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 378
XP Series Disk Array Configuration Rules
Each XP Series disk array must be configured with redundant CA links,
each of which is connected to a different LCP or RCP card. To prevent a
single point of failure (SPOF), there must be at least two physical boards
in each XP for the CA links. Each board usually has multiple ports.
However, a redundant CA link must be connected to a port on a different
physical board from the board that has the primary CA link. When using
bi-directional configurations, where data center A backs up data center B
and data center B backs up data center A, you must have at least four
CA links, two in each direction. Four CA links are also required in
uni-directional configurations in which you want to allow failback.
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. With MC/ServiceGuard a cluster lock disk or Quorum
Server is used as the tie-breaker when quorum is exactly 50%. However,
with MetroCluster CA/XP, a Quorum Server is supported and a cluster
lock disk in not supported. Therefore, a quorum of 50% will require
access to a Quorum Server, otherwise all nodes will halt.