(I think the one that holds the thumbnails, but I'm not at my Mac and I'm not sure I'm remembering correctly). I was originally thinking that it only matters if iPhoto needs to access that file a lot ...

This would probably only affect performance when the drive begins to fill up. Nortons reported that my free space was in about 5,000 pieces before the defrag. Drive free space totaled about 50 gig, so ...