Managing Serviceguard 12th Edition, March 2006

Planning and Documenting an HA Cluster
Cluster Configuration Planning
Chapter 4 155
Heartbeat Subnet and Re-formation Time
The speed of cluster re-formation is partially dependent on the type of
heartbeat network that is used. If two or more heartbeat subnets are
used, the one with the fastest failover time is used.
Cluster Lock Information
The purpose of the cluster lock is to ensure that only one new cluster is
formed in the event that exactly half of the previously clustered nodes
try to form a new cluster. It is critical that only one new cluster is formed
and that it alone has access to the disks specified in its packages. You
can specify either a lock disk or a quorum server as the cluster lock.
A one-node cluster does not require a lock. Two-node clusters require the
use of a cluster lock, but the lock is recommended for larger clusters as
well. Clusters larger than 4 nodes can only use a quorum server as the
cluster lock.
Cluster Lock Disk and Re-formation Time
If you are using a lock disk, the acquisition of the cluster lock disk takes
different amounts of time depending on the disk I/O interface that is
used. After all the disk hardware is configured, but before configuring
the cluster, you can use the cmquerycl command specifying all the nodes
in the cluster to display a list of available disks and the re-formation
time associated with each. Example:
# cmquerycl -v -n ftsys9 -n ftsys10
Alternatively, you can use SAM to display a list of cluster lock physical
volumes, including the re-formation time.
By default, Serviceguard selects the disk with the fastest re-formation
time. But you may need to choose a different disk because of power
considerations. Remember that the cluster lock disk should be separately
powered, if possible.
Cluster Lock Disks and Planning for Expansion
You can add additional cluster nodes after the cluster is up and running,
but doing so without bringing down the cluster requires you to follow
some rules. Recall that a cluster with more than 4 nodes may not have a
lock disk. Thus, if you plan to add enough nodes to bring the total to
more than 4, you should use a quorum server.