APFS Internals
Also known as: apfs how it works, apple file system internals
APFS (Apple File System) is the default file system on iOS, iPadOS, and macOS. It uses copy-on-write metadata, space-sharing containers, snapshots, and clones to manage storage, which is why iPhone and Mac free space can shift even when you haven't added or deleted obvious files.
- APFS replaced HFS+ as the default file system, arriving with iOS 10.3 in 2017 and macOS High Sierra in 2017.
- All volumes in an APFS container share one free-space pool rather than using fixed partition sizes.
- Clones and snapshots use copy-on-write, so duplicating a file initially consumes almost no extra space.
How APFS stores your data
APFS organizes a disk as a single container that holds one or more volumes. Unlike older fixed partitions, the volumes inside a container share free space dynamically, so the System, Data, and other volumes draw from one common pool rather than fixed allocations.
APFS is built around copy-on-write. When a file or its metadata changes, APFS writes the new version to fresh blocks instead of overwriting in place, then updates pointers. This protects against corruption and enables features like clones (instant duplicate files that share data blocks until one is edited).
Why APFS changes your free-space numbers
Because volumes share space, the storage breakdown on an iPhone or Mac is an estimate that recalculates over time. Snapshots, local Time Machine backups, and purgeable caches all consume real blocks but can be reclaimed by the system, which is why available space sometimes grows on its own.
When a cleaner app removes files, APFS frees the underlying blocks back into the shared container pool. Reclaimed space may not appear instantly because APFS defers cleanup and the OS recomputes the System Data and Other categories on its own schedule.