What is System Data on a Mac, and why is it so big?

On this page

1. What usually ends up in it
2. Why it's so hard to see inside
3. How to look inside it
4. What you can safely shrink
5. What to leave alone
6. Why it grows and shrinks on its own
7. Where Spaceback fits

System Data is the part of the Storage bar that macOS couldn't put anywhere else. It isn't a folder and there's no single thing to delete. It's virtual memory, caches, logs, Time Machine snapshots, app data and a long tail of files that don't belong to apps, photos, documents or any of the other named categories.

That's not me guessing at Apple's intent. In macOS 27's own storage code, the label "System Data" is still stored under the key DISK_OTHER. It's the category that used to be called Other, with a less alarming name.

What usually ends up in it

Apple doesn't publish the exact recipe, so treat this as the usual suspects rather than an official list. The last column is what I measured on my own MacBook Air (M1, 455 GB, macOS 27.0) on September 28, 2026:

What	Where it lives	On my Mac
Virtual memory: swap files and the sleep image	The VM volume, plus /private/var/vm	8.6 GB
App caches	~/Library/Caches	4.4 GB
Logs	~/Library/Logs, /Library/Logs, /private/var/log	about 0.4 GB
Local Time Machine snapshots	The startup volume	none at the time
App data that isn't a document: virtual machines, Docker's disk image, offline media, models	~/Library/Application Support, ~/Library/Containers	varies wildly
iOS simulator runtimes, if you have Xcode	/System/Library/AssetsV2 and /Library/Developer/CoreSimulator	35 GB

Depending on your macOS version, Storage may count some Xcode data under Developer instead of System Data. Either way, on a developer's Mac the simulator runtimes are the first thing I'd check, because they sit in folders nobody browses and each one is 3 to 8 GB.

Why it's so hard to see inside

Storage settings won't break System Data down for you, and a lot of it lives where Finder doesn't go: system folders, other apps' private data, a separate volume for swap. On macOS 27 some of it is also protected from apps, which means disk maps and cleaners can see less than they used to. A tool that doesn't tell you what it couldn't read will show a smaller number and let you believe it.

How to look inside it

Four read-only commands get you most of the way. None of them change anything.

The biggest folders in your user Library, largest last:

du -h -d 1 ~/Library 2>/dev/null | sort -h | tail -12

Local Time Machine snapshots on the startup disk:

tmutil listlocalsnapshots /

Simulator runtimes and their sizes, if you have Xcode:

xcrun simctl runtime list

System-wide folders. This one needs your admin password because it reads folders owned by the system, but it still only reads:

sudo du -h -d 1 /Library /private/var 2>/dev/null | sort -h | tail -15

If you'd rather click than type, a disk map does the same job visually. Run it with Full Disk Access if it's a direct download, and be sceptical of any tool that shows a neat total without mentioning folders it was refused.

What you can safely shrink

• Local snapshots. They expire on their own, but you can ask macOS to thin them now with tmutil. The details are in the purgeable space post, because snapshots are also why deleting things sometimes doesn't return space straight away.
• Simulator runtimes, simulator devices and device support files. Often the biggest single win on a developer's Mac. See where Xcode's disk space goes.
• Caches of apps you no longer use, or one cache that has grown absurd. See how to clear the cache.
• Virtual machine and container disk images. Docker Desktop, Parallels and UTM keep a disk image that grows as you use it and rarely shrinks by itself. For Docker, docker system df shows what's using the space and docker system prune removes stopped containers, unused networks, dangling images and unused build cache. Deleting a VM you no longer need is usually worth more than everything else on this list.
• Data left behind by apps you deleted. Look in ~/Library/Application Support and ~/Library/Containers for names you recognise from apps that are gone.

What to leave alone

• Swap files and the sleep image. macOS creates and removes them as memory pressure changes. Deleting them by hand on a running Mac is asking for trouble; a restart resets them properly.
• /private/var/folders. Per-user temporary files and caches that macOS manages. Some of it is cleared when you restart.
• The system snapshot. If you run diskutil apfs list you'll see a snapshot marked "Sealed" mounted as /. That's macOS itself on its read-only system volume, not a Time Machine snapshot, and it isn't something to delete.
• Anything under /System. It's sealed and read-only, and it's where the macOS category comes from, not System Data.

Why it grows and shrinks on its own

A System Data figure that jumps by 20 GB overnight and drops back a day later is usually normal. With Time Machine turned on, macOS takes local snapshots roughly every hour and removes them as they age. Swap grows under memory pressure: after a week of Xcode and simulators on a 16 GB Mac, mine was 8.6 GB. macOS also downloads and stages updates in the background, and some apps park large temporary files in /private/var/folders while they work.

What's worth investigating is a System Data figure that only ever grows, on a Mac with no Xcode, no virtual machines and no Docker. That's usually one app's data folder or cache that nobody is keeping in check, and the commands above will find it in a few minutes.

Where Spaceback fits

Spaceback's Storage Map and Junk scan show the parts of System Data that live in your user Library, like caches, logs and developer folders, with their real sizes, and they list the folders macOS wouldn't let them read. The Mac App Store version can't see system-wide folders such as /Library/Caches; the direct-download edition can. The App Store version doesn't touch snapshots, swap or the system volume, and I think that's the right call for an app that runs without an administrator password.

Vlada Misici builds Spaceback, a storage cleaner for Mac and iPhone. Every number in these posts was measured on a real Mac, and every command was run before it went on the page. Found a mistake? Email vlada@queazy.app and it gets fixed.