When MTTDLs Are Not Good Enough: Providing Better Estimates of Disk Array Reliability

While Mean Time To Data Loss (MTTDL) provides an easy way to estimate the reliability of redundant disk arrays, it fails to take into account the relatively short lifetime of these arrays. The authors analyzed five different disk array organizations and compared the reliability estimates obtained using their mean times to data loss with the more exact values obtained by directly solving their corresponding Markov model. They observed that the conventional MTTDL approach generally provided a good estimate of the long-term reliability of arrays - with the exception of non-repairable arrays - while significantly underestimating the short-term reliability of these arrays.