Overview#

Availability Answers the questions of Are you there?

Availability of a Personal Entity#

Availability of a Personal Entity is often referred to as Presence

Availability of a system#

Availability of a system is typically measured as a factor of its Reliability - as reliability increases, so does Availability . Availability of a system may also be increased by the strategy on focusing on increasing testability & maintainability and not on reliability.

Improving maintainability is generally easier than reliability. Maintainability estimates (Repair rates) are also generally more accurate. However, because the uncertainties in the reliability estimates are in most cases very large, it is likely to dominate the availability (prediction uncertainty) problem, even while maintainability levels are very high.[1]

How Reliable Are Your Systems#

The technology for disaster recovery and business continuity plans differs dramatically depending on the percentage of uptime that must be delivered.

Any project planning should include some statement of acceptable availability which directly influences design considerations.

This table shows the amount of time your service would be allowed to be un-avalable for each level of Availability.

AvailabilityDowntime/yearDowntime/monthDowntime/weekDowntime/day
98.0000%7.30 days14.4 hours3.360 hours28.80 minutes
99.0000%3.65 days7.20 hours1.680 hours14.40 minutes
99.5000%1.83 days3.60 hours50.40 minutes7.200 minutes
99.9000%8.76 hours43.2 minutes10.10 minutes1.440 minutes
99.9900%52.3 minutes4.32 minutes1.010 minutes8.660 seconds
99.9990%5.26 minutes25.9 seconds6.050 seconds0.864 seconds
99.9999%31.5 seconds2.59 seconds0.065 seconds0.009 seconds

Do not Over Do it#

Moving up in reliability form one level to another is very costly as a percentage of the project cost.

You should balance the risk vs rewards vs the costs for each level.

More Information#

There might be more information for this subject on one of the following:

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« This page (revision-22) was last changed on 23-Jul-2016 12:33 by jim