Dropbox posted somewhat of a whitepaper about their exabyte storage system, which exceeds the largest Swift cluster by about 2 orders of magnitude. Here's a couple of fun quotes:
The Block Index is a giant sharded MySQL cluster, fronted by an RPC service layer, plus a lot of tooling for database operations and reliability. We’d originally planned on building a dedicated key-value store for this purpose but MySQL turned out to be more than capable.
Kinda like SQLite in Swift.
Cells are self-contained logical storage clusters that store around 50PB of raw data.
And they have dozens of those. Their cell has a master node BTW. Kinda like Ceph's PG, but unlike Swift.