I see, guess I'm wrong then! What's the disadvantage of Paxos Commit vs 2PC? Presumably there's a reason that the Spanner team didn't use it for distributed transactions.

edit: Looking at the paper, it looks like the tradeoff is increased amount of coordination needed.

"The Two-Phase Commit protocol is thus the degenerate case of the Paxos Commit algorithm with a single acceptor."

I suppose in Spanner, having more than one acceptor is redudant since each shard is a Paxos group anyways.