2008-04-08
An alternate take on availability numbers
The usual way of presenting availability numbers is to cover how much (or how little) downtime you have per year. As another way of understanding them, let's turn that around and ask how much less downtime you have as you move from one availability level to a higher one.
| 90% to 99% | two hours less in a day almost 33 days less a year |
| 99% to 99.9% | three days less in a year |
| 99.9% to 99.99% | just under eight hours less in a year |
| 99.99% to 99.999% | 47 minutes less in a year |
| 99.999% to 99.9999% | just under five minutes less in a year |
Looking at the numbers this way makes it clear why so few people pay for very high availability; objectively, you don't get that much for your large quantities of money. Towards the higher end of the chart, you have to be asking how much money you are making (or losing) per minute in order for an availability bump to make any sense.
(Conversely, you get a lot of value at the low end of the availability numbers, assuming that you are willing to ignore exceptional disasters that require very costly things like replicated data centers.)