Managing Serviceguard 11th Edition, Version A.11.16, Second Printing June 2004

Understanding Serviceguard Hardware Configurations
Redundancy of Cluster Components
Chapter 236
Redundancy of Cluster Components
In order to provide a high level of availability, a typical cluster uses
redundant system components, for example two or more SPUs and two
or more independent disks. This redundancy eliminates single points of
failure. In general, the more redundancy, the greater your access to
applications, data, and supportive services in the event of a failure.
In addition to hardware redundancy, you must have the software support
which enables and controls the transfer of your applications to another
SPU or network after a failure. Serviceguard provides this support as
follows:
• In the case of LAN failure, Serviceguard switches to a standby LAN
or moves affected packages to a standby node.
• In the case of SPU failure, your application is transferred from a
failed SPU to a functioning SPU automatically and in a minimal
amount of time.
• For failure of other monitored resources, such as disk interfaces, a
package can be moved to another node.
• For software failures, an application can be restarted on the same
node or another node with minimum disruption.
Serviceguard also gives you the advantage of easily transferring control
of your application to another SPU in order to bring the original SPU
down for system administration, maintenance, or version upgrades.
The current maximum number of nodes supported in an Serviceguard
cluster is 16. Fast/Wide SCSI disks or disk arrays can be connected to a
maximum of 4 nodes at a time on a shared (multi-initiator) bus. Disk
arrays using fibre channel and those that do not use a shared bus — such
as the HP SureStore XP Series and the EMC Symmetrix — can be
simultaneously connected to all 16 nodes.
The guidelines for package failover depend on the type of disk technology
in the cluster. For example, a package that accesses data on a Fast/Wide
SCSI disk or disk array can failover to a maximum of 4 nodes. A package
that accesses data from a disk in a cluster using Fibre Channel or HP
SureStore XP or EMC Symmetrix disk technology can be configured for
failover among 16 nodes.