[abadid]

Let's say you have a transaction that writes data located in different quorums atomically. This is called a "multi-home" transaction in SLOG. Most of the machinery in the paper is dealing with that case. SLOG can do that with half-round trip latency. No idea how long it would take the version of Spanner you described to do that, but presumably at least two full round trips across quorums for two-phase commit.