Sectors Reserved for the Logical Volume Manager (LVM)

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If a physical volume is part of a volume group, the physical volume is used by the LVM and contains two additional reserved areas. One area contains the volume group descriptor area/volume group status area and follows the first 128 reserved sectors. The other area is at the end of the physical volume reserved as a relocation pool for bad blocks that must be software-relocated. Both of these areas are described by the LVM record. The space between these last two reserved areas is divided into equal-sized partitions.

The volume group descriptor area (VGDA) is divided into the following:

The volume group header. This header contains general information about the volume group and a time stamp used to verify the consistency of the VGDA.
A list of logical volume entries. The logical volume entries describe the states and policies of logical volumes. This list defines the maximum number of logical volumes allowed in the volume group. The maximum is specified when a volume group is created.
A list of physical volume entries. The size of the physical volume list is variable because the number of entries in the partition map can vary for each physical volume. For example, a 200 MB physical volume with a partition size of 1 MB has 200 partition map entries.
A name list. This list contains the special file names of each logical volume in the volume group.
A volume group trailer. This trailer contains an ending time stamp for the volume group descriptor area.

When a volume group is varied online, a majority (also called a quorum) of VGDAs must be present to perform recovery operations unless you have specified the force flag. (The vary-on operation, performed by using the varyonvg command, makes a volume group available to the system).

Attention: Use of the force flag can result in data inconsistency.

A volume group with only one physical volume must contain two copies of the physical volume group descriptor area. For any volume group containing more than one physical volume, there are at least three on-disk copies of the volume group descriptor area. The default placement of these areas on the physical volume is as follows:

For the first physical volume installed in a volume group, two copies of the volume group descriptor area are placed on the physical volume.
For the second physical volume installed in a volume group, one copy of the volume group descriptor area is placed on the physical volume.
For the third physical volume installed in a volume group, one copy of the volume group descriptor area is placed on the physical volume. The second copy is removed from the first volume.
For additional physical volumes installed in a volume group, one copy of the volume group descriptor area is placed on the physical volume.

When a vary-on operation is performed, a majority of copies of the volume group descriptor area must be able to come online before the vary-on operation is considered successful. A quorum ensures that at least one copy of the volume group descriptor areas available to perform recovery was also one of the volume group descriptor areas that were online during the previous vary-off operation. If not, the consistency of the volume group descriptor area cannot be ensured.

The volume group status area (VGSA) contains the status of all physical volumes in the volume group. This status is limited to active or missing. The VGSA also contains the state of all allocated physical partitions (PP) on all physical volumes in the volume group. This state is limited to active or stale. A PP with a stale state is not used to satisfy a read request and is not updated on a write request.

A PP changes from stale to active after a successful resynchronization of the logical partition (LP) that has multiple copies, or mirrors, and is no longer consistent with its peers in the LP. This inconsistency can be caused by a write error or by not having a physical volume available when the LP is written to or updated.

A PP changes from stale to active after a successful resynchronization of the LP. A resynchronization operation issues resynchronization requests starting at the beginning of the LP and proceeding sequentially through its end. The LVDD reads from an active partition in the LP and then writes that data to any stale partition in the LP. When the entire LP has been traversed, the partition state is changed from stale to active.

Normal I/O can occur concurrently in an LP that is being resynchronized.

Note: If a write error occurs in a stale partition while a resynchronization is in progress, that partition remains stale.

If all stale partitions in an LP encounter write errors, the resynchronization operation is ended for this LP and must be restarted from the beginning.

The vary-on operation uses the information in the VGSA to initialize the LVDD data structures when the volume group is brought online.