APFS Snapshots
Also known as: apfs snapshot, local snapshot storage
An APFS snapshot is a read-only, point-in-time image of a volume that records its exact state at a moment. Snapshots share data blocks with the live volume, so they cost little at first but grow as files change, often appearing as hidden System Data that fills storage on iPhone and Mac.
- APFS snapshots are read-only and immutable once created, capturing the volume's exact state at that moment.
- macOS keeps local Time Machine snapshots for roughly 24 hours, deleting them as free space gets low.
- A new snapshot shares blocks with the live volume, so it starts at near-zero size and grows only as files change.
What a snapshot is and how it works
A snapshot preserves the state of an APFS volume at a specific instant. Thanks to APFS copy-on-write, a fresh snapshot shares all data blocks with the live volume and uses almost no extra space. As you modify or delete files afterward, the old blocks the snapshot still references can no longer be freed, so the snapshot's real footprint grows over time.
macOS Time Machine creates local snapshots between full backups, and the OS uses snapshots during updates so a failed install can roll back. These are normal and the system removes them automatically, typically keeping local Time Machine snapshots for up to 24 hours.
Why snapshots inflate storage and how cleanup helps
Because snapshot space is held by the system rather than visible files, it commonly shows up under System Data or "Other" in storage breakdowns. A device can report low free space even when your photos and apps don't account for it, because old snapshots are pinning blocks that would otherwise be reclaimed.
Snapshots are usually purgeable: macOS deletes them as free space runs low (the data becomes purgeable), and on iOS the system manages them invisibly. Freeing large files lets stale snapshots release their referenced blocks, which is why deleting content can unlock more space than the files themselves occupied.