Cloud sync
Also known as: file sync, synced files, how cloud sync uses storage
Cloud sync keeps a folder of files identical across your devices and an online account, so a change in one place appears everywhere. By default synced files usually exist both in the cloud and on each device, so syncing does not automatically save local space.
- Keeps files identical across devices and the cloud
- Standard sync stores copies locally too, using device space
- On-demand modes keep originals online to save local space
How sync differs from backup
Sync mirrors a live set of files: edit, add, or delete on one device and the change propagates to the others. A backup, by contrast, is a one-way copy made for recovery and not kept in lockstep with your day-to-day changes.
Because sync keeps everything matched, deleting a synced file on one device deletes it everywhere — which is convenient but also means sync is not a safety net the way a backup is.
Sync and local storage
With standard sync, each synced file is downloaded and stored locally as well as in the cloud, so it counts against device storage. Many services add an on-demand or "optimize" mode that keeps full copies in the cloud and only smaller placeholders or previews on the device.
That on-demand mode is what actually frees space: the original lives online and downloads when you open it, while everyday browsing uses the lightweight local copy.