Know Your Nines
If you're negotiating a service agreement, sooner or later you have to confront the question of how many "nines" you want to buy.
Often, businesses want at least "five nines" of uptime, meaning the service will be up 99.999% of the year. (Though it often isn't clear what exactly that means.) Certain businesses, like financial firms, may require more availability than this.
Five nines, though, can be expensive. Often money saved from cutting the service level down a nine could better be used on something else. Joel Spolsky has a nice, non-theoretical discussion the issue here. Here is another helpful analysis.
Unless you live and breathe these issues, those articles are probably worth rereading before negotiating your next SLA. At the least, it's not a bad idea to know the approximate meaning of the various service levels for annual uptime. For instance:
- 99.9999% uptime means about 30 seconds of downtime.
- 99.999% uptime means about 5 minutes of downtime.
- 99.99% uptime means about 1 hour of downtime.
- 99.9% uptime means about 9 hours of downtime.
- 99.0% uptime means about 90 hours of downtime.
I find it much easier to think of these issues it terms of how much approximate downtime I am "buying." Then the question becomes, "Can I live with that?" Often it's not easy to say, for the reasons Spolsky points out on his blog. But at least it clarifies the problem.
(By the way, if you haven't had enough of the number nine yet, you can always get John August's highly entertaining and enigmatic movie on DVD.)
