Note: this is supported for rootvg and concurrent vgs from AIX 6.1 TL 4. See IBM technote IZ80021 http://bit.ly/cmHjmy
Resizing the rootvg disk
I tried to increase rootvg on an LPAR running AIX 5.3 TL 11 and hit the following error:
aix53_lpar # chvg -g rootvg
0516-1380 chvg: Re-sizing of the disks is not supported for the rootvg.
0516-732 chvg: Unable to change volume group rootvg.
Looks like the volume group needed to be varied off and varied on again. For rootvg, that means a reboot.
No reboot on AIX 6.1
In AIX 6.1 (from TL 4 - use oslevel -s to check your AIX level), you can increase rootvg on the fly.
Sounds like yet another reason to migrate to AIX 6.1.
aix61_lpar:/# chvg -g rootvg
0516-1164 chvg: Volume group rootvg changed. With given characteristics rootvg can include up to 16 physical volumes with 2032 physical partitions each.