[pornel]

I agree that 128 bits is on the lower end of "never", but you still need to store trillions of hashes to have a one-in-a-trillion chance to see a collision (and that's already the overall probability, you don't multiply it by the number of inserts to get 1:1 chance :) I don't think anybody in the world has ever seen a collision of a cryptographically strong 128-bit hash that wasn't a bug or attack.

Birthday paradox applies when you store the items together (it's a chance of collision against any existing item in the set), so overall annual hashing churn isn't affected (more hashes against a smaller set doesn't increase your collision probability as quickly).

[hansvm]

Based on currently available public estimates, Google stores around 2^75 bytes, most of that backed by a small number of very general-purpose object stores. A lot of that is from larger files, but you're still approaching birthday-paradox numbers for in-the-wild 128-bit hash collisions.