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Cluster Hardware and Software Preparation 89
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 -Qn vgname command, or by using the smit chvg fastpath.
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.