Good Tool from NetApp (thank you)
NetApp developed a closed-form RAID 6 equation that calculates the reliability of data stored on NetApp RAID-DP® technology and other, more general RAID 6 (or double-parity) RAID schemes. Compared with the original mean time to data loss (MTTDL) RAID equation that was formulated by Gibson and Patterson in 1993, the NetApp RAID 6 equation is much more realistic in both assumptions and outcomes. It considers failures that can be either operational (that is, complete disk failures) or transient, such as read errors (also called latent defects).
The equation operates with nonexponential distributions for time to failure and repair that can vary over time. The equation also accounts for restorative features of modern RAID implementations, including NetApp Data ONTAP® systems such as media and RAID parity scrubbing, which proactively repair transient errors.
See the Tool here