What is System Data on Mac and how to reduce it safely

By Pruneo TeamPublished: October 22, 2026Updated: October 25, 2026

You open the Storage panel and a grey bar called System Data takes tens of gigabytes. There is no list of files and no delete button. This guide explains what is inside that category, how to measure it with Apple’s own tools and what you can reduce without breaking macOS.

What Apple says System Data is

In Change Storage settings on Mac, Apple describes System Data as the category that “contains files that don’t fall into the categories listed here”, and adds that you cannot manage its contents. In How to free up storage space on your Mac, Apple says it measures the space used by Apple and third party files that don’t belong to a more specific category.

In other words, System Data is a bucket for everything else. It is not one folder: it is the sum of many places on the disk.

What usually ends up there

Apple does not publish a full list. In practice, these are the most common parts:

• Caches from macOS and from apps, for example in ~/Library/Caches.
• Logs and diagnostic reports, in ~/Library/Logs and /Library/Logs.
• Temporary files in /private/var/folders, which macOS uses for each user.
• Swap files (virtual memory) in /System/Volumes/VM, used when RAM is full.
• Time Machine local snapshots, copies of your startup disk that Time Machine keeps on the Mac.
• Data of large apps that is not counted as Documents, for example disk images of virtual machines, Docker or Xcode simulators.

Some of this is space macOS gives back by itself when it needs it. That is why the bar can change from one day to the next without you doing anything.

Step 1: measure before you act

You cannot delete System Data from the Storage panel, but you can see which part is big. All these commands only read.

Local snapshots

tmutil listlocalsnapshots /

It lists the Time Machine snapshots on the startup volume, with names like com.apple.TimeMachine.2026-10-22-101500.local. If it only prints “Snapshots for disk /”, you have none. See every option with man tmutil in Terminal.

Swap

sysctl vm.swapusage

It shows how much swap macOS is using. Swap grows when apps need more memory than the Mac has, and macOS removes it when it is not needed. Closing heavy apps or restarting is the normal way to reduce it.

Caches, logs and app data

du -sh ~/Library/Caches ~/Library/Logs 2>/dev/null
du -hd 1 ~/Library 2>/dev/null | sort -h | tail -10

du (see man du) shows the size of each folder in your Library. Look for unusual ones: Developer (Xcode), Containers (sandboxed apps, including Docker) or Application Support.

Step 2: what you can reduce

Time Machine local snapshots

Apple explains in About Time Machine local snapshots that Time Machine saves a snapshot of the startup disk about every hour and keeps it for 24 hours. It also saves one before each macOS update. Apple adds that your Mac counts the space used by snapshots as available storage, and removes them as they age or when space is needed.

So usually you do not need to do anything. If you still want to remove them, Apple’s method is the safest:

1. Open System Settings > General > Time Machine.
2. Click Options, set Backup Frequency to Manually and click Done.
3. Wait a few minutes, then set the frequency back to what you had.

If you prefer Terminal, tmutil can delete snapshots by date.

# Replace the date with one from tmutil listlocalsnapshots /
sudo tmutil deletelocalsnapshots 2026-10-22-101500

The man tmutil page also documents thinlocalsnapshots, which asks macOS to free a given amount of bytes by thinning snapshots. Most people do not need it.

User caches

Caches in your own Library are made to be rebuilt. Apple’s developer guide says the Caches folder holds data “that can be regenerated as needed” (File System Programming Guide). Quit the app, then move its folder in ~/Library/Caches to the Trash. We explain the details in Is it safe to delete the Library/Caches folder?.

Safe mode once

Apple says in Start up your Mac in safe mode that safe mode clears some system caches, which are created again as needed. It is a safe way to free some space, for example before a macOS update. Restart normally to exit.

Old logs and diagnostic reports

You can open them with the Console app. Files in ~/Library/Logs belong to your apps and can be moved to the Trash with the app closed. They are usually small, so they are rarely worth the effort.

Large app data

This is often the real reason behind a huge System Data number for developers and creators:

• Xcode: DerivedData, simulators and device support files. See Delete Xcode DerivedData without breaking anything.
• Docker: images and build cache inside its virtual disk. See Docker prune to free disk space on Mac.
• Video and audio apps: render files and caches. See Free up space on Mac for video editors and musicians.

Clean these with each tool’s own commands, not by deleting their folders by hand.

When NOT to touch anything

• Inside /System, /usr, /bin, /sbin and /var. They are protected by System Integrity Protection and macOS needs them. Do not try to disable it to clean space.
• /System/Volumes/VM. Never delete swap files by hand. macOS manages them.
• /private/var/folders. These are live temporary files of running apps. Deleting them can make apps fail. A restart is the safe way.
• Right after a macOS update. Give the system a day: it removes the extra snapshot and temporary files when space is needed.
• When free space is not a problem. If macOS counts the space as available, the number in the bar does not stop you from saving or installing anything.

Why the number does not match du

The Storage panel and du count in different ways. du measures folders you can read. The panel also counts snapshots and space that macOS considers purgeable, and folders that need special permissions. Use du to find which of your folders grew, and the panel for the global picture. Do not try to make both numbers match.

Summary

1. System Data is everything that does not fit in another category.
2. Measure it with tmutil listlocalsnapshots /, sysctl vm.swapusage and du.
3. Remove local snapshots the Apple way if you really need the space.
4. Clean user caches and large app data with the apps closed.
5. Leave /System, swap and temporary system folders to macOS.

For the full cleaning plan, read How to free up space on Mac, or download Pruneo for free.

FAQ