Understanding and managing Time Machine snapshots

Finding out how much free space there is on an APFS disk is notoriously difficult. Depending on where you look, its size comes and goes, and it’s not uncommon to see it apparently change by tens or even the odd hundred GB in the space of a few minutes, without the user doing anything. One important factor is the presence of snapshots: although they officially occupy “available storage”, their size does affect some of the figures you will see for free space.

Few users make snapshots themselves. The most common creator is Time Machine, which has made them routinely on all APFS volumes which it backs up since the file system’s introduction in High Sierra. At that time, Time Machine used snapshots to work out what files it needed to back up, using a procedure known as a snapshot diff, which works out what has changed between two snapshots made…