everyday mac tools

Purgeable space on a Mac: what it is and why it will not free up

· 6 min read

Purgeable space is disk that is currently occupied, but by things macOS believes it can throw away the moment something needs the room: Time Machine local snapshots, cached iCloud files, temporary files and caches. APFS counts it as available, so Finder tells you it is free while the blocks are still in use.

It does not release on a schedule and it does not release because you are looking at it. It releases when the file system is genuinely asked for space, which is why the number can sit there for days while you wonder what is wrong.

Where the word shows up

Select your startup disk in Finder, then File, Get Info. The line reads something like “42 GB available (20 GB purgeable)”. Disk Utility shows the same figure when a volume is selected.

Terminal does not agree, and this is the source of most of the confusion:

df -h /System/Volumes/Data

That reports available space without purgeable folded in. When Finder says 42 GB and df says 22 GB, neither is lying. Finder is telling you what you could have; df is telling you what you have right now.

The Storage pane in System Settings, General, Storage does not use the word at all. It rolls most of the same content into System Data, which is why that block is so often the biggest thing on the bar. What System Data is on a Mac covers the rest of that category.

What is actually in it

There is no command that lists the contents of purgeable space, and that is a genuine limitation rather than an oversight in this article. macOS treats it as an internal accounting figure. What you can do is check the three things that make up nearly all of it.

Time Machine local snapshots. Usually the largest component by a wide margin. Run tmutil listlocalsnapshots /. If that prints a list, you have found most of your purgeable space, and Time Machine local snapshots covers how to thin it.

Files stored in iCloud with a local copy. If Optimize Mac Storage is on for iCloud Drive or Photos, macOS keeps local copies of things it could re-download and counts them as purgeable. You get the speed of a local file and the accounting of a free one, until the day the network is not there.

Caches and temporary files that macOS has marked as reclaimable, including those written by apps that use the system’s cache directories properly.

Note that all three are your own recent history, not junk. Purgeable is not wasted space, and a Mac with none of it is a Mac with no local restore points.

Why it will not free up

Reclamation is demand driven. The file system frees purgeable blocks when a write needs them and not before, and the threshold at which macOS starts working hard at it is lower than most people expect. On a disk sitting at 90 percent, macOS is content. On a disk that has just been asked for another 30 GB, it acts.

The practical version: you cannot ask nicely. You have to either remove the underlying items yourself or create real demand.

There is a second reason it looks stuck. The Storage pane is calculated in the background and lags, sometimes for hours. Free 40 GB and the graph may show the same picture until you restart. Check with df -h /System/Volumes/Data, which is current.

The programs that refuse to trust it

This is the part that turns an accounting curiosity into an actual problem. Several things ask for real free space and will not proceed on a promise:

  • macOS installers and updates, which check free space before they start and again partway through
  • creating or expanding a disk image
  • partitioning, or adding a volume to a container
  • virtual machine and container tools that pre-allocate their disks
  • some backup and imaging tools

So Finder can say 60 GB available and an installer can tell you there is not enough room for a 20 GB update, and both are correct. If that is where you are, not enough space to update macOS is the more specific case.

How to make macOS release it

In order, from the one that works most reliably.

  1. Thin the snapshots. sudo tmutil thinlocalsnapshots /System/Volumes/Data 20000000000 4 asks macOS to reclaim about 20 GB, oldest first, at the highest urgency. Adjust the byte figure to what you need. On most Macs this is the whole answer.
  2. Restart. This clears a number of temporary files outright and forces the free space figures to be recalculated rather than estimated.
  3. Empty every Trash, including Photos’ Recently Deleted album, which holds items for 30 days, and the Trash on any external drive.
  4. Create real demand. Copy a genuinely large file you already have, a video or a disk image, onto the disk. macOS will reclaim purgeable space to make room. Delete the copy afterwards. This is a blunt instrument and it is slow, but when nothing else has moved the number it does.
  5. Turn off Optimize Mac Storage for Photos or iCloud Drive if you want a permanent, honest picture of what is on the disk. Be clear about the trade: everything then downloads and lives locally, so used space goes up while purgeable goes down.

If you want the underlying items named rather than inferred, Crumb audits the whole Mac read-only and reports local snapshots, swap, caches, temporary files and iOS backups separately, so you can see which part of the purgeable figure is which before deciding what to remove.

What not to do

Do not reformat or reinstall to clear purgeable space. It is not corruption and it is not a fault.

Do not run Disk Utility First Aid expecting it to help. First Aid checks the file system’s structure; purgeable space is a healthy file system doing what it was designed to do.

And do not treat a large purgeable figure as a problem in itself. If your disk has room and nothing is refusing to install, the space is working for you: it is the reason you can pull back this morning’s version of a document without connecting a backup drive. Only chase it when something concrete is blocked. If nothing here accounts for your missing space, the Mac says storage is full, but the folders do not add up covers the other places to look.

Questions

Can I disable purgeable space entirely? Not as a setting. You can shrink what feeds it: turn off Optimize Mac Storage, and stop or thin Time Machine local snapshots. There is no switch that makes APFS report space the way a decade-old file system did.

Why does the purgeable figure keep changing? Because its contents keep changing. Every hour of Time Machine, every iCloud download, every cache write moves it. A figure that varies by a few gigabytes through the day is normal.

I deleted 100 GB and purgeable went up by 100 GB. That is the snapshots. A snapshot taken before the deletion still refers to those blocks, so the space moved from used to purgeable rather than to free. Thinning the snapshots completes the deletion.

Does an external drive have purgeable space? It can, if it holds snapshots or synced content, but in practice the figure is almost always about the internal drive. Get Info on the external volume will tell you.