Apple software updates creating APFS snapshots on macOS High Sierra

As part of macOS High Sierra, Apple has added a new feature to Apple software updates which require a restart. When these updates are installed onto a boot drive which is using Apple File System (APFS), an APFS snapshot is automatically created on the boot drive prior to installing the software update. An APFS snapshot is a read-only copy of the state that the boot drive was in at a certain point in time, so it can be used as a backup in case something goes wrong with the update.

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Update 11-2-2017: Apple has a KBase article which references this behavior:

https://support.apple.com/HT204015

The KBase article notes that a snapshot is made before macOS updates are made, which may mean that not all updates that require a restart will generate a snapshot.

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In the event that the Apple software update causes post-installation issues, you can boot to Recovery HD and use the Time Machine restore functions available in Recovery to access the snapshot and restore the affected drive to the state it was in before the software update was installed.

Something to be aware of is that this functionality does not apply to all Apple software updates. Instead, the automated snapshot creation appears to be specifically tied to Apple’s macOS updates.

The automated snapshot creation process does not require Time Machine to be configured for the Mac in question and a separate Time Machine backup drive is not needed. The snapshot is stored on the affected boot drive and does not require anything other than sufficient free space on the boot drive to store the snapshot. For more details, see below the jump.