  We found that a reliability model commonly used to estimate
Mean-Time-To-Data-Loss (MTTDL), while suitable for modeling RAID 0 and RAID 5,
fails to accurately model systems having a fault-tolerance greater than 1.
Therefore, to model the reliability of RAID 6, Triple-Replication, or k-of-n
systems requires an alternate technique. In this paper, we explore some
alternatives, and evaluate their efficacy by comparing their predictions to
simulations. Our main result is a new formula which more accurately models
storage system reliability.
