Every time I've worked with mySQL I've seen some irksome behavior... just the same, setting up failover options is miles ahead of PostgreSQL. And when you already have in-house talent, it becomes even more obvious.

My only thought was that using a clustered database (such as Cassandra) as the store with the data itself might have been better. domain/url (minus querystring) would hash/distribute fairly well, and with even a relatively small cluster with 2 replica nodes for the shard would be pretty effective. Also, it would be easier to manage a replicated database, in my mind, than tracking sites to pairs of static servers. GoDaddy is/was moving to something similar with new development on one of their applications when I worked there, and able to serve a huge number of static requests (hundreds of thousands per second) off of a relative few servers with a sub 10ms response time, for content not backed by cdn.

In the end it just goes to show that serving static content on modern hardware can scale really well, with a number of options for technology. Which is why I'm somewhat surprised that something hasn't taken over the tide of poorly configured Wordpress blogs.