A Nasty Bug Is Discovered In macOS High Sierra Related To APFS Disk Images

The quality issues with Apple software keep popping up. Last night I became aware of a new one that while it would be a bit of an edge case, is still pretty serious. Mike Bombich of Bombich Software who make the popular Carbon Copy Cloner backup software discovered a pretty bad bug when it comes to disk images formatted for Apple’s shiny new APFS file system. Before I get to the bug, let me explain what disk images are.

In short, disk images are basically files that behave like hard disks. You can store thousands of files in them and mount and unmount them like hard disks. In other words, it’s a pretty cool way to back up stuff as it’s a pretty easy concept to understand for most users. Disk images on the Mac platform have been around forever and even Apple uses them with its Time Machine backup application. Thus, you might have used a disk image and not even been aware of it.

Now here’s the bug as described by Bombich:

Earlier this week I noticed that an APFS-formatted sparsebundle disk image volume showed ample free space, despite that the underlying disk was completely full. Curious, I copied a video file to the disk image volume to see what would happen. The whole file copied without error! I opened the file, verified that the video played back start to finish, checksummed the file – as far as I could tell, the file was intact and whole on the disk image. When I unmounted and remounted the disk image, however, the video was corrupted. If you’ve ever lost data, you know the kick-in-the-gut feeling that would have ensued. Thankfully, I was just running some tests and the file that disappeared was just test data. Taking a closer look, I discovered two bugs in macOS’s “diskimages-helper” service that lead to this result.

Well, that’s a #fail and a pretty bad one. He then tested on disk images formatted for HFS+ which is Apple’s previous file system and didn’t get this result. Thus he believes that this was an oversight rather than a regression (a regression is something that started out working fine and then broke at some point). More on that in a moment. But because this was a serious enough bug, he took the step of putting out an update to Carbon Copy Cloner that stops users from using APFS formatted disk images as well as filing a bug report with Apple. He also recommends that nobody on planet Earth use APFS formatted disk images until this issue is addressed.

This is clearly a QA fail as I would expect that a test case would have been built around testing an APFS formatted disk image to see if it had the same functionality of an HFS+ disk image. Clearly that didn’t happen here and it underlines the issues that Apple clearly has with the quality of their software. Now earlier this week I tweeted out a story from Bloomberg about a how Apple will address these systemic issues:

Hopefully that yields results as the current state of affairs is not that good.


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