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High availability clusters (HAC) improve availability of applications by failing them over or
switching them over in a group of systems as opposed to High Performance Clusters which improve performance of
applications by allowing them to run on multiple systems simultaneously.
Most Veritas Cluster Server implementations attempt to build availability into a cluster,
eliminating single points of failure by making use of redundant components like multiple network cards, storage
area networks in addition to the use of VCS.
High-availability clusters (also known as HA Clusters or Failover Clusters) are computer
clusters that are implemented primarily for the purpose of providing high availability of services which the
cluster provides. They operate by having redundant computers or nodes which are then used to provide service when
system components fail.
Normally, if a server with a particular application crashes, the application will be unavailable until someone
fixes the crashed server. HA clustering remedies this situation by detecting hardware/software faults, and
immediately restarting the application on another system without requiring administrative intervention, a process
known as Failover. As part of this process, clustering software may configure the node before starting the
application on it. For example, appropriate filesystems may need to be imported and mounted, network hardware may
have to be configured, and some supporting applications may need to be running as well.