Mean time to meaningless: MTTDL, Markov models, and storage system reliability

Abstract

Mean Time To Data Loss (MTTDL) has been the standard reliability metric in storage systems for more than 20 years. MTTDL represents a simple formula that can be used to compare the reliability of small disk arrays and to perform comparative trending analyses. The MTTDL metric is often misused, with egregious examples relying on the MTTDL to generate reliability estimates that span centuries or millennia. Moving forward, the storage community needs to replace MTTDL with a metric that can be used to accurately compare the reliability of systems in a way that reflects the impact of data loss in the real world. 1