1.705: Why am I having trouble adding another disk to my VG?

In some instances, the user will experience a problem adding
a new disk to an existing volume group or in the creation of
a new volume group.  The warning message provided by LVM will
be:

	Not enough descriptor space left in this volume group.
	Either try adding a smaller PV or use another volume group.

On every disk in a volume group, there exists an area called the
Volume Group Descriptor Area (VGDA).  This space is what allows
the user to take a volume group to another AIX system and
"importvg" that volume group into that AIX system.  The VGDA
contains the names of disks that make up the volume group, their
physical sizes, partition mapping, logical volumes that exist in
the volume group, and other pertinent LVM management information.

When the user creates a volume group, the "mkvg" command
defaults to allowing the new volume group to have a maximum
of 32 disks in a volume group.  However, as bigger disks have
become more prevalent, this 32 disk limit is usually not
achieved because the space in the VGDA is used up faster, as
it accounts for the capacity on the bigger disks.  This
maximum VGDA space, for 32 disks, is a fixed size which is
part of the LVM design.  Large disks require more management
mapping space in the VGDA, which causes the number and size
of available disks to be added to the existing volume group
to shrink.  When a disk is added to a volume group, not only
does the new disk get a copy of the updated VGDA, but all
existing drives in the volume group must be able to accept
the new, updated VGDA.

The exception to this description of the maximum VGDA is
rootvg.  In order to provide AIX users more free space, when
rootvg is created, "mkvg" does not use the maximum limit of
32 disks that are allowed into a volume group.  Instead in
AIX 3.2, the number of disks picked in the install menu of
AIX is used as the reference number by "mkvg -d" during the
creation of rootvg.  For AIX 4.1, this "-d" number is 7 for
one disk and one more for each additional disk
picked. i.e. you pick two disks, the number is 8.  you pick
three disks, the number is 9, and so on.....  This limit does
not mean the user cannot add more disks to rootvg in the
post-install phase.  The amount of free space left in a VGDA,
and thus the number of size of the disks added to a volume
group, depends on the size and number of disks already
defined for a volume group.  However, this smaller size
during rootvg creation implies that the user will be able to
add fewer disks to rootvg than compared to a non-rootvg
volume group.

If the customer requires more VGDA space in the rootvg, then
they should use the "mksysb" and "migratepv" commands to
reconstruct and reorganize their rootvg (the only way to
change the "-d" limitation is recreation of the rootvg).

Note:  It is always strongly recommended that users do not place
user data onto rootvg disks.  This separation provides an extra
degree of system integrity.