“System Data” on a Mac: what it is and how to reduce it

01What “System Data” contains

Since macOS Ventura, System Settings › General › Storage calls “System Data” (formerly “Other” or “System”) everything that does not fit the other categories. It includes in particular:

• local Time Machine snapshots, safety copies kept on the disk between two backups;
• virtual memory (swap files) and the sleep image;
• system and app caches and logs;
• the Docker Desktop disk image and part of Xcode’s data (simulators, device symbols);
• the Spotlight index and various app data.

02Time Machine snapshots

When Time Machine is on, macOS keeps local snapshots, usually for 24 hours. They count as purgeable space: macOS deletes them itself when space runs low. That is why the Finder and System Settings do not always show the same available space.

To list them, in Terminal:

To delete one (replace the date with the one shown):

Only do this if you need space right away: these snapshots let you recover a file changed by mistake.

03Virtual memory

When memory runs short, macOS writes swap files to the disk, in /System/Volumes/VM. They can reach several GB after days without a restart. Never delete them by hand: restart the Mac and they are recreated at the size needed.

04Docker and Xcode

Docker Desktop keeps images, containers and volumes in a single disk image that does not shrink by itself. See what it contains, then delete what is no longer used:

The maximum size of the disk image is set in Docker Desktop › Settings › Resources.

For Xcode, simulators for old iOS versions are removed with:

Device symbols (~/Library/Developer/Xcode/iOS DeviceSupport) are rebuilt the next time an iPhone is connected. More in Clean up developer caches.