Catalina's usage of 15-plus GB in 'Other Volumes' versus Sierra
I have two Macbook Airs each with an SSD of about 120 GB, with the difference being that one has Sierra and the other has Catalina. The user data are virtually identical (though the one of Catalina has somewhat fewer and smaller user caches). The two Airs are of models that are about two years apart.
Both SSDs are 120 GB and each is in an MBA albeit of models that are two years apart. The differences:– The SSD with Sierra shows Used 103.41 GB with Purgeable less than a gig, and Free 15.76 GB. The SSD with Catalina (new MBA) shows Used only 98.23 GB (because I nuked a good few files and apps) with Purgeable less than a half-gig, and Free 7.08 GB. The difference is 'Other Volumes' with 15.81 GB. If this comprises System files and caches, well, it should be on the SSD with Sierra too but it's not. Indeed, all caches should be greater in size on the Sierra SSD because I purposely didn't copy a good many user caches over to the new Air's disk.
Also, this MBA with Catalina came with the SSD partitioned into 5 volumes, unlike the one with Sierra. Why? Finally, /private/var on the old one is at 4.88 GB and the new one is at 4.91 GB so that's not the problem either.
For background please see closed question Sierra to Catalina - System Bloat - Paring System Files - Disk Usage
So my question is, what is 'Other Volumes' with 15.81 GB and can this space be recovered? These 75 GB is missing under 'Other volumes' category What is 'Other Volumes'? go into a bit of it.
Now I could do diskutils on the two Airs and draw inferences but they'd be just that -- inferences. What I am seeking is definitive answers from experts who know the facts.
I know about du, df, and the various disk-usage reporting utilities and already ran OmniDiskSweeper (and it didn't tell me anything new though even in root it 'sees' only 100.8 GB of an about 120 GB SSD; my guess is it's because volume(s) of about 19 GB weren't mounted). So please don't refer me to such utilities as my question isn't "Where's my disk space gone?" but is along the lines of, "What is Catalina doing to the SSD that Sierra didn't/doesn't."
------ Added 09-05-2020 -------
Output of diskutil apfs list :
APFS Container (1 found)
|
+-- Container disk1 C78E5D0E-464C-4704-980A-7F4122C7909B
====================================================
APFS Container Reference: disk1
Size (Capacity Ceiling): 121123069952 B (121.1 GB)
Capacity In Use By Volumes: 113780142080 B (113.8 GB) (93.9% used)
Capacity Not Allocated: 7342927872 B (7.3 GB) (6.1% free)
|
+-< Physical Store disk0s2 5F73E550-435E-4253-9F0D-3B77BF79B255
| -----------------------------------------------------------
| APFS Physical Store Disk: disk0s2
| Size: 121123069952 B (121.1 GB)
|
+-> Volume disk1s1 70F87157-C6ED-42AE-BB4E-FD70F566B001
| ---------------------------------------------------
| APFS Volume Disk (Role): disk1s1 (System)
| Name: Macintosh HD (Case-insensitive)
| Mount Point: /
| Capacity Consumed: 10767216640 B (10.8 GB)
| FileVault: No
|
+-> Volume disk1s2 C74FD6D6-48B4-4F19-ACFD-F02460714B80
| ---------------------------------------------------
| APFS Volume Disk (Role): disk1s2 (Data)
| Name: Macintosh HD - Data (Case-insensitive)
| Mount Point: /System/Volumes/Data
| Capacity Consumed: 97966219264 B (98.0 GB)
| FileVault: No
|
+-> Volume disk1s3 1CD24A7E-B82F-4169-8858-F6A80A12C2E8
| ---------------------------------------------------
| APFS Volume Disk (Role): disk1s3 (Preboot)
| Name: Preboot (Case-insensitive)
| Mount Point: Not Mounted
| Capacity Consumed: 81256448 B (81.3 MB)
| FileVault: No
|
+-> Volume disk1s4 AF3723F8-1F7A-4FE9-BC25-C8CDDFFB1121
| ---------------------------------------------------
| APFS Volume Disk (Role): disk1s4 (Recovery)
| Name: Recovery (Case-insensitive)
| Mount Point: Not Mounted
| Capacity Consumed: 542916608 B (542.9 MB)
| FileVault: No
|
+-> Volume disk1s5 839A52DD-021E-4538-817E-741BA79DC84E
---------------------------------------------------
APFS Volume Disk (Role): disk1s5 (VM)
Name: VM (Case-insensitive)
Mount Point: /private/var/vm
Capacity Consumed: 4296101888 B (4.3 GB)
FileVault: No
------ Added 11-05-2020 -------
More info if anyone is interested. It's diskutil list that did the trick.
Sierra has 3 partitions; Catalina, 5. The data partition and 2 very small partitions of only megs are common to both. Here's what's additional in Catalina:
1: APFS Volume Macintosh HD 10.8 GB disk1s1
5: APFS Volume VM 4.3 GB disk1s5
So this probably accounts for those 15 gigs that went MIA from Sierra to Catalina, and this is what Catalina is doing to the SSD that Sierra doesn't.
Catalina splits the system volume into two separate parts: the System volume, which is mounted as read-only, for security; and the Data volume, which contains the rest of the 'disk'. These two parts are presented in the Finder as one disk.
So, you have a System volume of 10.8 Gb: this contains the OS and Apple bundled apps.
Then, a Data volume: this contains everything else on the main volume.
The VM volume stores swap data and sleep images.
In short: these are all things that in Sierra were all on one volume.
Why the difference, if the two Macs are identical? Well, it may just be down to different reporting methods in the OS, and what constitutes 'free' space. Either way, both disks are very full and I'd be looking to offload even more user data to increase the free space.