Syntax:
ALTER LOGFILE GROUP logfile_group
    ADD UNDOFILE 'file_name'
    [INITIAL_SIZE [=] size]
    [WAIT]
    ENGINE [=] engine_name

This statement adds an UNDO file named 'file_name' to an existing log
file group logfile_group. An ALTER LOGFILE GROUP statement has one and
only one ADD UNDOFILE clause. No DROP UNDOFILE clause is currently
supported.

*Note*:

All NDB Cluster Disk Data objects 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 an undo log file with the same name, or an undo log file
and a data file with the same name.

The optional INITIAL_SIZE parameter sets the UNDO file's initial size
in bytes; if not specified, the initial size defaults to 134217728 (128
MB). You may optionally follow size with a one-letter abbreviation for
an order of magnitude, similar to those used in my.cnf. Generally, this
is one of the letters M (megabytes) or G (gigabytes). (Bug #13116514,
Bug #16104705, Bug #62858)

On 32-bit systems, the maximum supported value for INITIAL_SIZE is
4294967296 (4 GB). (Bug #29186)

The minimum allowed value for INITIAL_SIZE is 1048576 (1 MB). (Bug
#29574)

*Note*:

WAIT is parsed but otherwise ignored. This keyword currently has no
effect, and is intended for future expansion.

The ENGINE parameter (required) determines the storage engine which is
used by this log file group, with engine_name being the name of the
storage engine. Currently, the only accepted values for engine_name are
"NDBCLUSTER" and "NDB". The two values are equivalent.

URL: https://dev.mysql.com/doc/refman/5.7/en/alter-logfile-group.html

