[mindajar]

HFS+ is an oddball filesystem among FSes that anyone actually uses. Since the earliest days of the Mac, it's been able to e.g. track file identity even as a file is moved around, and the metadata to support this lives in a volume-wide tree called the Catalog File. So, there's your global lock. (Add an SSD as necessary for better performance.)

OS X already has some pretty high-level file and metadata APIs not found on other systems, so maybe Apple's future plans don't look like a traditional Unix file system at all. They've already demonstrated they know how to make a very weird, non-Unix filesystem look like one. ;)