Performance Testing and analogies

Talking with a tester at my new company and mentioning how analogies can often help you to look at things differently. Today this got me thinking about performance testing and scuba diving. Are there concepts or practices from scuba diving that can be applied to performance testing?

One of the rules for technical divers is called The Rule of Thirds. The idea behind this rule is that you should keep one third of your air as an emergency reserve. If I understand how much air I use for a given dive I should multiply it by 1.5 to handle unforeseen emergencies. For example, if I calculate my air consumption to require 80 cubic feet of air then I should take an additional 40 cubic feet of air. Thus 120 cubic feet of air and one third (40 cubic feet) held in reserve. If nothing goes wrong, I'll end the dive with 40 cubic feet of air. If an emergency happens, I have the extra air.

So how does this relate to performance testing? Usually when you are doing performance testing, you have a set of criteria. For example, one criteria with be that there will be 450 concurrent users. What happens if some unforeseen situation occurs? You do all your testing with 450 concurrent users and it passes with a nice margin. You throw it into production and something no one anticipate occurs. Your application gets a huge hit. Apply The Rule of Thirds and use 600 concurrent users. If your tests pass with 600 concurrent users you know it will pass with 450 concurrent users and you are prepared if some unforeseen circumstance occurs.

If your application is say a web store and some celebrity mentions your product on the red carpet at the Oscars, do you want your web site to go down when 50% more users than you could have ever anticipated tries to buy your product?