So to get back to Spanner's availability (if you need it), you need the Calvin sequencer Paxos groups to span data centers. Since you're not exploiting the commutativity structure of transactions you need either a leader-based consensus implementation, which will have latency stalls when a leader becomes unavailable (amplified by the all-to-all communication), or you can use old-school 2 RTT Paxos, and your end latency ends up the same as Spanner.
Lest it seem like I'm not actually a fan of the log-based approach, let me point out a way in which Calvin crushes Spanner: write contention. So long as the Calvin transactions can encode the application logic, they can support extremely high write rates on contended objects in the database. Spanner's 2PL, on the other hand, has single-object object update rates visible to the naked eye.
Lest it seem like I'm not actually a fan of the log-based approach, let me point out a way in which Calvin crushes Spanner: write contention. So long as the Calvin transactions can encode the application logic, they can support extremely high write rates on contended objects in the database. Spanner's 2PL, on the other hand, has single-object object update rates visible to the naked eye.