I don't believe CockroachDB provides SERIALIZABLE consistency for the entire keyspace, rather for subsets of the keyspace that reside in the same Raft cluster.
In practice this means no cross key serializability without including a serialization token in transactions.
I may be mistaken however but that is my understanding from reading CockroachDB docs and Aphyrs post.
Cockroach Labs employee here. We provide serializable consistency for the whole keyspace. It's just that it's easier when you stay in one Raft ensemble as less can go wrong.
In practice this means no cross key serializability without including a serialization token in transactions.
I may be mistaken however but that is my understanding from reading CockroachDB docs and Aphyrs post.