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 4 129
* Configurations with two arbitrators are preferred because they provide
a greater degree of availability, especially in cases when a node is down
due to a failure or planned maintenance.
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 Symmetrix
units. There must be either one or two arbitrators in 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:
• Provides local failover capability to applications running on the
arbitrator.
• Protects against more MPOF (Multiple Points of Failure).
• Provides for planned downtime of 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.
66 2*
77 1
77 2*
Table 4-2 Possible Number of Nodes in a Three Data Center Configuration
Primary Data
Center A
(with
Symmetrix)
Primary Data
Center B
(with
Symmetrix)
Arbitrator Third
Location
(No Symmetrix)