Outshining Mirrors: MTTDL of Fixed-Order Spiral Layouts

We evaluate the reliability of storage system schemes consisting of n data disks and n parity disks where each parity disk contains the exclusive or (XOR) of two of the n data disks. These schemes are instances of the so-called SSPiRAL (survivable storage using parity in redundant array layouts). Even though they offer the simplicity of mirroring and parity schemes, we show that they approach the performance of much more complex schemes based on erasure coding. In particular, we show that a SSPiRAL scheme defined across six disks offers an MTTDL superior that of three pairs of mirrored disks. Our scheme also offers a higher MTTDL than any scheme capable of surviving the loss of two disks.