At the time of importing VG(Volume Group), VGDA(Volume Group Descriptor Area) is parsed by importvg command.
On a single disk VG, 2 VGDAs exist.
With second disk being added to the VG, first disk has two VGDAs and second disk has one.
With third disk being added, the VGDA of first disk moves to third disk.
Henceforth every additional disk being added to VGDA will have one VGDA.
Quorum is a state of VG in which 51% or more VGDAs are available. Each VGDA acts as a vote for Quorum and ensures accessibility of PV(Physical Volume). Quorum is used by varyonvg command to decide whether a volume group should be varied on or not.
If a VG has Quorum enabled then >= 51% of VGDAs should be available for VG to varyon. Otherwise VG will not be varied on. That means minimum 51% of Pvs should be available. If VGDAs count falls below 51%, VG gets varied off automatically.
If a VG has Quorum disabled then all of the VGDAs should be available for VG to varyon. Once VG is varied on, then to keep it varied on only 1 VGDA is sufficient.
Importance of Quorum
The importance of Quorum concept lies in fact that if quorum is enabled and VGDAs fall below 51% then the data on VG may be corrupted if VG remains varied on. Thats why quorum enabled VG is varied off if quorum falls below 51%. This way data is safe on VG because disk I/O is stopped due to VG being varied off.
Command To Enable/Disable Quorum For VG
chvg -Qn vg_name
n–> To Turn off Quorum.
y–> To Turn on Quorum