As processor speeds have increased dramatically over the past few years, the I/O speeds of most disk systems have become a greater bottleneck to improved overall system performance. (While CPU speeds have increased at the rate of close to 50% per year, disk performance has increased by only 10-20% per year.) Moreover, as the number of disks in a system increases, the likeli- hood of a disk failure also increases, and the traditional approach of taking a failed disk off-line and restoring it from a back-up is not adequate for a system with high availability requirements.

RAID (redundant array of inexpensive disks) technology has been developed to address these performance and reliability issues. These arrays can provide both higher I/O rates and greater reliability than individual drives. Several RAID "levels" have been defined, each describing different disk array configurations with different performance characteristics. At the UUASC May meeting, member Marc Furon will review the concepts of RAID, the varying RAID levels, and the types of RAID systems available in the market today.