Node Group. A node group consists of one or more nodes, and stores partitions, or sets of replicas
(see next item).
The number of node groups in a MySQL Cluster is not directly configurable; it is a function of the
number of data nodes and of the number of replicas (NoOfReplicas  configuration parameter),
as shown here:
[number_of_node_groups] = number_of_data_nodes / NoOfReplicas
Thus, a MySQL Cluster with 4 data nodes has 4 node groups if NoOfReplicas  is set to 1
in the config.ini file, 2 node groups if NoOfReplicas  is set to 2, and 1 node group if
NoOfReplicas  is set to 4. Replicas are discussed later in this section; for more information
about NoOfReplicas , see Section 18.104.22.168, “Defining MySQL Cluster Data Nodes”.
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