A Simple Equation for Estimating Reliability of an N+1 Redundant Array of Independent Disks (RAID)

This paper develops an equation that provides a good approximation of the expected number of double-disk failures (data losses) in an (N+1) redundant array of inexpensive disks (RAID) as a function of time. This paper includes the statistical bases for the equation, sources of error and inaccuracies due to approximations, and limitations of its use. The equation is simple and can be evaluated using a hand held calculator or basic spreadsheet. Accuracy depends on four input distributions, which include operational failures, operational failure restorations, latent defects, and data scrubbing. Failure and restoration distributions may represent non-homogeneous Poisson processes and, therefore, not have constant rates. Results of the equations are compared to two other hand calculation methods, to a highly accurate Monte Carlo simulation, and to actual field data for more than 10,000 RAID groups composed of 14 drives.