System Data is the space macOS cannot file under Apps, Documents or Photos. It holds snapshots, swap, caches, logs, iPhone backups and the data apps keep. Some of it clears itself, some you can clear safely, and some you should leave.
When Time Machine is on, macOS keeps snapshots of your disk on the disk itself, and a macOS update leaves one behind while it installs. A snapshot holds the old copy of every file that has changed since.
tmutil listlocalsnapshots / in Terminal.Your startup disk is one APFS container with several volumes in it. Preboot, Recovery and Update share its free space with your data, and Storage settings counts them as System Data.
Leave them. Your Mac needs them to start up, recover and install updates.
When memory runs short, macOS moves some of it to swap files on the disk. They grow with long sessions and many open apps.
Save your work and restart. macOS shrinks swap files after a restart.
Files macOS can remove by itself when space is needed, such as iCloud copies it can download again and some caches.
Nothing to do. macOS frees it when an app needs the space.
Apps and tools keep downloads and computed results so the next time is faster. Browsers, package managers and Xcode keep the most.
npm cache clean --force, brew cleanup, pip cache purge, or xcrun simctl delete unavailable for old simulators.~/Library/Caches, not the folder itself.What Mail, Messages, Docker and other apps keep for you in ~/Library/Containers and ~/Library/Application Support.
docker system prune.Full backups made when you back up a device to this Mac. Each one can be as large as the device's data.
System and app logs, and crash reports, in /Library/Logs and ~/Library/Logs.
Old logs are safe to remove. macOS rotates its own logs.
Leave these alone: the other volumes, the swap files in use right now, and containers you did not create. DiskKind names every part on your own Mac, shows what would really come back from each, and offers a button only where a tool makes the data again.Not as a whole. System Data mixes parts that clear themselves (snapshots, swap, purgeable space), parts you can clear safely (caches, logs, device backups you no longer need) and parts you should leave (other volumes and the data apps keep). Clear the parts, not the bar.
Usually local Time Machine or macOS update snapshots, swap after a long time without a restart, developer caches, or iPhone backups. Each can reach tens of gigabytes.
Often, yes. A restart lets macOS shrink swap files and can let it remove snapshots and temporary files it no longer needs.
Caches and snapshots refill as you work, so the size goes up and down on its own. A steady climb usually points to one part, such as iPhone backups or a developer cache, which a disk analyzer can find.
DiskKind measures your disk the way APFS counts it, names every part above, and shows what you would really get back.
