Mar 28, 2018
The new Apple File System (APFS) was one of the most important and exciting changes introduced in macOS High Sierra. However, with all its new power, APFS also brought a number of new questions for Mac users, which we see asked in numerous online forums and in support tickets that we receive.
Here at DaisyDisk team, we took APFS very seriously and spent a lot of time researching into the new file system. APFS is still quite scarcely documented, and many nuances could only be discovered by trial and error. We even had to reach out to Apple engineers for the most difficult bits.
And finally, we have implemented all our findings in DaisyDisk, releasing the new version 4.5 (free update for existing users).
Most questions people ask about APFS are in fact caused by the so called “local snapshots of Time Machine”.
Time Machine is a great backup utility built in macOS. Every hour or so it takes a “snapshot” of your entire disk and stores it locally, i.e. on the same disk. Later the snapshots are copied to an external permanent storage, such as NAS, and eventually deleted from the local disk.
Note that unlike the external backup, local snapshot is not an actual copy of data, but rather a catalog of changes since the last snapshot. So if you don’t make sizeable changes to your data, new local snapshots do not take additional disk space. But if you do make significant changes, such as when you delete a big file, the snapshot grows by the corresponding amount.
The space occupied by local snapshots is also called “purgeable”. When the snapshots grow too big or become older than 24 hours, or apps demand more space, macOS quickly deletes them and reclaims the purgeable space. The whole process is automatic, no user action is required.
However, sometimes the local snapshots become a problem and a source of questions for Mac users. Namely:
Question #1: Mysteriously disappearing disk space
Suppose you want to backup your iPhone to iTunes, or create a new disk image with Disk Utility, but suddenly you can’t, because the app reports insufficient free space on disk. This is confusing, because Finder on the other hand displays quite a lot of “available space”. This happens because Finder counts the local snapshots of Time Machine toward the available space, assuming that they can be reclaimed quickly upon demand. However if an app checks the amount of ACTUALLY free space, BEFORE the operation – which seems the case – then it fails, and you cannot proceed until you somehow remove the local snapshots.
Question #2: Deleting files doesn’t produce free space, or at least not immediately
You notice that despite you have deleted tens of GBs of files, the disk’s free space hasn’t grown at all. This happens because Time Machine moves your files to the local snapshots, so the space is not physically freed until the snapshots are deleted. This will eventually happen automatically, but what if you need the space NOW?
Question #3: System taking too much space on macOS High Sierra
Wondering what’s taking up your disk space, you open About This Mac > Storage and see that the yellow “System” segment appears unrealistically large – tens of GBs, or up to 80% of your disk. This is because on macOS High Sierra, this System segment is not only a catch-all category for the system files, but also includes the “purgeable space”, i.e. the local snapshots. This is neither obvious nor useful when you need to free up space.
Question #4: How can I remove local snapshots of Time Machine? And where they are located?
In certain circumstances you may need to forcedly remove the local snapshots of Time Machine, to make room for a more important operation. However, macOS doesn’t seem to provide an obvious way to do it. Unlike in previous macOS versions, the content of local snapshots in APFS is entirely unreachable to the user, even if you search with raised access permissions. The APFS snapshots are like dark matter on your disk – you know it’s there, but can’t see the files. The only way to view and delete the snapshots (beside DaisyDisk ;) is to use the tmutil command-line tool in Terminal.
Question #5: Why Apple had to make things so complicated?
Some people dislike the fact that APFS impudently takes control over their disk space, as if it knows better what the user needs. We’ve asked Apple engineers regarding this. They see the free disk space as a resource that is otherwise underused. Why, they ask, you’d want to have most of your disk stay empty most of the time and not utilized for something useful? With APFS and its instant snapshots, the free disk space can now be used for temporary backups, which adds a layer of safety for your data, almost free of charge, of course as long as it “just works”.
How DaisyDisk helps solve these problems
DaisyDisk scans your entire disk and builds a size-oriented map of your files and folders. It can reach even restricted and system folders by scanning with raised access permission. The remaining used space, if any, DaisyDisk displays as well, labeling it as “hidden space”. The latter is useful to detect “dark matter” on your disk, such as the local snapshots of Time Machine. Starting with the recently released version 4.5, DaisyDisk can additionally single out the “purgeable space”, i.e. the local snapshots, displayed under the “hidden space”. In addition, you can forcedly delete that purgeable space by a simple drag-and-drop, as you’d delete a regular file in DaisyDisk.