why system data is so large on your mac, and how to shrink it
by sumit paul · last updated
system data on a mac is a catch-all for files that don't fit apple's other storage categories: caches, logs, time machine local snapshots, virtual memory swap, and the hidden data of apps like xcode and docker. when it's too large, restart first, then clear local snapshots, old iphone backups and developer caches, which are usually the biggest parts. never delete system folders by hand to shrink it.
what's inside system data
apple's own storage settings describe system data as files that don't fall into the listed categories, and say you can't manage its contents there. so you have to look elsewhere. the usual contents:
| what | where it lives | safe to remove? |
|---|---|---|
| time machine local snapshots | inside the disk, listed by tmutil | yes, with tmutil or by pausing backups |
| app caches | ~/Library/Caches | yes, with the app quit; they come back |
| logs and diagnostic reports | ~/Library/Logs, /private/var/log | user logs yes; leave system logs to macos |
| virtual memory swap | /System/Volumes/VM | no; restart instead |
| xcode build data and simulators | ~/Library/Developer | yes, with xcode or xcrun |
| docker images and containers | ~/Library/Containers/com.docker.docker | yes, with docker's own commands |
| virtual machines (parallels, utm and others) | wherever the vm files are stored | yes, delete vms you no longer use from the app |
iphone and ipad backups usually show up separately as ios files, but they're the same kind of hidden bulk, so they're covered below too.
step 1: restart and let storage finish counting
restart your mac. this clears swap files and temporary files. then open apple menu › system settings › general › storage and wait until the bar stops calculating. the numbers shown while it's still counting are often wrong, and system data tends to absorb whatever hasn't been sorted yet.
step 2: see which folders are big
storage settings won't open system data, but terminal can show you where the space actually is. this lists the biggest folders in your user library, smallest first:
du -sh ~/Library/* 2>/dev/null | sort -h | tail -15
terminal may need full disk access to read every folder (system settings › privacy & security › full disk access). the usual winners are caches, developer, containers, application support and mobile documents. mobile documents is your icloud drive, so treat it as your files, not junk.
step 3: clear time machine local snapshots
time machine saves a snapshot of your disk about every hour and keeps it for 24 hours, plus one of your last successful backup, plus one before each macos update. apple counts this space as available and deletes snapshots as space is needed, but they still inflate system data and can confuse apps that check free space.
to see them:
tmutil listlocalsnapshots /
to delete all local snapshots on your startup disk:
sudo tmutil deletelocalsnapshots /
apple's own method works too: system settings › general › time machine › options, set backup frequency to manually, click done, wait a few minutes, then set it back. time machine keeps its backup disks.
step 4: old iphone and ipad backups
if you back up an iphone to your mac, each backup can be large. connect the device (or select it in the finder sidebar), go to the general tab, click manage backups, then control-click old backups and choose delete. don't delete the backup folder in finder; use finder's own manage backups window.
step 5: developer tools
xcode is often the single biggest part of system data on a developer's mac.
- derived data (build output): quit xcode and delete the contents of ~/Library/Developer/Xcode/DerivedData. xcode rebuilds it as needed.
- simulator runtimes: xcode › settings › components (called platforms in some versions) lists them with their size; remove the ones you don't test on.
- simulators for runtimes you no longer have:
xcrun simctl delete unavailable. - archives in ~/Library/Developer/Xcode/Archives: keep any you might need to symbolicate a crash report.
docker desktop keeps all linux images and containers in one disk image file. it grows but doesn't shrink on its own when you delete files inside a container. run docker system df to see what's using space, then docker system prune to remove stopped containers, unused networks, dangling images and build cache. settings › resources › advanced shows the disk image's size and location.
step 6: caches and logs
quit the app first, then open ~/Library/Caches in finder (go › go to folder) and delete the contents of folders for apps you recognize, such as browsers, spotify or adobe apps. they rebuild. delete old files in ~/Library/Logs the same way. apple also notes that starting up in safe mode clears certain system caches, and that macos clears caches and logs that are safe to delete on its own when space runs low.
what never to delete
- anything in /System, /Library or /private/var. these belong to macos and deleting them can stop your mac from starting.
- /System/Volumes/VM (swap). restart instead.
- ~/Library/Mobile Documents. that's icloud drive; deleting there deletes your files everywhere.
- ~/Library/Containers or ~/Library/Group Containers as a whole. they hold app data like mail, messages and notes, not just cache.
- your photos library's inner folders. manage photos from the photos app.
- time machine backups through finder. use tmutil or time machine's own settings.
when it isn't system data
if system data is now reasonable and the disk is still full, the bar usually points at documents, photos or icloud drive. those are your own files, and the fix is moving them somewhere else rather than cleaning. for large videos, raw photos and archives, our guide to getting more storage without a new mac compares external ssds, icloud and cloud drives, including feather drive, which streams files from google drive or dropbox instead of storing them on your disk.
related
- mac storage full: what to do
- how to free up space on a macbook air
- does icloud drive take up space on your mac?
- how to get more storage on your mac without buying a new one
common questions
what is system data on a mac?
it is apple's catch-all storage category for files that don't fit the other categories, such as caches, logs, time machine local snapshots, virtual machine disk images and app support files. macos doesn't let you browse or delete it from the storage settings.
is it safe to delete system data?
not as a whole. caches, logs, old backups, xcode build files and docker images are safe to clear with the right tool. never delete folders in /system, /library or /private/var by hand, or the containers and mobile documents folders in your user library.
why does system data go down after a restart?
a restart clears virtual memory swap files and some temporary files, and gives macos a chance to remove expired time machine snapshots. if system data drops a lot after a restart, those were most of it.
do time machine local snapshots take up space?
yes, but apple counts that space as available. time machine keeps hourly snapshots for 24 hours and deletes them as they age or when the space is needed for something else.
should i use a cleaner app to shrink system data?
you don't need one. everything a cleaner removes can be found with the storage settings, finder and a couple of terminal commands, and doing it yourself means you know exactly what was deleted.