To quantify the amount of availability achieved, you have to perform some calculations: Committed hours of availability (A)This is usually measured in terms of number of hours per month, or any other period suitable to your organisation.Example: 24 hours a day, 7 days a week = 24 hours per day x 7 days x 4.33 weeks per month (average) = approximately 720 hours per month Outage hours (B)This is the number of hours of outage during the committed hours of availability. If high availability level is desired, consider only the unplanned outages. For continuous operations, consider only the scheduled outages. For continuous availability, you should consider all outages.Example: 9 hours of outage due to hard disk crash, 15 hours of outage for preventive maintenance Next you can calculate the amount of availability achieved as follows:Achieved availability = ((A-B)/A)*100 percent For the statistics in the examples above, here's each calculation:
- High availability = ((720-9)/720)*100 percent = 97.92 percent availability
- Continuous operations = ((720-15)/720)*100 percent = 98.75 percent availability
- Continuous availability = ((720-24)/720)*100 percent = 96.67 percent availability
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