Managing HP Serviceguard for Linux, Eighth Edition, March 2008

Serviceguard for Linux at a Glance
What is Serviceguard for Linux?
Chapter 1 19
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.”)