Existing Solutions

This territory has been tread before by many people. I just want to highlight a few examples of existing solutions. This is not meant to be a comprehensive list. I am mostly interested in noting some different approaches.

Name Link Concept Notes
IBM DB2 for i link_1
(unconfirmed) Weird name. I don’t have any firsthand knowledge of this. But Wikipedia describes it as a “database file system”, so perhaps it is similar to what I am calling database-like filesystem. Or, more likely, it is a database overlay on an existing filesystem.
WinFS link Database overlay Perhaps the most famous database-like filesystem that never was. This was a Microsoft project to revolutionize file handling that was never released. It used a database layer on top of the existing hierarchical NTFS filesystem.
DBFS link Database overlay This was an open source project to create a database-like interface to files. It uses a database layer on top of the existing hierarchical filesystem.
Many audio players Database overlay Many audio players treat your existing 2-D store of audio files as a data resource. They read all those files and create a database of music. When you want to play something, you search in the database. The software plays the correct underlying files.
Many image viewers Database overlay Images collections can be large, so developers have been tempted to roll out database overlays for managing them. Your files are still stored in a hierarchical fashion underneath, but you interface with a database to find what you are looking for.
ID3 link File-level metadata ID3 tags are a success story of file-level metadata. These are a way of tagging audio files with metadata such as artist, title, track, album, and so on. This metadata can then be used audio players to give more info about a track. And many players also allow you to sort and filter tracks by their metadata.
EXIF link File-level metadata EXIF is file-level metadata for images. These are a successful way of tagging images with many different attributes, such as date, camera type, camera settings, and so on. Applications that deal with images have varying degrees of support for this metadata.
IPTC link File-level metadata IPTC was conceived as file metadata for a variety of file types, but found fame as metadata in photos by professional and news photographers.
XMP link File-level metadata XMP is another kind of metadata that can be applied to images. This one was conceived to be applied to different sorts of documents, but in reality it cannot be used on just any file.
xattr link File-level metadata Extended file attributes (xattr) are interesting because they are file-level metadata that can be applied to any file. These were conceived as a way to store a few extra attributes of the file, not massive amounts of metadata. There are not many tools that make use of this metadata. If the file is transferred to an unfriendly filesystem, the metadata is lost.
this list is really just a starting point

Some Links