Saturday, 3 August 2013

AIX LVM QUORUM mysteries revealed

Technote (FAQ)


Why can't I varyon a Volume Group when one or more physical volumes are not available?


Varyonvg requires 100% of it's physical volumes be available and accessible in order to successfully vary on the Volume Group without using the force option.


A common misconception is that the QUORUM setting of an LVM Volume Group can affect one's ability to varyon a volume group, when, in fact, the Volume Group QUORUM setting (enabled or disabled) has no bearing on the varyon process. This misconception is further enhanced by the following varyonvg error message...

0516-052 varyonvg: Volume group cannot be varied on without a quorum. More physical volumes in the group must be active.

This message indicates a "quorum" of physical volumes must exist in order to varyon the Volume Group and is unrelated to the Volume Group's QUORUM setting.

The Volume Group QUORUM setting is a concept that applies only to currently varied on Volume Groups in order to force varyoff of the Volume Group should it lose more than 51% of it's disks. With QUORUM disabled on the Volume Group, loss of one or more disks will not cause the Volume Group to varyoff. If QUORUM is enabled on the Volume Group, LVM will force varyoff the Volume Group if less than 51% of it's disks are available and accessible. For a two disk Volume Group with QUORUM enabled, LVM will check the number of VGDAs on each disk and varyoff the Volume Group should it lose QUORUM (if it loses the disk with two active VGDA's).

The Volume Group's QUORUM setting has no meaning for a Volume Group which is currently varied off. Varyonvg does not look at how many VGDAs a disk has, it ONLY looks at the number of physical volumes which are available and accessible. Without the -f (force) flag, ALL physical volumes in a Volume Group must be available and accessible. If one or more physical volumes is unavailable, the Volume Group may be forced online with varyonvg -f.

Excerpts from the varyonvg man page...
"The varyonvg will fail to varyon the volume group if a majority of the physical volumes are not accessible (no Quorum). This condition is true even if the quorum checking is disabled. Disabling the quorum checking will only ensure that the volume group stays varied on even in the case of loss of quorum."

"If the volume group cannot be varied on due to a loss of the majority of physical volumes, a list of all physical volumes with their status is displayed. To varyon the volume group in this situation, you will need to use the force option."

"-f Allows a volume group to be made active that does not currently have a quorum of available disks. All disk that cannot be brought to an active state will be put in a removed state. At least one disk must be available for use in the volume group."

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