Managing HP Serviceguard for Linux Ninth Edition, April 2009

Figure 1-1 Typical Cluster Configuration
In the figure, node 1 (one of two SPU's) is running 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 copy of the data. Note that both nodes
are physically connected to disk arrays. 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).
Disk arrays 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.”)
24 Serviceguard for Linux at a Glance