Wow, ditching the b-trees is a bold move.

Will this make obnam less sensitive to roundtrip latency? Sounds like you can stream out data into a bag and then do essentially one update at the end of the metadata..

The bag.1 format seems possibly suboptimal, if only part of a bag needs to be accessed to restore, you'd need to download the whole thing. An offset allowing random access might be better?

I suspect this will also make pruning old generations more expensive, since some bags will need to be rewritten with parts removed.