title: Erlang's 99.9999999% (nine nines) reliability
body: <p><a href="http://www.erlang.org/">Erlang</a> was reported to have been used in production systems for over 20 years with an uptime percentage of 99.9999999%.</p>
  
  <p>I did the math as the following:</p>
  
  <pre><code>20*365.25*24*60*60*(1 - 0.999999999) == 0.631 s
  </code></pre>
  
  <p>That means the system only has less than one second of downtime during the period of 20 years. I am not trying to challenge the validity of this, I am just curious about how we can shut down a system (on purpose or by accident) for only 0.631 second. Could anyone who are familiar with large software system explain this to us? Thank you.</p>
  
  <hr>
  
  <p>Does anyone know how to calculate the downtime of a service over a cluster of processing units (or machines)?</p>