Managing Serviceguard 13th Edition, February 2007

Serviceguard at a Glance
What is Serviceguard?
Chapter 1 25
In Figure 1-1, node 1 (one of two SPU's) is running failover package A,
and node 2 is running package B. Each package has a separate group of
disks associated with it, containing data needed by the package's
applications, and a mirror copy of the data. Note that both nodes are
physically connected to both groups of mirrored disks. In this example,
however, only one node at a time may access the data for a given group of
disks. In the figure, node 1 is shown with exclusive access to the top two
disks (solid line), and node 2 is shown as connected without access to the
top disks (dotted line). Similarly, node 2 is shown with exclusive access to
the bottom two disks (solid line), and node 1 is shown as connected
without access to the bottom disks (dotted line).
Mirror copies of data provide redundancy in case of disk failures. In
addition, a total of four data buses are shown for the disks that are
connected to node 1 and node 2. This configuration provides the
maximum redundancy and also gives optimal I/O performance, since
each package is using different buses.
Note that the network hardware is cabled to provide redundant LAN
interfaces on each node. Serviceguard uses TCP/IP network services for
reliable communication among nodes in the cluster, including the
transmission of heartbeat messages, signals from each functioning
node which are central to the operation of the cluster. TCP/IP services
also are used for other types of inter-node communication. (The
heartbeat is explained in more detail in the chapter “Understanding
Serviceguard Software.”)
Failover
Any host system running in a Serviceguard cluster is called an active
node. Under normal conditions, a fully operating Serviceguard cluster
monitors the health of the cluster's components on all its active nodes.
Most Serviceguard packages are failover packages. When you configure a
failover package, you specify which active node will be the primary
node where the package will start, and one or more other nodes, called
adoptive nodes, that can also run the package.
Figure 1-2 shows what happens in a failover situation.