command succeeds. If exactly half the copies are available, as with two of four, quorum is not achieved and the varyonvg command fails.
3.4.5.2 Quorum after Vary On
If a write to a physical volume fails, the VGSAs on the other physical volumes within the volume group are updated to indicate that one physical volume has failed. As long as more than half of all VGDAs and VGSAs can be written, quorum is maintained and the volume group remains varied on. If exactly half or less than half of the VGDAs and VGSAs are inaccessible, quorum is lost, the volume group is varied off, and its data becomes unavailable.
Keep in mind that a volume group can be varied on or remain varied on with one or more of the physical volumes unavailable. However, data contained on the missing physical volume will not be accessible unless the data is replicated using LVM mirroring, and a mirror copy of the data is still available on another physical volume. Maintaining quorum without mirroring does not guarantee that all data contained in a volume group is available.
Quorum has nothing to do with the availability of mirrored data. It is possible to have failures that result in loss of all copies of a logical volume, yet the volume group remains varied on because a quorum of VGDAs/VGSAs are still accessible.
3.4.5.3 Disabling and Enabling Quorum
Quorum checking is enabled by default. Quorum checking can be disabled using the chvg
Quorum Enabled
With quorum enabled, more than half of the physical volumes must be available and the VGDA and VGSA data structures must be identical before a volume group can be varied on with the varyonvg command.
With quorum enabled, a volume group will be forced offline if one or more disk failures cause a majority of the physical volumes to be unavailable. Having three or more disks in a volume group avoids a loss of quorum in the event of a single disk failure.
Quorum Disabled
With quorum disabled, all the physical volumes in the volume group must be available and the VGDA data structures must be identical for the varyonvg command to succeed. With quorum disabled, a volume group will remain varied on until the last physical volume in the volume group becomes unavailable. This section summarizes the effect quorum has on the availability of a volume group.
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