundant arrays of independent disks - RAID requires further investigation. We report on a tool that can be used for the performance evaluation of k disk failure tolerant (kDFT) disk arrays. RAID0 is a 0DFT. RAID5 is a 1DFT utilizing a single check disk. RAID6, EVENODD, and RDP are 2DFTs with two check disks and a minimum level of redundancy, while RM2 is a 2DFT with a slightly higher redundancy level. The capacity overhead is less important for high capacity modern disks than the disk access overhead to update parities, especially the small write penalty. Degraded mode operation and rebuild processing incur extra overhead on surviving disks. The RAID level, configuration, operating mode (normal, degraded with 1 or 2 failures, and rebuild mode with various options), and workload characteristics have a significant effect on RAID performance, which can be evaluated using our tool. We consider here only discrete requests and derive cost functions to determine: (a) the volume of data to be transmitted; (b) the maximum attainable throughput for given disk characteristics; (c) the mean response time given the execution plans of RAID operations specified as dags (directed acyclic graphs). Analytic solutions are possible when arrivals are Poisson with the FCFS policy. Some interesting results obtained by the tool are given to illustrate its capabilities.