The cluster infrastructure provided as a part
of i5/OS™, called cluster
resource services, provides resiliency for your critical resources. These
resources can include data, applications, devices, and other resources accessed
by multiple clients. 
If a system outage or a site loss occurs, the functions
that are provided on a system within a cluster can be accessed through other
systems that have been defined in the cluster. There are two models in which
this data can be accessed: primary-backup model and peer model. For more details
on cluster resource groups (CRGs) that you can create based on these models,
see Cluster resource group.