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.