Having updating to macOS High Sierra I’ve mostly been impressed, except about one thing, disk space usage.
The new APFS creates snapshots during local Time Machine backups, and this can mean, especially if you’be been dealing with some large files, that when you delete them and empty the wastebasket, you don’t get your disk space.
I noticed this because I’ve been on vacation and created some rather large files from all the photos I’ve been taking. Freeing up 200-400GB of disk space, and then realizing I no longer have the space available. On a 2TB disk, 400GB is a lot to lose. Even more so when you think you’ve deleted the files that *were* using up space.
The key is to look at your local backups, and the easiest method for that is the tmutil command. In particular, check the list of local snapshots:
$ tmutil listlocalsnapshots / com.apple.TimeMachine.2017-10-06-163649 com.apple.TimeMachine.2017-10-07-065814 com.apple.TimeMachine.2017-10-11-165349 com.apple.TimeMachine.2017-10-11-19345 com.apple.TimeMachine.2017-10-11-203645 com.apple.TimeMachine.2017-10-12-003803 com.apple.TimeMachine.2017-10-12-124712
These are normally managed automatically using a combination of the date/age of the backup and the space they are using compared to how much disk space you need. All this happens automatically in the background for you.
But, if you’ve just done some house cleaning, or you’ve come back from using a lot of disk space and want to free it up, you’ll need to get rid of those old snapshots. You can do them individually using:
$ tmutil deletelocalsnapshots <snapshot_date>
But it’s easier to just purge the snapshots and specify how many you many want to get rid of. That will leave you with some recent snapshots but still recover some diskspace, for that, use this command:
$ tmutil thinlocalsnapshots <mount_point> [purgeamount] [urgency]
That [purgeamount] is how much space you want to recover in the process, and the [urgency] is a number (1-4) of how quickly you want the space recovered. Both are optional.
For me, I just ran the thinning and that left me with:
$ tmutil thinlocalsnapshots / Thinned local snapshots: 2017-10-06-163649 2017-10-07-065814
For me the difference was that I went from using 1.2TB on a 2TB disk from all those photos and the snapshots that went with them, to having 1.43TB free!