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12
High Availability under Linux
This chapter contains a short overview of the key concepts and tools from the area of
high availability under Linux. It also offers suggested further reading for all the topics
mentioned.
High availability describes systems that can mask certain malfunctions—in particular,
the failure of individual computers—so the service can be made available to the user
again after only a short downtime. Hardware and software are carefully coordinated
and laid out for redundancy, enabling an automatic switch to the other components in
the event of a malfunction. High availability differs from "error tolerance" because the
service is temporarily unavailable for the short service switchover phase, which can be
noticed in delays or short losses in connection.
A high availability system particularly means when the overall availability of the service
is between 99.999 percent and 99.99999 percent. This corresponds to a downtime of
between five minutes and three seconds over an entire year. The most important factor
is not just the software and hardware side, but, primarily, well-conceived system admin-
istration with well-documented and understandable processes for minimizing faults. In
every case, it involves weighing risks and costs. Different requirements and solutions
may be appropriate, depending on the application scenario. Your Novell partner will
be happy to advise you.
12.1 Important Terms
Here are a few important terms related to high availability:
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Troubleshooting
