Glossary
APFS
Apple File System. Copy-on-write, snapshots, clones, sealed system volumes — the default filesystem on every modern Apple device.
APFS (Apple File System) is the default filesystem on every Mac, iPhone, iPad, Apple Watch, and Apple TV shipped since 2017. It replaced HFS+ — which had been Apple's primary filesystem since 1998 — across all four major platforms in a remarkably short window.
Three defining properties:
- Copy-on-write everywhere. Every write goes to a fresh block; old blocks live until nothing references them.
- Snapshots are nearly free. A snapshot is just "stop garbage-collecting these blocks." No copy is performed.
- Containers share free space. Multiple volumes (Macintosh HD, Macintosh HD - Data, Preboot, Recovery, VM) live in one container and share the free pool dynamically.
The APFS driver itself is closed-source; XNU contains only the VFS plumbing that bridges POSIX syscalls into APFS's vnop_* callbacks.
apple-oss-distributions/xnubsd/vfs/vfs_syscalls.cPOSIX syscall → VFS entry points.View on GitHub(line —) apple-oss-distributions/xnubsd/sys/clonefile.hclonefile(2) — APFS cloning exposed to userspace.View on GitHub(line —)
Apple publishes the on-disk format in the APFS Reference PDF.
See also: Sealed System Volume, vnode.