System Data is not one thing. It’s everything on your startup disk that macOS doesn’t sort into Applications, Documents, Photos, Mail and its other categories. It’s often tens of gigabytes, and on a Mac that swaps heavily, builds software or runs Docker, it can pass 100 GB.
The good news: most of it is made of a handful of parts, each of which you can measure. Some shrink on their own, some are safe to clear, and a few you should leave alone.
Where to see it
Apple menu › System Settings › General › Storage. Hover over the grey part of the bar, or scroll down the list, to see System Data’s size. macOS shows it as a single number and gives no way to open it, which is why it’s confusing: you can’t click your way to what’s inside.
What’s inside System Data
1. Swap files
When apps need more memory than your Mac has, macOS writes some of it to the disk. Those swap files live in /System/Volumes/VM, in 1 GB pieces. On an 8 GB Mac with a browser and a few chat apps open, several gigabytes is common.
How to check: open Terminal and run:
sysctl vm.swapusageWhat to do: don’t delete swap files by hand. Quitting apps you aren’t using lets macOS shrink swap over time, and a restart clears it completely. See what Swap Used means for when it’s a problem.
2. The sleep image
A copy of memory macOS keeps so it can wake from deep sleep without losing your work. It lives at /private/var/vm/sleepimage and is about the size of what’s in memory, often 1 to several GB. macOS manages it. Leave it.
3. Time Machine local snapshots
If you use Time Machine, macOS keeps snapshots on your startup disk between backups, so you can restore files even when the backup drive isn’t connected. They’re counted as System Data, and after a busy day they can be large.
How to check:
tmutil listlocalsnapshots /What to do: macOS deletes these on its own when space runs low, so they rarely cause a real problem. If you need the space now, this removes the snapshots on your startup disk (your backups on the Time Machine drive are not touched):
sudo tmutil deletelocalsnapshots /4. Caches and temporary files
Apps keep caches in ~/Library/Caches, and macOS keeps each account’s temporary files in a hidden folder under /private/var/folders. Both rebuild when deleted, so they’re safe to clear, but clearing them rarely makes a Mac faster. See how to clear caches, and when not to.
One case is worth knowing: Google Chrome copies itself into that temporary folder while it updates, and sometimes leaves the copies behind. On one 8 GB M1 Mac we measured, old Chrome copies held 13 GB. They’re safe to remove, but only while Chrome is closed.
5. Developer tools
Xcode’s build data and iOS simulator runtimes, Docker’s disk image and package manager caches can each run to tens of gigabytes, and some of them show up as System Data. If you write code, check these first:
6. What deleted apps left behind
Dragging an app to the Trash leaves its data in your Library folder. Years of that adds up. See how to uninstall apps completely.
How to find what’s big yourself
In Finder, choose Go › Go to Folder, type ~/Library and press Return. Switch to list view, choose View › Show View Options and tick Calculate all sizes, then sort by Size.
Or, in Terminal, list the largest folders in your Library:
du -sh ~/Library/* 2>/dev/null | sort -h | tail -15Some folders only show their real size if Terminal has Full Disk Access (System Settings › Privacy & Security › Full Disk Access).
What not to do
- Don’t delete anything in
/System, or swap and sleep files by hand. - Don’t empty
/private/var/folderswholesale while apps are running. Clear what you recognise, then restart. - Be wary of scripts or apps that promise to “clear System Data” in one click without showing you what they delete.
Why it matters for speed
Swap lives on the same disk. When System Data and everything else leave little free space, macOS can’t swap comfortably, and a Mac that was a little slow starts to stall. Keeping 10 to 20% of the disk free matters more than any single number in the Storage bar. See how to free up space safely.
Unslow’s Storage view does this measuring for you: it splits System Data into swap, the sleep image, temporary files and Time Machine snapshots, sizes each, and says what removing it would cost before anything moves.