Interesting article on Digg and PHP’s scalability and performance. The article mentions 8 slave database servers for just 3 webservers, which seemed an odd configuration.
I posted a question but the author didn’t elaborate much aside from linking to an interesting set of slides about LiveJournal. This walks through increasingly elaborate scaling scenarios. (Edit: these slides are from 2004, a later 2005 presentation has more information)
One interesting techique is lumping sets of users together into clusters, so that you can have a master DB for each cluster. Beyond that, they’ve built a master-master setup, where you perform your writes to twin masters. Looking at these techniques, it becomes a little clearer how you might have 3 webservers with 8 db servers.