To optimize storage space, the OS analyzes the content of a user’s hard drive and sorts files into two camps: “purgeable” and everything else.
What Does “Purgeable” Mean?
“Purgeable” describes files that macOS has decided could be deleted, if necessary. This is similar to “deleting” data from a hard drive: data is marked as garbage, but it’s not gone until it’s overwritten. Purgeable data still exists as real, accessible files, but macOS has deemed those files expendable. As of now, it’s not possible to see which files, exactly, the OS has marked as purgeable. You also can’t clear the purgeable space yourself unless you resort to some Terminal hacking. But we do have a general idea of what can get marked as purgeable.
What Sort of Files Does Sierra Consider Purgeable?
Two main factors affect whether a file is purgeable: the date it was last opened and whether the file is available in iCloud. If the file can be downloaded from iTunes, Photos, or iCloud on demand, and it hasn’t been opened in a while, there’s a good chance it will get marked as purgeable. Turning on iCloud Desktop and Documents vastly expands the potential pool of files that can be marked purgeable. These folders probably contain most of your files. Once they’re backed up on iCloud, any of these files could technically be a purgeable candidate. User files that are not backed up to iCloud should never be marked as purgeable. Other candidate files include special cases like TV shows and movies in your iTunes library that you’ve already watched, foreign language dictionaries and large, non-Latin fonts that have never been used.
Exploring Optimized Storage
It doesn’t end at purgeable files. Sierra also offers a few features to help you better manage your storage space. By design, Sierra’s storage optimization operations are hidden from the user’s view. You shouldn’t need to take any direct action to benefit from the basics, but you can take a look under the hood to get some more control. If you open up “About This Mac” under the Apple menu and click on the Storage tab, you’ll see a breakdown of your disk storage. Click the button labeled “Manage” to open up a System Information window.
There’s a couple things going on in the resulting window.
On the left, you have a few categories of files, including Applications, Documents and Mail, along with the disk space each category takes up.
Clicking on the Documents tab, for example, will show you most of your files, sorted by size. Based on my tests, “Documents” actually includes the Documents, Downloads, and Desktop folders.
This is probably the most useful feature of the bunch. It helped me locate some long-lost, disk-hogging audio files, which was great. But not all of these menu options are as useful. Clicking on GarageBand, for example, tells you exactly nothing.
Under the “Recommendations” tab (which is the default view) you have a few other options. The first is called “Store in iCloud.”
While the description reads “store all files in iCloud,” “all” is a bit of an overstatement. This switch enables iCloud Desktop and Documents which backs up the content of your Desktop and Documents folders to iCloud. Any files that are safely stored in iCloud are then candidates for the purgeable flag. The same goes for your Photos library. Any full-res photos that also exist in iCloud can be flagged as purgeable as well, but optimized JPGs will stay on the system.
The second is somewhat inaccurately labelled “Optimize Storage.” It removes any iTunes movies and TV shows that you’ve already watched.
These videos can be re-downloaded and are still marked as purchased, but the local version is removed from your hard drive.
The third, “Empty Trash Automatically,” will automatically delete files that have been in the Trash for more than thirty days.
Finally, clicking “Review Files” under “Reduce Clutter” opens up the Documents tab that we saw before.
Conclusion
It’s a little scary to have the operating system manage storage for you, and as of now you cannot opt out. The best you can do is store as little as possible in iCloud which will reduce the number of potentially-purgeable files available. The implementation is designed to be conservative, so data wiping mistakes should be rare.
