There’s a great more to storage management than freeing up space as outlined by Apple. Although the Storage section in System Settings is becoming increasingly useful, neither that nor the support note even mentions snapshots, and their coverage of caches is vague at best. Search Google instead and you’ll be offered expensive subscriptions to housekeeping apps or, if you’re really unlucky, a link to a ClickFix attack.

Storage management has been built into Mac operating systems since classic times, and when QuickLook was introduced in Mac OS X 10.5 Leopard 19 years ago, one of its essential features was maintaining its thumbnail cache. Yet some recent comments about purging caches blame them on APFS, a relative newcomer, but that is at least responsible for the addition of snapshots to storage management.

macOS and its constituent processes are prolific cachers, and have been for many years. Those caches are a double-edged sword, accelerating many features in the human interface and beyond, at a modest cost of a little disk space. Without both caching to disk and caching in memory, features like the Finder’s Gallery view would be unusable, as would much of QuickLook itself.

Custom thumbnails were introduced in Classic Mac OS around 1988-91, when they were generated for images by the apps that edited them, then saved to an ICN# resource in that file’s Resource fork, which was effectively a cached thumbnail. To render a view containing multiple thumbnails, the Finder then had to look in each file’s Resource fork and convert its ICN# resource for display. Other types of document were out of luck, though, as apps didn’t save their thumbnails as they did with images.

QuickLook took on the task of converting the contents of a wide range of different file types to thumbnail images, using qlgenerator plug-ins, and saving those in its cache. When the Finder wanted multiple thumbnails to display in a view, instead of looking in each file individually, all it had to was to find their cached thumbnails, a task performed by iconservicesd. That’s fast enough for a responsive interface until it comes to scrolling through larger thumbnails in a Gallery view. For that, not only do all those thumbnails need to be cached to disk, but as many as possible need to be cached in memory as well.

The most accessible caches are those used by web browsers like Safari. The benefits they bring can be shown most readily too, by simply emptying their caches repeatedly when trying to access some familiar websites. The command to Empty Caches is available in Safari’s Develop menu, added by enabling Show features for web developers in its Advanced settings. You’ll soon appreciate the value added by its caching.

Snapshots are one of the neater features in APFS, particularly for those whose laptops can’t always be within range of their backup storage. However, because they include almost every file in a volume, it’s all too common for them to capture huge and ever-changing data such as virtual machines. With a little careful planning, those can be stored in volumes that aren’t included in snapshots, and backed up separately when they’re inactive and offline.

Our increasingly jaundiced view of storage management in macOS hasn’t been helped by the opaque System Data category in Storage settings, when it’s able to complete calculating successfully. Its remit is so broad that it might be better named Anything and Everything Else. I think I’ve got a reasonably good idea of what’s meant by purgeable space in Disk Utility, but I still haven’t a clue to the scope of what System Data includes, other than Apple’s unforthcoming definition as “the storage space used by all Apple and third-party files that don’t belong to any more specific category.”

When free space is running low and you turn to Storage settings to see where it has gone, 50 GB of System Data is sufficient to put even an advanced user into a panic. Would deleting some snapshots help? Do some caches need to be deleted, and if so, which and how? Look for those answers in Apple’s documentation and you’ll be disappointed.

But the underlying problem is that storage management shouldn’t be about such crises, just as financial management isn’t about what to do when you’re overspent. There’s just so little reliable and detailed information about storage management, particularly from Apple.