Вам не нужна справочная служба для знания пароля конечного пользователя. То, что можно сделать:
Существует несколько вопросов, которые поднимает Ваш вопрос:
NDB has some limitations for large tables due to it's in-memory model. For you situation, is does not make sense.
Recently, we tested Percona Cluster with some very promising results. It supports master-master replication, and has full InnoDB ACID compliance. One thing to note is the speed of queries is limited to the slowest node in the cluster - which many MySQL installations that are Master/Slave usually have a much faster Master.
If you need really fast access, you could also convert the really large data sets to MongoDB or Cassandra. Both of these are considerably faster than RDBMS like MySQL. Clustering in these NoSQL databases is native.