New volume groups can be created with the mkvg
command. Physical volumes are added to a volume group using the extendvg
command and can be removed using the reducevg command. It is important to
remember that the rootvg has attributes that differs from the user defined
volume groups, particularly, it cannot be imported or exported.
Just as the PVID is a soft serial number for the disk, the Volume Group also
has an identifier called the Volume Group Identifier (VGID). It is a soft serial
number for the volume group and is the number that is referenced by the
low-level LVM commands not the volume group name. It also forms the base
for all logical volume IDs created for the logical volumes in the volume group.
When a volume group is created, a major number is assigned. This can be
specified (mkvg -V major_number) or the next lowest available number will be
assigned. The lvlstmajor command will show which major numbers are
It is recommended in multi-host environments that the major numbers are
consistent across the cluster, particularly, if file systems are being cross
mounted. This is because the NFS uses the major number as part of the file
handle, which is used when remote systems refer to the file.
The VGDA contains information about the volume group, the logical volumes
that reside in the volume group, and the physical volumes that make up the
volume group. These VGDAs are also used in quorum voting. The status
information pertaining to the physical volume status is limited to active or
As the VGDA contains information about each of the disks in the volume
group, only one disk need to be specified when the importvg command is
being used to import a volume group into a system. The importvg command
will go to the specified physical volume, read the VGDA, find out what other
physical volumes make up the volume group (by their PVID), and import them
into the system and the ODM as well.
As the VGDA contains information about logical volumes in the volume group,
any changes to a logical volume (creation, extension, or deletion) will mean
that the VGDA on all disks is updated.
Big VGDA or VGDA expansion
The original limitation to LVM of 32 disks per volume group was becoming a
problem with systems with a large number of disks. Especially with the
increased use of LVM mirroring, the need for larger volume groups grew.
As the result, AIX Version 4.3.2 introduced the -B option. This changes the
maximum number of disks that can be included into a volume group from 32
to 128 disks. And, the number of logical volumes that can exist in that volume
group has grown from 256 to 512 logical volumes. AIX Version 4.3.3
introduced the -G option, which allows the VGDA to include up to 1016*1024
The obvious penalty is that the bigger VGDA is taking up more space on the
physical volume; thus, the usable space on each physical volume in the
volume group is reduced.
The opportunity has also been taken with the expansion of the VGDA to
include some additional information. In particular, if the permissions of the
special logical volume files have been changed, then this information can be
stored in the VGDA. Thus, when a big VGDA volume group is imported, the
modified permissions are preserved.