The Weak Link in Drupal Scalability (99.9% of you can ignore this article)

Drupal is as scalable as the applications it relies on. While there's nothing preventing Drupal itself from being scaled across any number of web servers, scalability of open-source databases such as MySQL and PostgreSQL is an ongoing issue. While these databases do support replication, that's not always enough. For a very small percentage of websites, daily user visits are counted not in hundreds or thousands but in hundreds of thousands. Chances are pretty good that you're site is not in this minority, but if it is (or if you anticipate it will be), you will need to consider the limitations imposed by resources that are necessarily shared by the tools you are using. It doesn't do much good to have 100 redundant webservers if they are sharing a single NFS mount. Likewise, open-source database technology currently suffers from a lack of "shared nothing" capability. Dana Blankenhorn discusses this in his article Shared nothing coming to open source. I have confidence that by the time the other ninety-nine percentile need to start worrying about this problem that it will be addressed by the open source community. If you end up needing to solve the problem before then, be sure to let us know how you did it :)