NDB Cluster Disk Data Tables Undo log files are used only by Disk Data tables, and are not needed or used by  NDB  tables that are stored in memory only. NDB Cluster tablespaces and log file groups are not implemented as files. Although not all Disk Data objects are implemented as files, they all share the same namespace. This means that  each Disk Data object  must be uniquely named (and not merely each Disk Data object of a given type). For example, you cannot have a tablespace and a log file group both named  dd1 . Assuming that you have already set up an NDB Cluster with all nodes (including management and SQL nodes), the basic steps for creating an NDB Cluster table on disk are as follows: Each of these tasks can be accomplished using SQL statements in the  mysql  client or other MySQL client application, as shown in the example that follows. To add  undo_2.log  to the log file group, use the following  ALTER LOGFILE GROUP  statement: Some items of note: There can exist at most one log file group in the same NDB Cluster at any given time. 4499 •When you add an undo log file to a log file group using  ADD UNDOFILE ’ filename ’ , a file with the name  filename  is created in the  ndb_ node_id _fs  directory within the  DataDir  of each data node in the cluster, where  node_id  is the node ID of the data node. Each undo log file is of the size specified in the SQL statement. For example, if an NDB Cluster has 4 data nodes, then the  ALTER •Every  CREATE LOGFILE GROUP  and  ALTER LOGFILE GROUP  statement must include an  ENGINE option. The only permitted values for this option are  NDBCLUSTER  and  NDB . •The  .log  file extension used here is not required. We employ it merely to make the log files easily recognizable. 1.  We create a log file group named  lg_1  using  CREATE LOGFILE GROUP . This log file group is to be made up of two undo log files, which we name  undo_1.log  and  undo_2.log , whose initial sizes are 16 MB and 12 MB, respectively. (The default initial size for an undo log file is 128 MB.) Optionally, you can also specify a size for the log file group’s undo buffer, or permit it to assume the default value of 8 MB. In this example, we set the UNDO buffer’s size at 2 MB. A log file group must be created with an undo log file; so we add  undo_1.log  to  lg_1  in this  CREATE LOGFILE GROUP  statement: 3. Create a Disk Data table that uses this tablespace for data storage. 2. Create a tablespace; assign the log file group, as well as one or more data files, to the tablespace. 1. Create a log file group, and assign one or more undo log files to it (an undo log file is also sometimes referred to as an  undofile ). CREATE LOGFILE GROUP lg_1 ADD UNDOFILE ’undo_1.log’ INITIAL_SIZE 16M UNDO_BUFFER_SIZE 2M ENGINE NDBCLUSTER; ALTER LOGFILE GROUP lg_1 ADD UNDOFILE ’undo_2.log’ INITIAL_SIZE 12M ENGINE NDBCLUSTER; Important