In the past two weeks, I have been called in for some near-emergency tuning of two rails applications that recently went into production. One was for a state-level government agency, the other was for a startup that recently went into production and found performance problems as their app grew in popularity. In both cases, the first place I looked was the innodb table settings for mysql - and in both cases, I found things that could immediately help the application in question.
I'm going to walk you through that thought process now, potentially teach you something about your own rails app, and hopefully improve the performance of your rails application.