Introducing the OpenSSI Project
Richard Ferri and Brian J. Watson
Although we often think of Linux clusters synonymously with High Performance
Linux clusters, there are, in fact, many different types of Linux clusters.
In addition to the well-known High Performance Computing (HPC) Linux cluster
type, in the technical paper "Open Single System Image (OpenSSI) Linux Cluster
Project", Bruce Walker lists five other cluster types:
1. Load leveling -- Clusters that move processes from highly loaded nodes
to more lightly loaded nodes, as in openMosix (see related article "The Secrets
of openMosix" in this issue)
2. Web serving -- Typically a cluster with one or more front-end nodes sending
HTTP requests to back-end Web serving nodes
3. Storage -- Clusters that provide coherent views of modern cluster-wide
file systems
4. Database -- Clusters that provide coherent access to databases
5. High Availability (HA) -- Clusters that provide resource redundancy and
failover in case of failure
Not that there is any definitive list of Linux cluster types, mind you. In
Linux Clustering, Charles Bookman mentions another type of cluster, the
distributed cluster, in which loosely coupled workstations cooperate to solve
grand problems, such as the search for extraterrestrials. However, Bookman groups
MOSIX (and by association, openMosix) in with the distributed class, and puts
Web-serving solutions in the load-leveling class. Walker put MOSIX in the load-leveling
class and put Web serving in a class by itself. To muddy the waters further,
Greg Pfister, in In Search of Clusters, classifies clusters as "a subparadigm
of the parallel/distributed realm". But wait, didn't Bookman classify distributed
and parallel as types of clusters? So, are clusters subcategories of distributed
and parallel computing, or are distributed and parallel types of clusters?
Clearly, classifying the different types of clusters into the greater realm
of computers is a slippery slope.
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