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How to calculate availability

How can I mathematically calculate availability if I add a standby system?

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A reader asked Evan the following question:


Example: Two system operating availability 97%.

What is the availability if I add a standby system (n+1) and how shall I calculate it?

Here is Evan's answer:

There are two different ways to interpret your question. The first is simply how you calculate the availability of a system that has been in operation for some time. For that, I offer the simple formula:

     Uptime + Downtime

- A = the percentage of time that the system is operational
- Uptime = the amount of time during the period in question that the system was up
- Downtime = the amount of time during that same period that the system was down

It's pretty simple really.

What I suspect you were asking in reality, though, is how you can predict availability. That's not quite as simple. In fact, it's effectively impossible to predict availability. I cannot tell you how many percentage points you can increase your availability if you cluster two or more systems together. I can tell you that if you do it properly you should see an improvement in your availability and it should be fairly significant.

If you have one system with historic 97% availability as you suggest and you cluster with an identical system from which it is reasonable to expect the same levels of availability, that second system should cover you 97% of the time that the first system is down. 97% of 3% is 2.91%. The two clustered machines, if clustered properly, and if there are no calamities that affect both systems, should be available 97 + 2.91 or 99.91% of the time.

If you reread that last paragraph, you'll find a lot of ifs. That's because there are a lot of conditions that must be met before you can hope for anything that even resembles a guarantee of availability. So be careful what you promise. Those calculations also do not take external factors like networks and storage into account; protecting those assets requires a different approach.

On the other hand, 97% availability translates to a total downtime of nearly 11 days a year, while 99.91% availability is a little less than eight hours over a year. What would it be worth to your organization to reduce your downtime by more than 10 1/2 days a year?

I hope this is helpful.

Evan L. Marcus


This was first published in August 2003

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