Yes, Availability vs Durability is the difference between temporary and permanent data loss. Four nines of availability allows for 4.5 minutes of inaccessibility per month (~ 1 hour per annum).
What I find interesting is that their numbers (500PB & 500,000,000 customers) works out to ~ 1TB/customer. Of course this is raw footprint, so there is quite a bit of protection overhead, but still that seems quite high.
Biting the hand that feeds IT
