Managing HP Serviceguard for Linux, Tenth Edition, September 2012

Figure 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.”)
20 Serviceguard for Linux at a Glance