Role abstractions

Clusters commonly consist of groups of many machines that behave the same way. The large majority of nodes are usually used to compute scientific algorithms, run database applications, or provide high availability or failover services for web servers and the like. However, a smaller set of nodes in the cluster usually have auxiliary functions necessary to the operation of the cluster.

A typical cluster may have administration nodes, front-end nodes, login nodes, and nodes dedicated to providing filesystem I/O to the rest of the cluster. For these nodes, it is desirable to have a root filesystem image that is identical to the main `compute' nodes. Although using the same filesystem image for `admin' or `IO' nodes is not required, for example as for `compute' nodes, consistency helps ease administration and reduce the overall complexity of the system.