[nehan]

serializability != one giant lock around your database.

concurrency and serializability go together great. it's only if transactions actually read/write or write/write the same data that they are done one at a time (and even that's not exactly true, depending on the concurrency control algorithms). MVCC often provides better performance because read-only transactions will not block writers.

and achieving serializability across database nodes is DEFINITELY NOT impossible.