A General Reliability Model for Data Storage Systems

Typical models for analysis of storage system reliability assume independent and exponentially distributed times to failure. Also the rebuild time periods are often assumed to be deterministic or to follow an exponential distribution, or alternatively a Weibull distribution. As a first step towards a generalization of these models, we consider more general non-exponential distributions for failure and rebuild times while still retaining the independence assumption. It is shown that the mean time to data loss (MTTDL) of storage systems is practically insensitive to the actual failure distribution when the storage nodes are generally reliable, that is, when their mean time to failure is much larger than their mean time to repair. This implies that MTTDL results previously obtained in the literature by assuming exponential node failure distributions may still be valid despite this unrealistic assumption. In contrast, it is shown that the MTTDL depends on the characteristics of the rebuild distribution.