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.

The optional INITIAL_SIZE parameter sets the UNDO file's initial size
in bytes; if not specified, the initial size default to 128M (128
megabytes). 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 (for megabytes) or G
(for gigabytes).

On 32-bit systems, the maximum supported value for INITIAL_SIZE is 4G.
(Bug#29186 (http://bugs.mysql.com/29186))

*Note*: WAIT is parsed but otherwise ignored, and so has no effect in
MySQL 5.1 and MySQL Cluster NDB 6.x. It 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. In MySQL 5.1 and MySQL Cluster NDB 6.x, the only
accepted values for engine_name are "NDBCLUSTER" and "NDB". The two
values are equivalent.

URL: http://dev.mysql.com/doc/refman/5.1/en/alter-logfile-group.html

