Last Week on My Mac: Snapshot hostages

Like it not, many of us will still be using hard disks with our Macs for some years to come. For users who need ample storage space for backups and archives, such as those who pay dearly for M1 Macs with 4 and 8 TB internal SSDs, there’s really little option. Now we’ve switched over to Time Machine backing up to APFS, this poses a problem. Given that hard disks are inherently liable to develop bad storage blocks, which can in turn damage those backups, how are we to repair them?

If your answer is Disk Utility’s First Aid, then maybe one day it will be able to unmount disks sufficiently reliably for repair. But even when it does, can the repair tool, fsck_apfs, which it relies on repair damaged snapshots?

Perhaps inevitably my attempts to answer that have drawn a blank. Apple doesn’t appear to give any indication whether that’s even possible at present. However, we know that snapshots are strictly read-only, and the only user experiences that I can discover confirm my suspicion that all fsck_apfs does when it finds an error in a snapshot is to throw its hands in the air, report an opaque error code, and not even attempt a repair.

For a backup snapshot, that’s fatal. All you can then do is delete the whole snapshot, knocking a hole in your backups which can never be replaced. Disk Utility’s typical response only rubs salt into the wound by telling the user to make a backup of the affected disk. As it’s currently impossible to copy backup snapshots to another disk, a single error on that storage compromises all your backups stored there: every single one of them, and there’s absolutely nothing that macOS offers to help that.

The inescapable conclusion is that making Time Machine backups to a hard disk exposes the user to a risk which they cannot mitigate without resorting to a RAID system, which only raises further issues, additional cost and yet more hard disks.

I know that many users are eagerly awaiting the first third-party utilities which can perform maintenance on APFS volumes and containers, to give them an alternative to Disk Utility. Given the high reliability of APFS on SSDs, improving on fsck_apfs, if that were to prove possible, is likely to be of fairly marginal benefit. For most users clearer benefits would only come with extending the ability to repair beyond normal volumes to snapshots, and to provide the user with a means of making a full copy of the snapshots which make up their backups.

Here Apple’s wilful neglect to provide adequate documentation of APFS takes effect: these issues could remain a blind spot for Apple’s own tools, but preventing others from addressing them can only expose Apple’s customers to unnecessary risk. Apple thus has a simple choice: either deliver the tools that we need to use our Macs properly, or empower others to do the job for you.