NDB Cluster Nodes, Node Groups, Fragment Replicas, and Partitions Checkpoint.   Generally speaking, when data is saved to disk, it is said that a  checkpoint  has been reached. More specific to NDB Cluster, a checkpoint is a point in time where all committed transactions are stored on disk. With regard to the  NDB  storage engine, there are two types of checkpoints which work together to ensure that a consistent view of the cluster’s data is maintained. These are shown in the following list: • Local Checkpoint (LCP) : This is a checkpoint that is specific to a single node; however, LCPs take place for all nodes in the cluster more or less concurrently. An LCP usually occurs every few minutes; the precise interval varies, and depends upon the amount of data stored by the node, the level of cluster activity, and other factors. • Global Checkpoint (GCP) : A GCP occurs every few seconds, when transactions for all nodes are synchronized and the redo-log is flushed to disk. NDB 8.0 supports partial LCPs, which can significantly improve performance under some conditions. See the descriptions of the  EnablePartialLcp  and  RecoveryWork  configuration parameters which enable partial LCPs and control the amount of storage they use. For more information about the files and directories created by local checkpoints and global checkpoints, see NDB Cluster Data Node File System Directory. This section discusses the manner in which NDB Cluster divides and duplicates data for storage. A number of concepts central to an understanding of this topic are discussed in the next few paragraphs. Data node.   An  ndbd  or  ndbmtd  process, which stores one or more  fragment replicas —that is, copies of the  partitions  (discussed later in this section) assigned to the node group of which the node is a member. Each data node should be located on a separate computer. While it is also possible to host multiple data node processes on a single computer, such a configuration is not usually recommended. It is common for the terms “node”  and “data node”  to be used interchangeably when referring to an  ndbd  or ndbmtd  process; where mentioned, management nodes ( ndb_mgmd  processes) and SQL nodes ( mysqld processes) are specified as such in this discussion. Node group.   A node group consists of one or more nodes, and stores partitions, or sets of  fragment replicas  (see next item). The number of node groups in an NDB Cluster is not directly configurable; it is a function of the number of data nodes and of the number of fragment replicas ( NoOfReplicas  configuration parameter), as shown here: Thus, an NDB 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. Fragment replicas are discussed later in this section; for more information about  NoOfReplicas , see Section 23.3.3.6, “Defining NDB Cluster Data Nodes” . All node groups in an NDB Cluster must have the same number of data nodes. Note 23.1.2 NDB Cluster Nodes, Node Groups, Fragment Replicas, and Partitions 3929 [# of node groups] = [# of data nodes] / NoOfReplicas