The EVENODD Code and its Generalization: An Efficient Scheme for Tolerating Multiple Disk Failures in RAID Architectures

This paper contains most of the original EVENODD paper [2] and partially its generalization given in [3]. We present a novel method, that we call EVENODD, for tolerating up to two disk failures in RAID architectures. EVENODD employs the addition of only two redundant disks and consists of simple exclusive-OR computations. This redundant storage is optimal, in the sense that two failed disks cannot be retrieved with less than two redundant disks. A major advantage of EVENODD is that it only requires parity hardware, which is typically present in standard RAID-5 controllers. Hence, EVENODD can be implemented on standard RAID-5 controllers without any hardware changes. The most commonly used scheme that employs optimal redundant storage (i.e. two extra disks) is based on Reed-Solomon (RS) error-correcting codes. This scheme requires computation over finite fields and results in a more complex implementation. For example, we show that the complexity of implementing EVENODD in a disk array with 15 disks is about 50% of the one required when using the RS scheme. We then extend the EVENODD scheme to multiple parities, and we show that it can be extended to 3 parities without losing the MDS property, but for more than 3 parities, the MDS property depends on the number of disks being considered.