Go reliability and durability at Dropbox
What does "Reliability of 99.9999999999% (twelve 9s)" even mean? Obviously you have to exclude large classes of user-visible failures (network outage, account over quota) to achieve that. I don't think they're claiming less than 0.00000000001% chance of a zombie apocalypse/Mad Max/ex Machina/asteroid impact end-of-times situation. So just what failures are counted?For comparison, public telephony systems aimed for five 9s. That was usually expressed as "20 minutes downtime over 40 years, combined hardware and software budget, for outages affecting more than 32 users." One software crash requiring human intervention would count for more than 20 minutes, so you were allowed <1 of these in 40 years system lifetime.