Grow your rootvg on the fly (from 6.1 TL 4)

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There are lots of good reasons for having spare disk for rootvg, as I looked at in the post make way for rootvg. With virtual disks you can resize your volume group on the fly:
 
Increase rootvg dynamically

If your rootvg “disk” is actually virtual, such as a SAN LUN or a logical volume on the VIO server, then it usually can be expanded on the SAN (or using extendlv on the VIOS) and then recognised on the AIX LPAR using the -g flag of the chvg command:
chvg -g rootvg

 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.
 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.
 Sounds like yet another reason to migrate to AIX 6.1.