During mksysb process you have the chance to encounter many errors. One of them is "An internal packing error occurred: incorrect size" and I will try below to aid you to get pass it.

The error:

# mksysb -eiX /backups-tmp/gzlpar-rootvg.sysb

Creating information file (/image.data) for rootvg.

Creating list of files to back up
Backing up 89961 files......................[color=red]An internal packing error occurred: incorrect size
./var/adm/wtmp will not be backed up.[/color]


0512-005 mksysb: Backup Completed.
        The backup command completed with errors.
        The messages displayed on Standard Error contained additional
        information.

In this case, add to /etc/exclude.rootvg any temporary directories which are anyway not needed in case of restore.

For example, related to the error above, you may exclude already /var/adm/wtmp. You can add also /tmp or any other temporary files/directories, to avoid other similar errors.

# mksysb -eiX /backups-tmp/gzlpar-rootvg.sysb

Creating information file (/image.data) for rootvg.

Creating list of files to back up 
Backing up 89951 files..............................
28302 of 89951 files backed up (31%)...........................

89951 of 89951 files backed up (100%)
0512-038 mksysb: Backup Completed Successfully.

In my example, excluding only /var/adm/wtmp avoided that error but if it doesn't work in your case, you can add option "-p" to disable software packing.

The man file describes the "-p" flag as follows:
-p Disables software packing of the files as they are backed up. Some tape
drives use their own packing or compression algorithms. This flag only
applies to AIX Version 4.2 or later.

# mksysb -eiXp /backups-tmp/gzlpar-rootvg.sysb

Creating information file (/image.data) for rootvg.

Creating list of files to back up 
Backing up 89961 files..............................
68259 of 89961 files backed up (75%).......

89961 of 89961 files backed up (100%)
0512-038 mksysb: Backup Completed Successfully.