What does 99.999% reliability really mean?

Michael Levin posted a Java coding challenge this morning to write a Java program to show what terms like e.g., 99.999% reliability actually means.

Here's my quick and dirty take (written while watching F1 :-):

/*
 * What does X nines reliability really mean?
 */

import java.io.*;

public class Nines
{
    private static int SPM = 60;
    private static int SPH = SPM * 60;
    private static int SPD = SPH * 24;
    private static int SPY = SPD * 365;

    public static void main (String[] args)
	{
	spit ("Nines of Reliability: (Hours / Minutes / Seconds)\n");
	spit (2, SPY / 100.0);
	spit (3, SPY / 1000.0);
	spit (4, SPY / 10000.0);
	spit (5, SPY / 100000.0);
	spit (6, SPY / 1000000.0);
	spit (7, SPY / 10000000.0);
	}
	
    private static void spit (int numNines, double seconds)
	{
	spit (numNines + " 9's (");
	for (int i = 0; i < numNines; i++)
	    {
	    if (2 == i)
		spit (".");
	    spit ("9");
	    }
	spit ("%) = up to ");
	spit ((seconds / SPH) + "h / ");
	spit ((seconds / SPM) + "m / ");
	spit (seconds + " seconds of downtime per year.\n");
	}
	
    private static void spit (String str)
	{
	System.out.print (str);
	}
}

Which tells us:

Nines of Reliability: (Hours / Minutes / Seconds)
2 9's (99%) = up to 87.6h / 5256.0m / 315360.0 seconds of downtime per year.
3 9's (99.9%) = up to 8.76h / 525.6m / 31536.0 seconds of downtime per year.
4 9's (99.99%) = up to 0.876h / 52.559999999999995m / 3153.6 seconds of downtime per year.
5 9's (99.999%) = up to 0.0876h / 5.256m / 315.36 seconds of downtime per year.
6 9's (99.9999%) = up to 0.00876h / 0.5256000000000001m / 31.536 seconds of downtime per year.
7 9's (99.99999%) = up to 8.76E-4h / 0.05256m / 3.1536 seconds of downtime per year.

So, 5 9's means less than 5 1/2 minutes of downtime per year. Hmm... Now, how long does it take your server just to boot once?