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

Disaster Tolerance and Recovery in an MC/ServiceGuard Cluster
Disaster Tolerant Architecture Guidelines
Chapter 136
It also uses network bandwidth, whereas most physical replication
methods use a separate data replication link. As a result, there may
be a significant lag in replicating transactions at the remote site,
which affects data currency.
• If the primary database fails and is corrupt, and the replica takes
over, the process for restoring the primary database so that it can be
used as the replica is complex. It often involves recreating the
database and doing a database dump from the replica.
• Applications often have to be modified to work in an environment
that uses a logical replication database. Logic errors in applications
or in the RDBMS code itself that cause database corruption will be
replicated to remote sites. This is also an issue with physical
replication.
• Most logical replication methods do not support personality
swapping, which is the ability after a failure to allow the secondary
site to become the primary and the original primary to become the
new secondary site. This capability can provide increased up time.
Ideal Data Replication The ideal disaster tolerant architecture, if
budgets allow, is the following combination:
• For performance and data currency—physical data replication.
• For data consistency—either a second physical data replication as a
point-in-time snapshot or logical data replication which would only
be used in the cases where the primary physical replica was corrupt.
Using Alternative Power Sources
In a high-availability cluster, redundancy is applied to cluster
components, such as PV links, redundant network cards, power supplies,
and disks. In disaster tolerant architectures another level of protection is
required for these redundancies.