> Based on Backblaze’s stats, high-quality disk drives fail at 0.5-4% per year. A 4% risk per year is a 2% chance in any given week. Two simultaneous failures would happen once every 48 years, so I should be fine, right?
Either I misunderstood or there are some typos but this math seems all kind of wrong.
A 4% risk per year (assuming failure risk is independent of disk age) is less than 0.1% by week. A 2% risk per week would be a 65% risk per year!!
2 simultaneous failures at the same week for just 2 disks (again with the huge assumption of age-independent risk) would in the order of magnitude of less than 1:10^6 , so more than 20k years(31.2 k years tbc)
Of course you either change your drives every few years so the age-independent AFR still holds or you have to model the probability of failure using some exponential distribution like Poisson's. Exercise for the reader to estimate the numbers in that case.
Either I misunderstood or there are some typos but this math seems all kind of wrong.
A 4% risk per year (assuming failure risk is independent of disk age) is less than 0.1% by week. A 2% risk per week would be a 65% risk per year!!
2 simultaneous failures at the same week for just 2 disks (again with the huge assumption of age-independent risk) would in the order of magnitude of less than 1:10^6 , so more than 20k years(31.2 k years tbc)
Of course you either change your drives every few years so the age-independent AFR still holds or you have to model the probability of failure using some exponential distribution like Poisson's. Exercise for the reader to estimate the numbers in that case.