Understanding and Designing Serviceguard Disaster Recovery Architectures

3 Extended Distance Cluster Configurations
Extended Distance Cluster configurations (also known as Extended Campus Cluster configurations)
are specialized cluster configurations, which allow a single Serviceguard cluster to extend across
two or three separate data centers for increased disaster recovery. These configurations provide
additional availability protection against the failure of an entire data center. These configurations
allow significantly increased distances between data centers, so we refer to them as Extended
Distance Cluster or Extended Serviceguard Cluster configurations.
An Extended Distance Cluster is a normal Serviceguard cluster that has alternate nodes located in
two different data centers separated by distance. Extended distance clusters are connected using
a high speed cable that guarantees network access between the nodes as long as all guidelines
for disaster recovery architecture are followed. Extended distance clusters were formerly known
as campus clusters, but that term is not always appropriate because the supported distance has
increased beyond the typical size of a single corporate campus. The maximum distance between
nodes in an Extended Distance Cluster is set by the limits of the data replication technology and
networking limits. An Extended Distance Cluster is shown in Figure 24.
Figure 24 Extended Distance Cluster
Node 1 Node 2
Node 3
Node 4
Disk
Mirroring
Data Center A Data Center B
Highly Available Network
pkg A pkg C
pkg B pkg D
The two types of Extended Distance Cluster configurations are Extended Distance Cluster and
Extended Distance Cluster for Oracle RAC. Both types use Serviceguard to create disaster recovery
High Availability clusters. The following describes in more detail the key differences between the
two types:
Extended distance clusters can be configured over shorter distances using Fibre Channel mass
storage, or over distances limit to 100 km using storage and networking routed over links extended
via WDM.
In extended distance architecture, each clustered server is directly connected to all storage in both
data centers. With direct access to remote storage devices from a local server, an Extended Distance
Cluster with up to four nodes can be designed with two data centers using dual cluster lock disks
for cluster quorum. If the cluster size is greater than four nodes, an Extended Distance Cluster can
be designed with two data centers and a third location housing arbitrator nodes or quorum server.
An Extended Distance Cluster for Oracle RAC merges Extended Distance Cluster with Serviceguard
Extension for RAC (SGeRAC). SGeRAC is a specialized configuration that enables Oracle Real
Application Clusters (RAC) to run in an HP-UX environment on high availability clusters. With the
presence of Oracle RAC in a Serviceguard environment you can maintain a single (Oracle) database
image that is accessed by the servers in parallel in an active/active configuration, thereby providing
greater processing power without the overhead of administering separate databases.
48 Extended Distance Cluster Configurations