Managing Serviceguard Eighteenth Edition, September 2010

Therefore, if all of the hardware in the cluster has 2 or 3 power inputs, then at least
three separate power circuits will be required to ensure that there is no single point of
failure in the power circuit design for the cluster.
Larger Clusters
You can create clusters of up to 16 nodes with Serviceguard. Clusters of up to 16 nodes
may be built by connecting individual SPUs via Ethernet.
The possibility of configuring a cluster consisting of 16 nodes does not mean that all
types of cluster configuration behave in the same way in a 16-node configuration. For
example, in the case of shared SCSI buses, the practical limit on the number of nodes
that can be attached to the same shared bus is four, because of bus loading and limits
on cable length. Even in this case, 16 nodes could be set up as an administrative unit,
and sub-groupings of four could be set up on different SCSI buses which are attached
to different mass storage devices.
In the case of non-shared SCSI connections to an XP series or EMC disk array, the
four-node limit does not apply. Each node can be connected directly to the XP or EMC
by means of two SCSI buses. Packages can be configured to fail over among all sixteen
nodes. For more about this type of configuration, see “Point to Point Connections to
Storage Devices ” (page 51).
NOTE: When configuring larger clusters, be aware that cluster and package
configuration times as well as execution times for commands such as cmviewcl will
be extended. In the man pages for some commands, you can find options to help to
reduce the time. For example, refer to the man page for cmquerycl (1m) for options
that can reduce the amount of time needed for probing disks or networks.
Active/Standby Model
You can also create clusters in which there is a standby node. For example, an eight
node configuration in which one node acts as the standby for the other seven could
easily be set up by equipping the backup node with seven shared buses allowing
separate connections to each of the active nodes. This configuration is shown in
Figure 2-5.
50 Understanding Serviceguard Hardware Configurations