Quorum

The quorum is one of the mechanisms that the LVM uses to ensure that a volume group is ready to use and contains the most up-to-date data.
A quorum is a vote of the number of Volume Group Descriptor Areas and Volume Group Status Areas (VGDA/VGSA) that are active. A quorum ensures data integrity of the VGDA/VGSA areas in the event of a disk failure. Each physical disk in a volume group has at least one VGDA/VGSA. When a volume group is created onto a single disk, it initially has two VGDA/VGSA areas residing on the disk. If a volume group consists of two disks, one disk still has two VGDA/VGSA areas, but the other disk has one VGDA/VGSA. When the volume group is made up of three or more disks, then each disk is allocated just one VGDA/VGSA.
A quorum is lost when enough disks and their VGDA/VGSA areas are unreachable so that a 51% majority of VGDA/VGSA areas no longer exists. In a two-disk volume group, if the disk with only one VGDA/VGSA is lost, a quorum still exists because two of the three VGDA/VGSA areas still are reachable. If the disk with two VGDA/VGSA areas is lost, this statement is no longer true. The more disks that make up a volume group, the lower the chances of quorum being lost when one disk fails.
When a quorum is lost, the volume group varies itself off so that the disks are no longer accessible by the LVM. This prevents further disk I/O to that volume group so that data is not lost or assumed to be written when physical problems occur. Additionally, as a result of the vary-off, the user is notified in the error log that a hardware error has occurred and service must be performed.
There are cases when it is desirable to continue operating the volume group even though a quorum is lost. In these cases, quorum checking can be turned off for the volume group. This type of volume group is referred to as a nonquorum volume group. The most common case for a nonquorum volume group occurs when the logical volumes have been mirrored. When a disk is lost, the data is not lost if a copy of the logical volume resides on a disk that is not disabled and can be accessed. However, there can be instances in nonquorum volume groups, mirrored or nonmirrored, when the data (including copies) resides on the disk or disks that have become unavailable. In those instances, the data might not be accessible even though the volume group continues to be varied on.