The seamless workflow that only a full-blown filesystem can offer- new revisions are created automatically when files are edited and saved.
I implemented something like VMS style versioning with Fuse and Perl for a talk/demo a couple of years ago - it simply created a new version on open() if any writeable flags were passed - not robust, but functional enough to demo.
edit Possessed by VMS, not multiple VMSes.
I'm not the biggest fan of FUSE filesystems(until highly-tuned, ala NTFS-3G), but they certainly do work.
* Using FUSE you gain the atomicity through synchronicity; when a write occurs, you stand between the disk and request. In so doing, you introduce the potential for latency (nevermind the latency inherent to FUSE).
* Using fsevents, you get notifications asynchronously, so there's no I/O performance hit. You may not get the events right away, but with fsevent APIs there's a bounded queue so you generally don't miss events, unless there are too many and the buffer is overflowed[1,2]. The events don't contain the actual file data, so you will lose intermediate edit data.
Thinking about this, it would be cool to exploit a Copy-on-Write filesystem (like ZFS) to scalably add the edit data to fsevent apis. I suppose you could preserve write buffers in memory temporarily instead of CoW, but that would have a nasty memory footprint.
[1]http://msdn.microsoft.com/en-us/library/system.io.filesystem... - Internal buffer size
[2]http://docs.oracle.com/javase/7/docs/api/java/nio/file/Watch... (see the watchkey queue and overflow)
† I tried answering that question myself, but your program kept crashing.
† Fixed. Sorry about that.