WTMemory guide

System Data taking up space on Mac: what it is and how to reduce it

Updated 13 October 2026

System Data on a Mac is everything macOS cannot put in a named category: caches, logs, Time Machine local snapshots, swap and sleep files, app support data, and on developer Macs large items like Xcode simulators, Docker's virtual disk and AI model folders. You shrink it by finding the biggest folders with du and clearing the regenerable ones, not by deleting system files.

macOS Monterey and later call this category System Data (in Ventura and later it appears in System Settings > General > Storage). Big Sur and earlier labelled it "Other". Apple does not let you open it to see what is inside, so this guide uses Terminal. If you prefer a window, WTMemory has a free Disk Explorer for the same job.

What is included in System Data on Mac?

The exact rules are Apple's and are not published, but these are the usual contributors:

Caches in ~/Library/Caches and /Library/Caches, plus temporary files in /private/var/folders.
Logs and crash reports in ~/Library/Logs and /private/var/log.
Time Machine local snapshots, kept on the internal disk while Time Machine is turned on.
Virtual memory: swap files and the sleep image in /System/Volumes/VM.
Developer data: Xcode DerivedData, iOS DeviceSupport, simulator devices and runtimes, package manager caches in hidden folders like ~/.npm and ~/.gradle.
Docker Desktop's VM disk, a single large file inside ~/Library/Containers/com.docker.docker.
Local AI models in ~/.ollama, ~/.cache/huggingface and ~/.lmstudio.
App data in ~/Library/Application Support and ~/Library/Containers, including leftovers from apps you removed.

Hidden folders and Library data tend to end up here because Storage settings classifies files by type and location, and these do not match Documents, Apps or Photos.

How do I see what is in System Data?

Give Terminal Full Disk Access first (System Settings > Privacy & Security > Full Disk Access), otherwise many folders report "Operation not permitted" and the totals come out low. Then measure the usual suspects:

du -sh ~/Library/Caches ~/Library/Logs ~/Library/Developer \
  ~/Library/Containers ~/Library/Application\ Support \
  ~/.ollama ~/.cache ~/.npm ~/.gradle 2>/dev/null | sort -h

For a whole-disk view, list the largest top-level folders on the data volume. -x keeps du on one volume and -d 1 limits the depth:

sudo du -xh -d 1 /System/Volumes/Data 2>/dev/null | sort -h | tail -15

Then drill into whatever is biggest, for example du -sh ~/Library/Developer/* | sort -h. Check the system-managed parts separately:

tmutil listlocalsnapshots /         # local snapshots
sysctl vm.swapusage                 # current swap in use
ls -lh /System/Volumes/VM           # swap files and sleep image
sudo du -sh /private/var/folders /private/var/log /Library/Caches 2>/dev/null

Expect the numbers not to match Storage settings exactly. Storage settings counts purgeable space, snapshots and APFS clones differently from du.

How do I reduce System Data safely?

1. Restart your Mac

A restart clears swap files and many temporary files in /private/var/folders. If sysctl vm.swapusage showed several gigabytes used, this alone can recover them. See the memory usage guide if swap keeps growing.

2. Thin Time Machine local snapshots

tmutil thinlocalsnapshots / 20000000000 4

This asks macOS to reclaim about 20 GB from local snapshots at the highest urgency. Snapshots are restore points; your backups on the Time Machine drive are unaffected.

3. Clear developer caches

Xcode is often the single biggest contributor. Clear ~/Library/Developer/Xcode/DerivedData, remove old iOS DeviceSupport folders, and delete simulators you no longer use:

xcrun simctl delete unavailable   # simulators for runtimes you removed
xcrun simctl runtime list         # installed simulator runtimes (Xcode 15+)

Remove unwanted runtimes from Xcode's Settings > Components (Platforms in older versions) or with xcrun simctl runtime delete. Details are in the Xcode DerivedData guide. Package caches have their own commands, covered in the npm, Yarn and pnpm guide.

4. Shrink Docker's disk with Docker's tools

Run docker system df to see what images, containers, volumes and build cache use, and docker system prune to remove unused items. Volumes are only removed if you add --volumes, so think before you do. The Docker disk space guide explains how to reclaim space without losing data.

5. Remove AI models you no longer use

Use ollama rm <name> for Ollama and the Hugging Face CLI cache commands for ~/.cache/huggingface/hub. Never delete ~/.cache/huggingface as a whole, because it can contain your access token. See where AI models are stored.

6. Clear caches and logs you can name

Folders inside ~/Library/Caches and ~/Library/Logs are safe to delete one at a time when the app that owns them is closed. The app rebuilds what it needs, so it may be slower on its next launch. Remove Application Support folders only for apps you are certain you have uninstalled.

7. Empty the Trash

Anything you moved to the Trash still counts toward used space until you empty it.

What should I not delete in System Data?

/System and everything under it, including /System/Volumes/VM. The system volume is sealed, and swap is managed by macOS. Restart instead.
/private/var wholesale. It holds databases, the dynamic linker cache and system state. Do not run sudo rm -rf on it, even on /private/var/folders while you are logged in.
~/Library as a whole, or ~/Library/Containers, ~/Library/Group Containers, ~/Library/Keychains and ~/Library/Mobile Documents (that last one is your iCloud Drive). These hold real data, not caches.
Docker's Docker.raw file. Deleting it wipes every image, container and volume.
iPhone backups in MobileSync/Backup. Remove them from Finder's Manage Backups window so you know which device and date you are deleting.

Why does System Data keep growing back?

Much of it is supposed to. Caches refill as you use apps, swap grows when memory is under pressure, and Time Machine creates a new local snapshot roughly every hour. macOS frees purgeable space and snapshots automatically when the disk gets close to full. Steady growth that never drops back usually points to something specific: a Docker disk that only expands, simulators added with each Xcode release, a model folder, or an app that writes huge logs. The du commands above will show which.

For the wider picture, including Downloads and large files, follow the free up disk space playbook.

Is there an app that shows this without Terminal?

WTMemory is a native menu-bar app for macOS 12 and later. Its free Disk Explorer lets you choose a folder such as ~/Library, compare its largest direct children, and list up to 50 of its largest files, including hidden ones, with Reveal in Finder. The free Storage and AI Tools views estimate supported developer caches and model folders. With Pro ($9 once for up to three Macs, 7-day trial) you can select supported items, read the consequences, and move them to the Trash after review. It never deletes automatically, and Docker VM data is excluded from cleanup. It does not clean system files, swap or snapshots; those are handled by macOS as described above.

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Frequently asked questions

What is System Data on a Mac?

System Data is the storage category for files macOS does not classify as apps, documents, photos or other named types. It includes caches, logs, Time Machine local snapshots, swap files, app support data and developer data such as Xcode simulators and Docker's disk image.

Is it safe to delete System Data?

Not as a whole. Clear specific regenerable items such as app caches, Xcode DerivedData, unused simulators and local snapshots. Do not delete /System, /private/var or ~/Library wholesale.

Why is System Data so large on my Mac?

On developer Macs the usual causes are Xcode simulators and DerivedData, Docker's virtual disk, local AI models, package manager caches and Time Machine local snapshots. Use du to find which one applies.

Does restarting reduce System Data?

Often a little. A restart clears swap files and many temporary files, which can recover several gigabytes if swap use was high.

What was System Data called before?

macOS Big Sur and earlier called this category Other. The name changed to System Data in later versions.