What does a

$ diskutil list

give you?

I’m not a mac user, though it says something about ~160GiB beeing purgeable.

How does it usually account for backups?

Also it seems as if APFS supports snapshots, how are those accounted?

As a ZFS user who uses snapshotting a lot in automated ways, I usually do not see the snapshots in a filemanager, but only by using the specialised ZFS tools.

Lets say du -sh ~ tells me that ~ were 230 GB, df -h ~ only says 209G due to compression, while a zfs list tells me this:

$ zfs list -o name,used,available,referenced,compressratio rpool/safe/home/nmelzer
NAME                      USED  AVAIL     REFER  RATIO
rpool/safe/home/nmelzer   302G  72.5G      209G  1.18x

As you can see, this subvolume refers to 209 G directly, while it has an overall size of 302G. The additional ~80G come from differences in the snapshots I keep for that volume.

Perhaps something similar is going on for your APFS?

Windows “Snapshots” are totally different, they are just files copied from one location to another. Snapshots on the filesystem level are totally different from that, They mark blocks on your harddisk as immutable and will only change a copy of them and pretend the immutable copy has been overwritten, but still keep a reference to it.

You can then either drop those old references or roll back to them.

On BTRFS or ZFS I can create a snapshot worth 500 GiB of data in split seconds, and then copy this snapshot over to my backup system. And the nice thing is, the system can stay active in that time.

Lets consider a postgres or mysql database, usually if you just do a cp of the data folder and move use it as backup, then the database itself will probably be corrupted due to the serialised copy. eg. the index file has been copied after the table itself, and the index has been changed since then.

The snapshout of BTRFS and ZFS though is atomic. All files are fixed in time. If you snapshot and copy this snapshot as a backup, you can restart a database using that snapshot, and it will recover from it similar to how it would recover from a power outage, as all files still are coherent in their state.