Why does my reported used and available disk space not equal the total reported via "df"?

Question

I'm running mac Big Sur (11.2.3). I get some strange disk space readings when I query free space on the command line ...

s davea$ df -h /
Filesystem       Size   Used  Avail Capacity iused      ifree %iused  Mounted on
/dev/disk1s5s1  234Gi   14Gi   18Gi    45%  568975 2448556385    0%   /

I'm confused -- the size of the disk listed (234 GB) seems to exceed the used + available by quite a lot. I've been noticing disk space issues even though there is 18GB free reported so I'm trying to track down what the problem is.

Edit: Per request, here's the diskutil output ...

$ diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *251.0 GB   disk0
   1:                        EFI ⁨NO NAME⁩                 209.7 MB   disk0s1
   2:                 Apple_APFS ⁨Container disk1⁩         250.8 GB   disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +250.8 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume ⁨Macintosh HD - Data⁩     205.8 GB   disk1s1
   2:                APFS Volume ⁨Preboot⁩                 426.9 MB   disk1s2
   3:                APFS Volume ⁨Recovery⁩                613.9 MB   disk1s3
   4:                APFS Volume ⁨VM⁩                      6.4 GB     disk1s4
   5:                APFS Volume ⁨Macintosh HD⁩            15.1 GB    disk1s5
   6:              APFS Snapshot ⁨com.apple.os.update-...⁩ 15.1 GB    disk1s5s1

Answer 1

It's because you have a 250.8 GB (= 233.6 GiB, thanks to the inconsistent units used for memory) APFS container (/dev/disk1) with a number of volumes in it, all taking up space. 15 GB (= 14 GiB) is just how much that one volume (technically a snapshot of a volume) is taking up, but the free space is the amount left after all volumes are taken into account.

diskutil list gives a more complete view of what's going on:

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +250.8 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume ⁨Macintosh HD - Data⁩     205.8 GB   disk1s1
   2:                APFS Volume ⁨Preboot⁩                 426.9 MB   disk1s2
   3:                APFS Volume ⁨Recovery⁩                613.9 MB   disk1s3
   4:                APFS Volume ⁨VM⁩                      6.4 GB     disk1s4
   5:                APFS Volume ⁨Macintosh HD⁩            15.1 GB    disk1s5
   6:              APFS Snapshot ⁨com.apple.os.update-...⁩ 15.1 GB    disk1s5s1

To see how much space is taken up by each volume, look at the SIZE column. The container has 250.8 GB of space, disk1s1 is taking up 205.8 GB of that, disk1s2 is taking 426.9 MB (= 0.4269 GB), etc

So the sum of space taken by the volumes in the disk1 container (in GB) is 205.8 + 0.4269 + 0.6139 + 6.4 + 15.1 = 228.34. Note that the 15.1 MB only counts once, because disk1s5s1 is just a snapshot of disk1s5, so they at-least-mostly are just different ways of viewing the same data and it's only stored once.

Now, 228.34 GB from 250.8 GB leaves 22.46 GB (= 20.92 GiB), which doesn't quite match up with the 18 GiB of free space reported by df. I'm not sure why there's a difference there. It may be there's some filesystem overhead that isn't counted in the space taken up by the individual volumes. Also, I suppose it's possible the snapshot of the startup disk actually has been modified slightly, so disk1s5 and disk1s5s1 don't completely share space (which would be weird, but who knows)?

Answer 2

It's because on Big_Sur / is a read only system volume mounted on a snapshot. The size of this volume is around 15 go. The free size is 18 go, so the volume "data" may be around 200 go. The command

diskutil list

Will give the real size of your system volumes.