Microsoft® SQL Server High Availability

Calculating “Availability”

Calculating what the availability of a system has been (or needs to be) is actually quite simple. You simply subtract the “time unavailable” from the “meantime between unavailability” and then divide this by the same “meantime between unavailability.” Use a common time factor as the basis of this calculation (like minutes). The “time available” is the actual time, if calculating what has already happened, or the estimated time, if doing this for the future—in addition, it is here that you add in all unplanned and planned downtime. The “meantime between unavailability” is the time since the last unavailability occurred.

Design Note A system that needs to be up 24 hours per day, 7 days a week, 365 days per year would ...