Cascading Failover in a Continentalclusters, December 2005

6. Re-establish the local mirror devices (device B’) in the secondary disk array.
Failure of Secondary Site within the Primary Cluster
When the secondary site fails, or all data replication links between the primary disk array and the
secondary disk array fail, unless data storage specific technology that will be suspend I/O operations
under such cases is used (for example, replication mode DATA for HP XP CA or Domino Mode for
EMC SRDF), 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 6.
Figure 6 - Failure of Secondary Site in Primary Cluster
Without the secondary site, the current configuration does not provide any means to replicate the new
data from the primary disk array directly to the recovery disk array. If the secondary site is down for
a long time, the data in the recovery disk array is very out-of-date. If the primary site fails during this
time, and the recovery site 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 replication pair between the primary disk array and the
secondary disk array will not be synchronized. If the local mirror in the secondary disk array
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 primary replication group before re-establishing the primary replication
pairs between the primary disk array and the secondary disk array. Use the following steps:
1. Split the local devices (device B’) in the secondary disk array from the primary replication
group.
2. Incrementally establish the primary replication pairs between the primary disk array and the
secondary disk array.
3. Check the state of the primary replication pair. Make sure that the data resynchronization has
completed from the primary site to the secondary site.
4. Once the copy completes, re-establish the local mirror devices in the secondary disk array
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