To deal with Drupal's growth, we're adding a second database server to drupal.org which is useful for at least two reasons. First, we'll be able to handle more SQL queries as we can distribute them between multiple database servers (load balancing). Secondly, this new server can act as a "hot spare" that can immediately take over if the other database server fails (high availability / fail-over).
![Scaling with MySQL replication](https://cdn-ak-scissors.b.st-hatena.com/image/square/e58cacbb112781a80345f951cfa83459dfa425de/height=288;version=1;width=512/https%3A%2F%2Fdri.es%2Fthemes%2Fcustom%2Fdries%2Fimg%2Fmugshot-dries.jpg)