Why not using DAX FS mode for Optane DC memory for instance and serialize the data structure by hand? I'm not sure how much performance you lose. That said the data store I'm working on serializes the huge persistent in-memory index to durable storage in a postorder traversal (mainly changed records in a data page Plus path copies) and atomically sets an offset to the main root, the UberPage to the new revision in a file. So, I'd say it's precisely what you describe :-)