Why macOS System Data takes up so much disk space

What is System Data in macOS Ventura Storage overview?

When you check the disk usage of your Mac in System Settings > General > Storage you might find the category “System Data” takes up huge amounts of disk space. Depending on your disk size, this can be way above 100 GB.

What is this mysterious data? It seems, Apple labels everything as “System Data” that is managed by the system like logs and system caches. Supposedly, macOS will automatically free this space when it is needed.

When System Data is very large on your Mac, what hides behind it are usually local Time Machine backups. You might not be aware of it, but macOS automatically keeps local snapshots of your data. This allows you to recover files even when you haven’t performed a Time Machine backup to an external disk recently.

These backups use the snapshot feature of the Apple File System (APFS). A snapshot basically freezes the state of your disk at a certain moment. A snapshot will initially take up no space. But as you change, add or delete files, it will record the difference to the snapshot state. So, each snapshot will take up as much space as you have made changes on the disk since it was taken (or up to the state of the next newer snapshot). You can directly see this effect when you delete large files. As you empty the trash, the size of the “System Data” category will increase by the size of the deleted files.

Free up the space of macOS System Data

The easiest way to reduce the space taken up by System Data is to simply make a Time Machine backup to an external disk. During the backup process, the system will thin out the local backups and free up the space. A restart of your Mac might be needed, however, to see the effect in the Storage overview of System Settings.

This can already reduce the System Data block by dozens of GB.

Delete System Data on Monterey and Ventura

If you want to free up even more disk space, you can manually delete all local Time Machine snapshots. These snapshots are usually invisible. But you can view and manage them in Disk Utility. To do so, select Show APFS Snapshots from the View menu in Disk Utility. (Please note: This feature is only available in macOS Monterey, Ventura, and newer).

When you select your disk in the sidebar, you will now see a list of all available snapshots and when they were taken.

It is safe to just select all the snapshots and press the minus button to delete them. (Make an external Time Machine backup before!) Note that deletion takes a few minutes and can freeze your Mac momentarily.

Deleting all snapshots can free up significant amounts of disk space on your Mac. Of course, you lose all local backups for the moment, but Time Machine will soon create new snapshots anyway (usually every hour).

Remember that the snapshots contain only the changes you make to your data. So, they will take up a lot of space after you delete large applications (like XCode or games) or when you work with large media like movies. If you mostly do web browsing and usually home and office use, the local snapshots will probably never take up much space.