Disclaimer: this question is exclusively about APFS-based backups, older hard-link based format of Time Machine is not considered in this question
When a Time Machine backup is running, there is a button called Skip This Backup which was later renamed into Stop This Backup (source). Counterintuitively, the button’s function is to pause the backup job as opposed to terminating it early.
How can Time Machine produce reliable and wholistic backups with this modus operandi i.e. stopping-resuming-stopping?
Here’s a trivial example. Around 5.30pm MacOS downloads OS update in the background. Time Machine starts a backup at 6pm. The backup daemon (process) is “around” system-related files. At 6.15 user interrupts the backup by restarting or shutting down the system. During the next boot, OS applies the update that it has previously downloaded. This includes overwriting/replacing some of the system files.
If some files were already copied by the Time Machine, continuing this backup makes no sense because the state of OS files themselves has changed. Otherwise you end up with inconsistent state i.e. part of the backup includes files before OS update was applied and part of the backup includes files after the OS update was applied.
Obviously, this had to be considered somehow by Apple engineers but I couldn’t find a definitive answer anywhere online.
