The INFORMATION_SCHEMA FILES Table The  FREE_EXTENTS  column in this row shows the total number of free extents available to all undo files belonging to a given log file group whose name and number are shown in the  LOGFILE_GROUP_NAME and  LOGFILE_GROUP_NUMBER  columns, respectively. Suppose there are no existing log file groups on your NDB Cluster, and you create one using the following statement: mysql>  CREATE LOGFILE GROUP lg1 ADD UNDOFILE ’undofile.dat’ INITIAL_SIZE = 16M UNDO_BUFFER_SIZE = 1M ENGINE = NDB; mysql>  SELECT DISTINCT FILE_NAME AS File, FREE_EXTENTS AS Free, TOTAL_EXTENTS AS Total, EXTENT_SIZE AS Size, INITIAL_SIZE AS Initial FROM INFORMATION_SCHEMA.FILES; mysql>  ALTER LOGFILE GROUP lg1 ADD UNDOFILE ’undofile02.dat’ INITIAL_SIZE = 4M ENGINE = NDB; mysql>  SELECT DISTINCT FILE_NAME AS File, FREE_EXTENTS AS Free, TOTAL_EXTENTS AS Total, EXTENT_SIZE AS Size, INITIAL_SIZE AS Initial FROM INFORMATION_SCHEMA.FILES; mysql>  SELECT FREE_EXTENTS AS ’Free Extents’, FREE_EXTENTS * EXTENT_SIZE AS ’Free Bytes’ FROM INFORMATION_SCHEMA.FILES WHERE LOGFILE_GROUP_NAME = ’lg1’ AND FILE_NAME IS NULL; +----------------+---------+---------+------+----------+ | File | Free | Total | Size | Initial | +----------------+---------+---------+------+----------+ | undofile.dat | NULL | 4194304 | 4 | 16777216 | | undofile02.dat | NULL | 1048576 | 4 | 4194304 | | NULL | 5223944 | NULL | 4 | NULL | +----------------+---------+---------+------+----------+ +--------------+---------+---------+------+----------+ | File | Free | Total | Size | Initial | +--------------+---------+---------+------+----------+ | undofile.dat | NULL | 4194304 | 4 | 16777216 | | NULL | 4184068 | NULL | 4 | NULL | +--------------+---------+---------+------+----------+ You can now see this  NULL  row when you query the  FILES  table: The total number of free extents available for undo logging is always somewhat less than the sum of the  TOTAL_EXTENTS  column values for all undo files in the log file group due to overhead required for maintaining the undo files. This can be seen by adding a second undo file to the log file group, then repeating the previous query against the  FILES  table: The amount of free space in bytes which is available for undo logging by Disk Data tables using this log file group can be approximated by multiplying the number of free extents by the initial size: 4798