The cluster lock disk (used only in HP-UX clusters) is a disk
that can be written to by all members of the cluster. When a node
obtains the cluster lock, this disk is marked so that other nodes
will recognize the lock as “taken.” This mark
will survive an off-on power cycle of the disk device unlike SCSI
disk reservations. A lock disk may be used for clusters of up to
four nodes.
The lock is created in a special area on a particular LVM
physical volume. The cluster lock volume group and physical volume
names are identified in the cluster configuration file.
The lock disk is not dedicated for use as the cluster lock;
thus, it can be employed as part of a normal volume group with user
data on it. The usable space on the disk is not impacted; the lock
disk takes no space away from the disk’s volume group.
Further, the activation of the volume group on one node does not
affect the ability of another node to acquire the cluster lock.
The lock area on the disk is not mirrored, even though the
physical volume may be a part of a volume group that contains mirrored
logical volumes.
The operation of the lock disk is shown in Figure 5.
The node that acquires the lock (in this case node 2) continues
running in the cluster. The other node halts.
Serviceguard periodically checks the health of the lock disk
and writes messages to the syslog file when a lock disk fails the
health check. This file should be monitored for early detection
of lock disk problems.
You can choose between two lock disk options—a single
or dual lock disk—based on the kind of high availability
configuration you are building. A single lock disk is
recommended where possible. With both single and dual
locks, however, it is important that the cluster lock be available
even if the power circuit to one node fails; thus, the choice of
a lock configuration depends partly on the number of power circuits available.
Regardless of your choice, all nodes in the cluster must have access
to the cluster lock to maintain high availability.
Single
Cluster Lock |
 |
It is recommended that you use a single lock disk. A single
lock disk should be configured on a power circuit separate from
that of any node in the cluster. For example, it is highly recommended
to use three power circuits for a two-node cluster, with a single,
separately powered disk for the cluster lock. For two-node clusters,
this single lock disk may not share a power circuit with either
node, and it must be an external disk. For three or four node clusters,
the disk should not share a power circuit with 50% or more of the
nodes.
Dual
Cluster Lock |
|
In an extended distance cluster, where the cluster contains
nodes running in two separate data centers, a single lock disk would
be a single point of failure should the data center it resides in
suffer a catastrophic failure. In this case only,
a dual cluster lock, with two separately powered disks, should be
used to eliminate the lock disk as a single point of failure. The
use of the dual cluster lock is further shown in “Use of
Dual Lock Disks in Extended Distance Clusters” on page 26.
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