Designing Disaster Tolerant High Availability Clusters, 10th Edition, March 2003 (B7660-90013)

Disaster Tolerance and Recovery in an MC/ServiceGuard Cluster
Types of Disaster Tolerant Clusters
Chapter 1 27
• The physical connection is one or more leased lines managed by a
common carrier. Common carriers cannot guarantee the same
reliability that a dedicated physical cable can. The distance can
introduce a time lag for data replication, which creates an issue with
data currency. This could increase the cost by requiring higher speed
WAN connections to improve data replication performance and
reduce latency.
• Tools such as Transaction Processing Monitors or database
replication tools that work across a WAN are needed to make sure
the data replication maintains data consistency.
• Operational issues, such as working with different staff with
different processes, and conducting failover rehearsals, are made
more difficult the further apart the nodes in the cluster are.
Continental Cluster With Cascading Failover
Another way of setting up your continental cluster is the use of
cascading failover, an option available for users of the EMC
Symmetrix. Cascading failover means that applications are configured to
fail over on nodes within two data centers in either cluster. They fail over
to a cluster if the entire alternate cluster is down. For example,
bi-directional failover allows a cluster to be both a recovery cluster and
a primary cluster for different packages. Data replication also follows
the cascading model. Data is replicated from the primary disk array to
the secondary disk array in the MetroCluster, then data is replicated to
the third disk array in the ServiceGuard recovery cluster.
A continental cluster with cascading failover uses three main data
centers distributed between a metropolitan cluster, which serves as a
primary cluster, and a standard cluster, which serves as a recovery
cluster. The configuration uses three EMC Symmetrix frames, two of
which are part of the metropolitan cluster and the other attached to the
recovery cluster.The data centers are distributed as follows:
• Primary Symmetrix—on the site that holds the primary copy of the
data, located in the primary cluster.
• Secondary Symmetrix—on the site that holds a remote mirror copy of
the data, located in the primary cluster.
• Arbitrator—a third location that contains the arbitrator nodes,
located in the primary cluster.