In an Serviceguard
cluster configuration, high availability is achieved by using redundant
hardware to eliminate single points of failure. This protects the
cluster against hardware faults, such as the node failure in Figure 1-1.
This architecture, which is typically implemented on one site
in a single data center, is sometimes called a local cluster. For some installations, the level of protection given by a local
cluster is insufficient. Consider the order processing center where
power outages are common during harsh weather. Or consider the systems
running the stock market, where multiple system failures, for any
reason, have a significant financial impact. For these types of installations,
and many more like them, it is important to guard not only against
single points of failure, but against multiple points of failure (MPOF), or against single
massive failures that cause many components to fail, such as the failure
of a data center, of an entire site, or of a small area. A data center, in the context of disaster recovery,
is a physically proximate collection of nodes and disks, usually all
in one room.
Creating clusters that are resistant to multiple points of failure
or single massive failures requires a different type of cluster architecture
called a disaster
tolerant architecture. This architecture provides you
with the ability to fail over automatically to another part of the
cluster or manually to a different cluster after certain disasters.
Specifically, the disaster tolerant cluster provides appropriate failover
in the case where a disaster causes an entire data center to fail,
as shown in Figure 1-2.
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