Availability


To explain the background of MTBF calculation

Image via Wikipedia

Availability

Availability is a measure of the accessibility of a system or application, not including scheduled down-time. It can be measured as a ratio of expected system up-time relative to the total system time, which includes uptime
and recovery time when the system is down.

Even if you’re on the right track, you’ll get run over if you just sit there.

Will Rogers

Source: http://quotes4all.net/quote_1235.html

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A = [MTBF/(MTBF + MTTR)] x 100%
Where
MTBF = Mean Time Between Failure; average elapsed time between failures
MTTR = Mean Time To Recover; average time required to recover from a
failure

High availability

High availability refers to the ability of a system or component to be operational and accessible when required for use for a specified period of time. The system or component is equipped to handle faults in an unplanned outage gracefully to continue to provide the intended functionality.

Continuous availability

Continuous availability refers to the ability of a system or component to be operational and accessible when required for use at all time. The system or component is designed such that a user experiences zero downtime.

Availability is usually in “9s” notation. The following table shows 6 classes of “9s” and their corresponding downtime hours.

9s

Percentage of uptime

Downtime per year

Downtime per week

Downtime per day

One 9

90.00%

36.5 days

16.9 hours

2.4 hours

Two 9s

99.00%

3.65 days

1.7 hours

14.4 minutes

Three 9s

99.9%

8.76 hours

10.1 minutes

1.4 minutes

Four 9s

99.99%

52 minutes

1 minute

8.6 seconds

Five 9s

99.999%

5 minutes

6 seconds

86.4 milliseconds

Six 9s

99.9999%

32 seconds

604.8 milliseconds

8.6 milliseconds

Source:

High Availability in
WebSphere Messaging
Solutions

– IBM RED BOOK. 

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