Good question. The structure is a B tree. When you traverse it, and arrive to a block, but don't see it in the reference counting base, you know that it is refe

Good question. The structure is a B tree. When you traverse it, and arrive to a block, but don't see it in the reference counting base, you know that it is referenced in the tree, so its reference count is at least 1. Then, if you want to free it, you have to delete all references to it in the tree. Maybe that blog post could have been more verbose about another feature of Sanakirja, the list of free pages.