THE PERFORMANCE OF PARITY PLACEMENTS IN DISK ARRAYS

Due to recent advances in CPU and memory system performance, I/O systems are increasingly limiting the performance of modern computer systems. Redundant arrays of inexpensive disks (RAID) have been proposed to meet the impending I/O crisis. RAID's substitute many small inexpensive disks for a few large expensive disks to provide higher performance (both transfer rate and I/O rate), smaller footprints, and lower power consumption at a lower cost than the large expensive disks they replace. Unfortunately, with so many small disks, media availability becomes a serious problem. RAID's provide high availability by using parity encoding of data to survive disk failures. As will be shown, the way parity is distributed in a RAID has significant consequences for performance. In particular, we investigate the performance of eight different parity placements via simulation and show that these eight parity placements span a limited space of parity placements. We show that although for small request sizes the choice of parity placement does not have a significant effect on performance, for relatively large request sizes of hundreds of kilobytes at low loads, the choice of parity placement significantly affects performance (up to 20%-30% for the typical disk array configurations that are common today). We also propose properties that are generally desirable of parity placements.