Monday, April 19, 2010

What does 99.999% mean?

When someone promises "99.999%" uptime, they're probably lying. It's a goal of many IT professionals, but seldom achieved.
Just do the maths to figure out why. 99.999% of 1 year is 365*0.99999 which is 364 days, 23 hours 54 minutes and 44.6 seconds. So if your server reboot takes longer than 5 minutes, you have already run out of time, even in a year. I have created a table with the allowable downtime for various percentages:

%Per YearPer Month
99.999%5 mins25 sec
99.99%52 mins4 min 22 sec
99.9%8 hrs 45 mins43 mins
99%3 days 15.6 hrs7 hrs 18 mins
95%18 days 6 hours 36.5 hours

Percentages are often used to hide unpleasant facts. For example, a condom that is 95% effective means that if you have sex more than 20 times, you might as well not bother, because 95% means 19 times out of 20 you're protected, and the twentieth time it will fail, if not sooner.


Anonymous said...

To be fair, you should not use the same condom more than 20 times :P

Donn Edwards said...

Brilliant comment! LOL

Anonymous said...

And you are wrong here, with the condom example. Because that is only what that is: a probability of not protecting you. It may break on you sooner than 20th time you use a condom (different one :D) and it may never break on you for 20 years of frequent using. It's just your luck. But that doesn't mean that every 20th condom will not work on you.

95% is just a mean value. You should properly understand that.

Donn Edwards said...

Actually, it does mean that *on average* every 20th condom will fail. That's what an average value means.

As you rightly point out, it could mean that the condom *never* fails, but that also implies that for some poor victim it *never works* either. Its obvious.

All I'm trying to say is that a "risk of 1 in 20" *seems* a lot more dodgy than "a 95% success rate", even if they mean the same thing.