A system in which data operations for a data replication group are received in-order, and buffered. When the operations are complete, they are stored in a non-volatile memory atomically with a group sequence number. The cache is preferably mirrored. This creates a persistent association between the data operation and the sequence number. After the atomic store is performed in at least one non-volatile cache, the data operation is propagated to another member of the data replication group along with the group sequence number. In the other member, the data operation is cached at least once in a non-volatile cache atomically with the group sequence number. In this manner, the set of group sequence numbers for a plurality of operations forms a continuous ordering of the data operations.