By Jason Snell

October 6, 2016 11:38 AM PT

When free space isn’t free: Purgeable storage in macOS Sierra

Note: This story has not been updated for several years.

For years, the storage inside our computers just kept increasing. But with the advent of solid-state drives, which are less prone to failure, dramatically faster, and more energy efficient than spinning hard drives, there came a tough transition. Instead of having a terabyte or two of disk space, maybe you’ve only got 250 or 500 GB. Maybe you used to never think about running out of space, and now every so often your Mac throws up that terrifying warning box that your disk is almost full.

Apple recognized this, too, and to its credit, it made this transition one of the main features of macOS Sierra. A lot of the stuff on our drives doesn’t need to be there. It either isn’t necessary (log files, used app installers, and the like), or it’s just a duplicate of something that’s stored in the cloud.

The result is a whole raft of features in macOS Sierra that are all about freeing up more space on your drive. One of the biggest changes affects about how free space is calculated and displayed.

Free space and purgeable space

macOS displays the amount of free space on your drive in a few places, including:

• If you’ve got the Status Bar turned on in the Finder (View: Show Status Bar or Command-slash), you’ll see it at the bottom of the window

• If you’ve got Finder preferences set to show hard disks on the Desktop, you can turn on Show Item Info from the View Options panel (Command-J)

• If you select your drive from the Desktop or from the Computer window (Go: Computer or Command-Shift-C) and choose Get Info (Command-I)

• You can choose About This Mac from the Apple menu and click the Storage tab.

• In macOS Sierra, you can activate Siri and ask it how much free space you’ve got.

The Storage tab in About this Mac has been redesigned for macOS Sierra to give you more information about what’s filling your drive. The mysterious “Other” area of the storage graph is gone, and there’s much more granularity about what’s using that space, including unexpected space hogs like GarageBand and iTunes backups of iOS devices. The new Manage button, above the right edge of the graph, can help you reduce that stuff.

But if you look at the right end of the graph you’ll find two blocks: Free Space, labeled in white, and Purgeable, labeled in white with a diagonal gray pattern. In macOS Sierra, there are two different kinds of free space: Free and Purgeable.

Free space is what we’ve always known it to be. It’s space on disk where there’s nothing¹, that’s ready to have data poured into it. Purgeable space is different. Purgeable space is a collection of files that are really on disk, ready to be read or modified or added to at any time—stuff like files stored in iCloud, dictionaries you haven’t used recently, certain large fonts (especially of Asian languages) that you may never or rarely use, movies and TV shows you’ve already watched (and are re-downloadable from iTunes), and photos and videos in that are synced with iCloud Photo Library (if the Optimize Mac Storage setting is turned on in Photos preferences).

These are real files, but Apple considers them expendable. They can be deleted immediately, without warning, in order to free up disk space, because they can always be downloaded again later.

Now here’s the big change in macOS Sierra: Apple adds the amount of truly free space to the amount of purgeable space, and that’s what is displayed on your Mac as the amount of free space on disk. As I write this, my boot drive has 51.3GB of free space—and 22.6GB of purgeable space. In the old days, this would be reported to me as 51.3GB free. In macOS Sierra, it’s reported as 73.9GB free.

According to Apple, if I were to try to copy a 60GB file onto my drive, it would just work. The system would work in the background to purge enough stuff to fit my file and keep a decent amount of free space on the drive so that my Mac wouldn’t slow to a crawl, which happens when your disk is nearly completely full.

Now, when I was writing my review of macOS Sierra, I ran into a few bugs on this front. My system became confused about the real amount of free space on my drive, and prevented me from performing an iOS device backup when I had plenty of space. None of my data was lost, but I was prevented from performing actions that shouldn’t have been a problem. At one point—you can see this in the screen shot at the top of my Sierra review—my Mac couldn’t decide how much free space it really had. Siri said 30GB, but the System Information app said 55GB².

I’m confident that Apple will squash these bugs in short order. But the new concept of free space is here to stay. In an ideal world, your Mac should treat that free space as real free space, and there’s no need to peek behind the curtain. In the shorter term, there will probably be bugs and incompatibilities, and if you’re someone who cares about what’s going on behind the scenes of your Mac, you should be aware that not all free space is the same.

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