Yeah, this "two-layer storage paradigm" is exactly what I'm getting at. There are a lot of other ways you could slice it, though. System services want simple APIs with high performance -- they'd prefer not to pay the cost of the higher-level user document representation.

Personally, I think user documents shouldn't be a kernel concept -- it should be a protocol implemented by user-space system services. This would let any app expose its storage as "documents" without having to change its internal representation.