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

Building a Metropolitan Cluster Using MetroCluster/CA
Designing a Disaster Tolerant Architecture for use with MetroCluster/CA
Chapter 3 77
NOTE In the campus or metropolitan environment, the same number of
systems must be present in each of the two data centers (Data Center A
and Data Center B) whose systems are connected to the XP disk arrays.
There must be either one or two arbitrators or a Quorum Server in a
third location.
Arbitrator Node Configuration Rules
Although you can use one arbitrator, having two arbitrators provides
greater flexibility in taking systems down for planned outages as well as
providing better protection against multiple points of failure. Using two
arbitrators:
• Provides local failover capability to applications running on the
arbitrator.
• Protects against multiple points of failure (MPOF).
• Provides for planned downtime on a single system anywhere in the
cluster.
If you use a single arbitrator system, special procedures must be followed
during planned downtime to remain protected. Systems must be taken
down in pairs, one from each of the data centers, so that the
MC/ServiceGuard quorum is maintained after a node failure. If the
arbitrator itself must be taken down, disaster recovery capability is at
risk if one of the other systems fails.
Arbitrator systems can be used to perform important and useful work
such as:
• Hosting mission-critical applications not protected by disaster
recovery software
• Running monitoring and management tools such as IT/Operations or
Network Node Manager
• Running backup applications such as Omniback
• Acting as application servers