In reply to Steve.

Steve, Amazon's "eleven nines" of "data durability" is among the most creative bits of marketing nonsense ever invented.

When asked to explain the math they used, Amazon has refused.

However, I set out to 'reverse engineer' the claim. It wasn't hard.

If you do the math on the chances that two cheap disk drives (in two different locations) will both fail >>at the same instant<<, and then you further assume that both of these instantaneous failures are catastrophic, such that it is impossible for a data-recovery service to do a forensic recovery from EITHER of the two drives, then this goofy 'eleven nines' number pops out of the equation quite unsurprisingly.

http://www.enterprisestorageforum.com/features/cloud-storage-marketing-hype-amazon-glaciers-big-claims.html

In terms of availability, Amazon claims "designed for four nines", but their SLA guarantee doesn't cost them anything until they drop down below three nines to two nines.

So, in reality the comparison Bill is giving us is perfectly valid.