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

Building a Metropolitan Cluster Using MetroCluster/SRDF
Designing a Disaster Tolerant Architecture for use with MetroCluster/SRDF
Chapter 4130
Arbitrator systems can be used to perform important and useful work
such as:
• Running mission-critical applications not protected by disaster
recovery
• IT/Operations or NetworkNodeManager
• Backup
• Application servers
Calculating a Cluster Quorum
When a cluster initially forms, all systems must be available to form the
cluster (100% Quorum requirement).
A quorum is dynamic and is recomputed after each system failure. For
instance, if you start out with an 8-node cluster and two systems fail,
that leaves 6 out 8 surviving nodes, or a 75% quorum. The cluster size is
reset to 6 nodes. If two more nodes fail, leaving 4 out of 6, quorum is 67%.
Each time a cluster forms, there must be more than 50% quorum to
reform the cluster. Cluster lock disks are normally used as the
tie-breaker when quorum is exactly 50%. However, cluster lock disks are
not supported with MetroCluster with Symmetrix SRDF. Therefore, a
quorum of 50% or less will cause the remaining nodes to halt.