Oracle 5.0 Reference Manual page 1539

MySQL Cluster Nodes, Node Groups, Replicas, and Partitions
The data stored by the cluster is divided into four partitions, numbered 0, 1, 2, and 3. Each partition is
stored—in multiple copies—on the same node group. Partitions are stored on alternate node groups:
• Partition 0 is stored on node group 0; a primary replica (primary copy) is stored on node 1, and a
backup replica (backup copy of the partition) is stored on node 2.
• Partition 1 is stored on the other node group (node group 1); this partition's primary replica is on node
3, and its backup replica is on node 4.
• Partition 2 is stored on node group 0. However, the placing of its two replicas is reversed from that of
Partition 0; for Partition 2, the primary replica is stored on node 2, and the backup on node 1.
• Partition 3 is stored on node group 1, and the placement of its two replicas are reversed from those
of partition 1. That is, its primary replica is located on node 4, with the backup on node 3.
What this means regarding the continued operation of a MySQL Cluster is this: so long as each node
group participating in the cluster has at least one node operating, the cluster has a complete copy of all
data and remains viable. This is illustrated in the next diagram.
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