In yesterday’s post, we discussed Checkpoint Maps, the simple linear-time data structures that APFS uses to manage persistent, ephemeral objects. Today, we will give a general overview of B-Trees and detail the layout and on-disk structures of B-Tree Nodes.

In our last post, we discussed NX Superblock Objects and how they can be used to locate the Checkpoint Descriptor Area in which they are stored. Today, we will discuss the other type of objects that are stored in the descriptor area, Checkpoint Maps, and how they can be used to find persistent, ephemeral objects on disk.

The NX Superblock Object is a crucial component of APFS. It stores key information about the Container, such as the block size, total number of blocks, supported features, and the object IDs of various trees and other structures used to track and maintain other objects. The on-disk nx_superblock_t structure is used as the root source of information to locate all other objects in the checkpoint. In this post, we will go into detail about this structure as well as discuss methodology that can be used to locate them on-disk.

APFS is a pooled storage, transactional, copy-on-write file system. Its design relies on a core management layer known as the Container. APFS containers consist of a collection of several specialized components: The Space Manager, the Checkpoint Areas, and the Reaper. In today’s post, we will give an overview of APFS containers and these components.