Comment by Intralexical

This is good thinking, but I think you are basically describing a Restic/Borg respository :)

- Deduplication? Check.

- Compact format? Check.

- Good read speed? Yep. (Proportional to backing store.)

- Custom purpose-built? Yeah, that's what backup programs are for.

- Custom data layout? Check. (Rabin/BuzHash content-defined chunking, SHA256 dedupe.)

- Adding additional versions? Yes. ("Incremental backups"— See above.)

- Virtual filesystem access? `restic mount`/`borg mount`.