I'm the not the knowledgeable about this but I'll bite. Journalling isn't a different format for the MacOS. It's a sits on top of the MacOS extended format and records keeps track of what happens on you hard drive so if there's an interuption, like a power outtage, it knows were to pick up from were it left off. To make a long story short, it helps you bounce back from crashes and stuff. You enable or disable journaling depending what you feel like in the disk utility application. Hopefully somebody will reply to this post and explain the performance dis/advantages to it.