Looking through the documentation, though, it's even worse than just saying every alter is an offline alter. There are a few, but significant, schema changes that one could want to make -- like when dealing with a table that has full text search -- that need to be done offline; you have to tread real real lightly in dealing with online alters. I'd personally argue that online alter is not ready for prime time with 5.6; there are too many other things they need to do with the database engines to make it really useful.
