NDB Cluster Nodes, Node Groups, Fragment Replicas, and Partitions You can add new node groups (and thus new data nodes) online, to a running NDB Cluster; see Section 23.5.7, “Adding NDB Cluster Data Nodes Online” , for more information. Partition.   This is a portion of the data stored by the cluster. Each node is responsible for keeping at least one copy of any partitions assigned to it (that is, at least one fragment replica) available to the cluster. The number of partitions used by default by NDB Cluster depends on the number of data nodes and the number of LDM threads in use by the data nodes, as shown here: [# of partitions] = [# of data nodes] * [# of LDM threads] When using data nodes running  ndbmtd , the number of LDM threads is controlled by the setting for MaxNoOfExecutionThreads . When using  ndbd  there is a single LDM thread, which means that there are as many cluster partitions as nodes participating in the cluster. This is also the case when using ndbmtd  with  MaxNoOfExecutionThreads  set to 3 or less. (You should be aware that the number of LDM threads increases with the value of this parameter, but not in a strictly linear fashion, and that there are additional constraints on setting it; see the description of  MaxNoOfExecutionThreads  for more information.) NDB and user-defined partitioning.   NDB Cluster normally partitions  NDBCLUSTER  tables automatically. However, it is also possible to employ user-defined partitioning with  NDBCLUSTER  tables. This is subject to the following limitations: See Section 23.4.3, “ndbmtd — The NDB Cluster Data Node Daemon (Multi-Threaded)” , for more information. For more information relating to NDB Cluster and user-defined partitioning, see Section 23.1.7, “Known Limitations of NDB Cluster” , and Section 24.6.2, “Partitioning Limitations Relating to Storage Engines” . Fragment replica.   This is a copy of a cluster partition. Each node in a node group stores a fragment replica. Also sometimes known as a  partition replica . The number of fragment replicas is equal to the number of nodes per node group. A fragment replica belongs entirely to a single node; a node can (and usually does) store several fragment replicas. The following diagram illustrates an NDB Cluster with four data nodes running  ndbd , arranged in two node groups of two nodes each; nodes 1 and 2 belong to node group 0, and nodes 3 and 4 belong to node group 1. Only data nodes are shown here; although a working NDB Cluster requires an ndb_mgmd  process for cluster management and at least one SQL node to access the data stored by the cluster, these have been omitted from the figure for clarity. 3930 Note 1. Only the  KEY  and  LINEAR KEY  partitioning schemes are supported in production with  NDB  tables. 2. The maximum number of partitions that may be defined explicitly for any  NDB  table is  8 * [ number of LDM threads ] * [ number of node groups ] , the number of node groups in an NDB Cluster being determined as discussed previously in this section. When running  ndbd  for data node processes, setting the number of LDM threads has no effect (since  ThreadConfig  applies only to  ndbmtd ); in such cases, this value can be treated as though it were equal to 1 for purposes of performing this calculation.