Understanding and Designing Serviceguard Disaster Tolerant Architectures Fourth Edition, December 2007 (T1906-90023)

Building an Extended Distance Cluster Using Serviceguard
Two Data Center Architecture
Chapter 268
Two Data Center Architecture
The two data center architecture is based on a standard Serviceguard
configuration with half of the nodes in one data center, and the other half
in another data center. Nodes can be located in separate data centers in
the same building, or even separate buildings within the limits of
FibreChannel technology. Configurations with two data centers have the
following requirements:
• There must be an equal number of nodes (1 or 2) in each data center.
• In order to maintain cluster quorum after the loss of an entire data
center, you must configure dual cluster lock disks (one in each data
center). Since cluster lock disks are only supported for up to 4 nodes,
the cluster can contain only 2 or 4 nodes. The Serviceguard Quorum
Server cannot be used in place of dual cluster disks, as the Quorum
Server must reside in a third data center. Therefore, a three data
center cluster is a preferable solution, if dual cluster lock disks
cannot be used, or if the cluster must have more than 4 nodes. When
using dual cluster lock disks, there exists a chance of Split Brain
Syndrome (where the nodes in each data center form two separate
clusters, each with exactly one half of the cluster nodes) if all
communication between the two data centers is lost and all nodes
remain running.
The Serviceguard Quorum Server prevents the possibility of split
brain, however the Quorum Server must reside in a third site.
Therefore a three data center cluster is a preferable solution, to
prevent split brain, and the only solution if dual cluster lock disks
cannot be used, or if the cluster must have more than 4 nodes.
• Two data center configurations are not supported if SONET is used
for the cluster interconnects between the Primary data centers.
• To protect against the possibility of a split cluster inherent when
using dual cluster locks, at least two (three preferred) independent
paths between the two data centers must be used for heartbeat and
cluster lock I/O. Specifically, the path from the first data center to
the cluster lock at the second data center must be different than the
path from the second data center to the cluster lock at the first data
center. Preferably, at least one of the paths for heartbeat traffic
should be different from each of the paths for cluster lock I/O.
• No routing is allowed for the networks between data centers.