if you are avoiding the use of foreign keys for the high and lofty goal of "performance" and 100% uptime while you run your fancy online migration utilities, SE

if you are avoiding the use of foreign keys for the high and lofty goal of "performance" and 100% uptime while you run your fancy online migration utilities, SERIALIZABLE isolation is the last thing you would be using as it does what it says, serialiazes transactions and locks things like crazy. lots of waiting on locks, lots of deadlock potential. Throw in InnoDB's quirky implementations of isolation levels and you'd be in for a world of fun.