John D. Mitchell's Blog

April 2005 Archives

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?

GCC turns 4.0

The GNU folks have released version 4.0 of the venerable GCC compiler with built-in support for the C, C++, Objective-C, Ada, Fortran, and Java programming languages.

The biggest general change is the completely new intermediate language representation based on tree SSA. SSA (Static, Single Assignment) is a modern approach to the intermediate representation of the parsed programs which allows for a much more sane and aggressive approach to optimization.

On the Java front, the GCJ sub-project has made major improvements including better support of AWT and Swing and a lot more of the other Java libraries such as java.util.regex. If you didn't know, GCC can generate native (machine-specific) binaries directly from Java code.

Check out the ChangeLog for more details.

This work is licensed under a Creative Commons License.