11 9s isn't uncommon. AWS S3 does 11 9s (upto 16 9s with cross region replication?) for data durability, too. AFAIK, AWS published papers about their use of formal methods to ascertain bugs from other parts of the system didn't creep in to affect durability/availability guarantees: https://blog.acolyer.org/2014/11/24/use-of-formal-methods-at...

This is a pretty neat and concise read on ObjectStorage in-use at BigTech, in case you're interested: https://maisonbisson.com/post/object-storage-prior-art-and-l...