How available do you want your services? A lot? Downtime is never desirable, so you want zero downtime, right? OK, that's impossible, so what is reasonable, and how much are you willing to pay for it?
Uptime is often measured in 9's, meaning how many nines in a percentage of uptime is acceptable? 99%, 99.9%, 99.99%, or even 99.95% if you want to be more precise. Of course, this is the desirable uptime — there will be years you are 100% available, then years you will fall far short of your goal.
Let's look at some uptime percentages:
| Uptime | Outage Duration per Year |
| 99% | 3.65 days |
| 99.5% | 1.83 days |
| 99.9% | 8.76 hours |
| 99.95% | 4.38 hours |
| 99.99% | 52.56 minutes |
| 99.999% | 5.26 minutes |
| 99.9999% | 31.50 seconds |
| 99.99999% | 3.15 seconds |