Stuff like this is cool, but it really points out just how poor the filesystem is for organizing certain types of data. The "Why does TMSU not detect file moves and renames?" question in the FAQ really highlights how this isn't helped along by the filesystem at all.
One comment mentioned BFS, which had some really cool stuff. There's an Ars Technica article that touches on some of it[0].
The secret to BFS, in my mind, is that applications use it. The Haiku Mail app, as noted in the article, used the filesystem as its email database by attaching its own attributes to messages. This is also an example used in the "Practical Filesystem Design with the Be Filesystem" book[1].
Unless the metadata becomes a first class citizen in the filesystem, any attempts to layer it on top will have problems. Either applications won't understand it or normal filesystem operations will cause the metadata database to become de-synced with the filesystem data.
MacOS does an OK job of helping the user find things with Spotlight, but it's not a full metadata system like BFS had.
Mail.app, for example, keeps each message in a separate file[0] (and probably has a cache or separate database of this to make displaying mailboxes quicker). This makes it easy for Spotlight to index, but all of the stuff that you'd think of as metadata is actually just regular data inside the .emlx file.
If Apple made a huge effort to start treating the metadata (assuming the infrastructure described in the Ars Technica article still exists) as a first class citizen and using it like BeOS did, maybe we can get there. This would be a drastic rethink though. It feels like files, in some ways, are becoming second-class citizens in the Mac world. Photos, for example, are managed in the Photos app - you do not go into the filesystem and organize your photos.
One big problem with filesystem metadata is how do you transfer it? The Ars article showed a sidecar file (._filename) being created when the file was copied to a non-HFS volume. Now the metadata is detached from the file and we're back to the same problem.
One comment mentioned BFS, which had some really cool stuff. There's an Ars Technica article that touches on some of it[0].
The secret to BFS, in my mind, is that applications use it. The Haiku Mail app, as noted in the article, used the filesystem as its email database by attaching its own attributes to messages. This is also an example used in the "Practical Filesystem Design with the Be Filesystem" book[1].
Unless the metadata becomes a first class citizen in the filesystem, any attempts to layer it on top will have problems. Either applications won't understand it or normal filesystem operations will cause the metadata database to become de-synced with the filesystem data.
[0] http://arstechnica.com/information-technology/2010/06/the-be...
[1] http://www.letterp.com/~dbg/practical-file-system-design.pdf...