@jamey I'm afraid I don't have much in the way of high level documentation.

I have https://doc.obnam.org/obnam/arch.html as a high level software architecture overview. If you're short of focus that might still be too detailed?

I have https://doc.obnam.org/obnam/obnam.html for detailed acceptance criteria and how to verify them, but that's too detailed for you.

@liw Those both look like good documents which, indeed, I'm too sleepy and distracted for at the moment.

I'll just say the main thing I would love to have out of a backup tool's storage layer is something that reliably makes backups even during times when the main storage is unavailable, such as when cloud storage is unreachable or an external backup drive is not plugged in, with the ability to sync updates to those stores later. The way I currently do that is to write my backups to the same disk that I'm backing up, then later manually trigger rsync from the backups to a remote server or to a USB disk. My guess is that backup software which is designed for this use could do better; perhaps keeping only an index of already-known chunks locally, for instance, rather than the complete backup.