Using Serviceguard Extension for RAC, 5th Edition, June 2007

Introduction to Serviceguard Extension for RAC
What is a Serviceguard Extension for RAC Cluster?
Chapter 1 17
RAC on HP-UX lets you maintain a single database image that is
accessed by the HP servers in parallel, thereby gaining added processing
power without the need to administer separate databases. Further, when
properly configured, Serviceguard Extension for RAC provides a highly
available database that continues to operate even if one hardware
component should fail.
Group Membership
Oracle RAC systems implement the concept of group membership,
which allows multiple instances of RAC to run on each node. Related
processes are configured into groups. Groups allow processes in
different instances to choose which other processes to interact with. This
allows the support of multiple databases within one RAC cluster.
A Group Membership Service (GMS) component provides a process
monitoring facility to monitor group membership status. GMS is
provided by the cmgmsd daemon, which is an HP component installed
with Serviceguard Extension for RAC.
Figure 1-2 shows how group membership works. Nodes 1 through 4 of
the cluster share the Sales database, but only Nodes 3 and 4 share the
HR database. Consequently, there is one instance of RAC each on Node 1
and Node 2, and there are two instances of RAC each on Node 3 and
Node 4. The RAC processes accessing the Sales database constitute one
group, and the RAC processes accessing the HR database constitute
another group.