Clustered RAID Arrays and Their Access Costs

RAID5 (resp. RAID6) are two popular RAID designs, which can tolerate one (resp. two) disk failures, but the load of surviving disks doubles (resp. triples) when failures occur. Clustered RAID5 (resp. RAID6) disk arrays utilize a parity group size G, which is smaller than the number of disks N , so that the redundancy level is 1/G (resp. 2/G). This enables the array to sustain a peak throughput closer to normal mode operation; e.g. the load increase for RAID5 in processing read requests is given by α = (G − 1)/(N − 1). Three methods to realize clustered RAID are balanced incomplete blocks designs and nearly random permutations, which are applicable to RAID5 and RAID6, and RM2 where each data block is protected by two parity disks. We derive cost functions for the processing requirements of clustered RAID in normal and degraded modes of operation. For given disk characteristics, the cost functions can be translated into disk service times, which can be used for the performance analysis of disk arrays. Numerical results are used to quantify the level of load increase in order to determine the value of G which maintains an acceptable level of performance in degraded mode operation.