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

Cascading Failover in a Continental Cluster
Data Replication Procedures
Chapter 8388
Scenario 2—Secondary Site within the Primary Cluster Fails
When the secondary site fails, or all SRDF links between the primary
Symmetrix and the secondary Symmetrix fail, unless domino mode is
used, the application running on the primary site is not aware of this
failure and continues to run on the primary site. This scenario is
illustrated in Figure 8-11.
Figure 8-11 Failure of Secondary Site in Primary Cluster
Without the secondary site, the current configuration doesn’t provide
any means to replicate the new data from the primary Symmetrix
directly to the recovery Symmetrix. If the secondary site is down for a
long time, the data in the recovery Symmetrix is very out-of-date. If the
primary site fails during this time, and the recovery takes over, the
customer will have to operate on an old copy of the data. Therefore, it's
important to fix and have the secondary site up and running as soon as
possible.
When the secondary site is fixed, the SRDF volume pair between the
primary Symmetrix and the secondary Symmetrix will be in
“Suspended” mode. If the BCV/R1 in the secondary Symmetrix contains
a good copy of the data, to protect this data from corruption in case of
rolling disaster, these devices must be split from the mirror group before
re-establishing the SRDF volume pairs between the primary Symmetrix
and the secondary Symmetrix. Use the following steps:
1. Split the BCV/R1 devices in the secondary Symmetrix from the
mirror group. From a host that connects to the primary Symmetrix:
# symmir -g <prisymdevgrpname> split -rdf