High Performance Linux Clusters with OSCAR, Rocks, OpenMosix, and MPI [Electronic resources] نسخه متنی

1.2 Types of Clusters

Originally,
"clusters" and
"high-performance computing" were
synonymous. Today, the meaning of the word
"cluster" has expanded beyond
high-performance to include high-availability (HA)
clusters and load-balancing (LB)
clusters. In practice, there is considerable
overlap among thesethey are, after all, all clusters. While
this book will focus primarily on high-performance clusters, it is
worth taking a brief look at high-availability and load-balancing
clusters.

High-availability clusters, also called
failover clusters, are often used in
mission-critical applications. If you can't afford
the lost business that will result from having your web server go
down, you may want to implement it using a HA cluster. The key to
high availability is redundancy. An HA cluster is composed of
multiple machines, a subset of which can provide the appropriate
service. In its purest form, only a single machine or server is
directly availableall other machines will be in standby mode.
They will monitor the primary server to insure that it remains
operational. If the primary server fails, a secondary server takes
its place.

The idea behind a load-balancing cluster is to provide better
performance by dividing the work among multiple computers. For
example, when a web server is implemented using LB clustering, the
different queries to the server are distributed among the computers
in the clusters. This might be accomplished using a simple
round-robin algorithm. For example,

Round-Robin
DNS could be used to map responses to DNS
queries to the different IP addresses. That is, when a DNS query is
made, the local DNS server returns the addresses of the next machine
in the cluster, visiting machines in a round-robin fashion. However,
this approach can lead to dynamic load imbalances. More sophisticated
algorithms use feedback from the individual machines to determine
which machine can best handle the next task.

Keep in mind, the term
"load-balancing" means different
things to different people. A high-performance cluster used for
scientific calculation and a cluster used as a web server would
likely approach load-balancing in entirely different ways. Each
application has different critical requirements.

To some extent, any cluster can provide redundancy, scalability, and
improved performance, regardless of its classification. Since
load-balancing provides greater availability, it is not unusual to
see both load-balancing and high-availability in the same cluster.
The

Linux Virtual Server Project
(LVSR) is an example of combining these two
approaches. An LVSR server is a high-availability server implemented
by distributing tasks among a number of real servers. Interested
readers are encouraged to visit the web pages for the Linux Virtual
Server Project (http://www.linux-vs.org) and the
High-Availability Linux Project
(http://www.linux-ha.org) and to
read the relevant HOWTOs. OSCAR users will want to visit the
High-Availability OSCAR web site
http://www.openclustergroup.org/HA-OSCAR/.