Datasheet

“main” (Installation and Administration) — 2004/6/25 — 13:29 — page 394 — #420
i
i
i
i
i
i
i
i
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. Hard-
ware 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 corre-
sponds to a downtime of between five minutes and three seconds over an
entire year. The most important factor is not just the software and hard-
ware side, but, primarily, well-conceived system administration with well-
documented and understandable processes for minimizing faults. In every
case, it involves weighing risks and costs. Different requirements and so-
lutions may be appropriate, depending on the application scenario. Your
Novell partner will be happy to advise you.
19.1 Important Terms
Here are a few important terms related to high availability:
SPOF Single Point of Failure: Component of a system whose failure impairs
the functioning of the whole system.
Failover Another similar system component automatically takes over the
function of a failed component.
Cold Standby The alternative hardware is on cold standby. The failover
must be performed manually, so the failure will be clearly apparent.
Warm Standby The backup system runs in the background, so the trans-
fer can take place automatically. The data on both systems is auto-
matically synchronized. For the user, the failover is like a very fast
automatic service reboot. However, the current transaction may be
aborted because it was not possible to synchronize the data prior to
failure.
Hot Standby Both systems permanently run in parallel — data on both
systems is one hundred percent synchronized. Users will not be
aware of any failures. This level cannot usually be reached without
making a corresponding modification to the client. To run both sys-
tems completely synchronously, the connections to the client must
394 19.1. Important Terms