AIX 5L Problem Determination - IBM Redbooks

7.1 LVM data
The Logical Volume Manager (LVM) data structures that are required for the
LVM to operate are stored in a number of structures. This logical layout is
described in the following sections.
7.1.1 Physical volumes
7.1.2 Volume groups
Each disk is assigned a Physical Volume Identifier (PVID) when it is first
assigned to a volume group. The PVID is a combination of the serial number of
the machine creating the volume group and the time and date of the operation.
The PVID is stored on the physical disk itself and is also stored in the Object
Data Manager (ODM) of a machine when a volume group is created or imported.
You should not use the dd command to copy the contents of one physical volume
to another, since the PVID will also be copied; this will result in two disks having
the same PVID, which can confuse the system.
Each volume group has a Volume Group Descriptor Area (VGDA). There are
(commonly) multiple copies of the VGDA in a volume group. A copy of the VGDA
is stored on each disk in the volume group. The VGDA stores information about
the volume group, such as the logical volumes and the disks in the volume
group.
The VGDA is parsed by the importvg command when importing a volume group
into a system. It is also used by the varyonvg command in the quorum voting
process to decide whether a volume group should be varied on.
For a single disk volume group, there are two VGDAs on the disk. When a
second disk is added to make a two disk volume group, the original disk retains
two VGDAs and the new disk gets one VGDA.
Adding a third disk results in the extra VGDA from the first disk moving to the
third disk for a quorum of three with each disk having one vote. Adding this
additional disk adds a new VGDA per disk.
A volume group with quorum checking enabled (the default) must have at least
51 percent of the VGDAs in the volume group available before it can be varied
on. Once varied on, if the number of VGDAs falls below 51 percent, the volume
group will automatically be varied off.
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