poking around in private view I found this old picture in a draft post from 3/3/13.
originally we had the app and mysql slave servers separate, but as we cut more sites over to kinja the slaves were sitting nearly idle while the apps were getting overloaded during deploys. so we made every app its own slave over loopback to spread the work out evenly across all of them.
that was meant to buy us time to finish the physical migration, but before we could finish we had already outgrown the 12 servers for each DC and needed to bump to 16. four of the servers in the red box stayed behind and 8 more got bought for the new site.
a year or so later 32 wasn't enough anymore so we bought 8 more to get to today's 40.
until last weeks performance improvements, we never really had enough cpu head room to go back and re-separate the mysql slaves. now we do, but bandwidth is another story.