Today, it is exactly 23 years, nine months, 19 days; or 8,693 days; or 751,075,200 seconds; or 12,517,920 minutes; or 208,632 hours; or just less than 1,241 weeks since the launch of the original IBM PC on Aug. 12, 1981. For those of you who might prefer the metric system, this is, of course, equivalent to 42.42 liters or 2,000 kilometers per femtosecond if we ignore leap years.

A wonderful service on timeanddate.com told us so. This site provides a number of almost useful calculators that determine such timely things as the duration between two dates, or when alternative birthdays (such as when you are 1 billion seconds old) will occur.

Other than providing you with yet another site on which to waste your highly valuable time when you should be doing far more productive things, we bring this up as a fairly thin, albeit not completely, uninteresting way for us to note how far PCs have come in the short time since their launch.

What brought this ooh-ah moment home for us was receiving a fantastic new book titled The Linux Enterprise Cluster by Karl Kopper.

The Linux Enterprise Cluster is a how-to book and explains how to convert two or more PCs into a high-reliability, high-availability cluster based on Linux and inexpensive hardware using free and mainly open source software - what would have been an unthinkable configuration back when mainframes ruled the earth.

The book starts by exploring what is meant when we talk about a "cluster" and offers the definition of a system that can be used as "a single computing resource" using "a local computing system comprising a set of independent computers and a network interconnecting them."

Key to the whole concept is that a cluster must not have a single point of failure. Should any of the individual computers in the cluster (the "nodes") fail, there must not be a failure of any service provided by the cluster. This means that any node in the cluster can fail and be rebooted without users of the cluster being aware of the events.

This leads to the four basic properties of a cluster, which are all about what we could quite reasonably, call "transparency":

• Users accessing cluster services don't know that they are using a cluster.
  
  

• Nodes that comprise the cluster don't need to be aware that they are part of a cluster.
  
  

• Applications running on nodes don't need to know they are running in a cluster environment.
  
  

• Servers that are not part of the cluster don't need to know when they are providing services to nodes in a cluster.
  
  

The basic architectural elements of a cluster are a load balancer, shared data storage and output devices. The load balancer sits between the nodes and the users and distributes the incoming workload to the node services. The shared data storage must support lock arbitration to ensure exclusive access for each process to items (files, blocks or bytes, as required) in the file system. The final basic architectural element, output devices, covers printers, fax lines, and so on.

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