Thumbnail cache
Also known as: thumbnail cache, thumbnails, preview cache
A thumbnail cache is a store of small preview images that lets photo galleries, file browsers, and apps render grids instantly instead of rebuilding each preview from the full file. It is rebuildable data, so clearing it is safe — the system simply regenerates previews as needed.
- Small previews that make grids load instantly
- Rebuildable data — safe to clear
- On iOS it falls under System Data
Why thumbnails are cached
Generating a preview from a full-size photo or video is expensive, so the system creates small thumbnails once and reuses them. This is what makes a photo grid scroll smoothly even with thousands of images. The previews are kept in a cache so they do not have to be recomputed every time you open the gallery.
On a large library this cache can grow to a noticeable size, and on iOS it is one of the things that lives inside the System Data category rather than appearing as its own line item.
Is it safe to clear?
Yes. A thumbnail cache contains no original files — only regenerable previews. Clearing it (or letting the system purge it when space is low) costs nothing but a brief slowdown the next time previews are rebuilt, and your actual photos and files are untouched.