Mojave and High Sierra in same APFS container

[treekram]

There was a thread in the High Sierra forum where the OP asked about partitioning a disk for using High Sierra and the Mojave beta.
https://forums.macrumors.com/threads/partioning-high-sierra-with-another-drive.2122360/

In that thread, I was wondering whether High Sierra and Mojave could be put in the same APFS container. I looked in this forum but couldn't find anything (sorry if I missed it in the 100+ threads that were there at the time). I looked in the Apple developer forums. I searched the web. The only thing I could find was an article in Italian.

So I decided to try it out (wasn't planning on using the Mojave beta previously). It turns out that you can put High Sierra and Mojave in the same APFS container and it turns out to be pretty trivial - maybe that's why there isn't much on this.

To do this, you obviously have to have free space in your APFS container.

Start the Disk Utility app.

- Select the APFS container where you want Mojave. In my case, that was "Container disk2", which is selected above.
- Either right-click and select "Add APFS Volumes" or from the top Disk Utility menu, select Edit -> Add APFS Volume.
- The dialog box "Add APFS volume to container?" will appear. In my case, I set a starting size of 40GB in "Size Options".
- Press the "Add" button.
(The "Mojave" volume already appears because the screenshot was taken after the Mojave installation.)

In the Mojave installation app, in the beta at least, only my existing High Sierra "rd400" appeared as the target volume. Press "Show All Disks..." and the new volume should appear.

When the installation is done, the Mojave volume should appear as an option in the Startup Manager (Option/Alt key at boot) and in the list of startup volumes in System Preferences.

The following is what diskutil list showed for the container before (in High Sierra) and after the Mojave installation. Notice there is only one Preboot and one Recovery volume. I don't think that the doubling in size of the Recovery and Preboot volumes is important - the other SSD in the computer which only has High Sierra on it has volume sizes which are about the same size and those sizes did not change because of the Mojave install.

High Sierra

/dev/disk2 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +255.9 GB   disk2
                                 Physical Store disk1s2
   1:                APFS Volume rd400                   96.2 GB    disk2s1
   2:                APFS Volume Preboot                 21.5 MB    disk2s2
   3:                APFS Volume Recovery                518.1 MB   disk2s3
   4:                APFS Volume VM                      3.2 GB     disk2s4


Mojave

/dev/disk2 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +255.9 GB   disk2
                                 Physical Store disk0s2
   1:                APFS Volume rd400                   102.7 GB   disk2s1
   2:                APFS Volume Preboot                 44.6 MB    disk2s2
   3:                APFS Volume Recovery                1.0 GB     disk2s3
   4:                APFS Volume VM                      2.1 GB     disk2s4
   5:                APFS Volume Mojave                  40.0 GB    disk2s5

I tested going into recovery mode. I was not given a choice of which recovery to go into even though I had High Sierra and Mojave on one SSD and High Sierra on the other. In recovery mode, I was poking around and in exiting MacOS Utilities, it froze at a grey screen so maybe it was Mojave recovery - I don't know.

Obviously the big advantage of putting High Sierra and Mojave in the same APFS container is that you don't have to do the old-style re-partition or worry about whether the size you're assigning to the volumes is going to be correct. Once you're done with the Mojave beta, zap the volume and it will be available to the other volumes. Or, upgrade the Mojave beta to release, migrate and then zap the High Sierra container.

The caution will need to be given - if the APFS container code proves unreliable, that could have catastrophic results for both High Sierra and Mojave volumes so proceed at your own risk. I installed Mojave on a SSD I'm using for testing purposes.

 

[treekram]

After poking around, I have more information on what happens in the Recovery and Preboot volumes in an APFS container after you install multiple OS's on it.

In both the Recovery and Preboot volumes, a separate directory having the name of the Disk/Partition UUID (available by using diskutil info) is created for each OS volume. Thus, even if you install mutliple OS's, you will only have one Recovery, Preboot and VM volume in the container (although I did not look into the VM volume). The size of these volumes in APFS will grow as you add more OS's. Note: If you don't use APFS in High Sierra, this convention is not used - there is a com.apple.recovery.boot directory in the Recovery volume where the files necessary to run recovery is stored.

If you go into Recovery mode by pressing Cmd-R, you are not given a choice of which recovery to use - it will start the Recovery which corresponds to the last selected (via System Preferences) start disk. I have not done testing to see if this changes if your last start was done by the Startup Manager (Cmd/Alt key at start).

As I mentioned in the OP, I have a second SSD where there is only High Sierra but it has Recovery and Preboot partitions of about the same size as the two-OS SSD. It appears that happened because I did a re-install of High Sierra on that SSD whereby the Disk/Partition UUID changed and the directory with the old UUID was not deleted in the re-install. (I restored from a TM backup which was not the backup of the disk it was being restored to, if that makes any sense.)