Performance consequences of parity placement in disk arrays

Due to recent advances in CPU and memory system performance, 1/0 systems are increasingly limiting the performance of modern computer systems. Redundant Arrays of Inexpensive Disks (RAID) have been proposed by Patterson et. al. [10] to meet the impending 1/0 crisis. RAIDs substitute many small inexpensive disks for a few large expensive disks to provide higher performance (both transfer rate and 1/0 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. RAIDs provide high availability by using parity encoding of data to survive disk failures. As will be shown by this paper, the way parity is distributed in a RAID has significant consequences for performance. In particular, we show that for relatively large request sizes of hundreds of kilobytes, the choice of parity placement significantly affects performance (up to 20-30 percent for the typical disk array configurations that are common today) and propose properties that are generally desirable of parity placements.