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Is it true that if we dual boot Mojave and Catalina, or if we dual boot Catalina and Big Sur, we can go the traditional way of creating two APFS partitions, and each partition is one container and in it, just have one volume and it is Mojave or Catalina, respectively. Each OS would then have a fixed size (for example, 250GB each, on a 500GB SSD drive).

But with APFS, we could also have one single APFS partition on the physical hard drive -- and so it is one container, and then create two volumes: one for Mojave and one for Catalina, and they can dual boot this way? This actually has the benefits that each OS is not limited to a fixed size, because Mojave can be 150GB and Catalina can grow to 350GB, and if Catalina goes down to 200GB, now Mojave can grow to 300GB -- dynamic flexible sizing?

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Below is an example of where the macOS versions named Mojave, Catalina and Big Sur (beta) have been installed in the same container. This is a triple boot arrangement. The output from diskutil list is shown below.

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *548.1 GB   disk0
   1:                        EFI ⁨EFI⁩                     209.7 MB   disk0s1
   2:                 Apple_APFS ⁨Container disk1⁩         547.9 GB   disk0s2

/dev/disk1 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +547.9 GB   disk1
                                 Physical Store disk0s2
   1:                APFS Volume ⁨MyMojave⁩                12.7 GB    disk1s1
   2:                APFS Volume ⁨Preboot⁩                 410.3 MB   disk1s2
   3:                APFS Volume ⁨Recovery⁩                1.7 GB     disk1s3
   4:                APFS Volume ⁨VM⁩                      1.1 MB     disk1s4
   5:                APFS Volume ⁨MyCatalina - Data⁩       4.5 GB     disk1s5
   6:                APFS Volume ⁨MyCatalina⁩              11.2 GB    disk1s6
   7:                APFS Volume ⁨MyBigSur - Data⁩         2.9 GB     disk1s7
   8:                APFS Volume ⁨MyBigSur⁩                14.9 GB    disk1s8
   9:              APFS Snapshot ⁨com.apple.os.update-...⁩ 14.9 GB    disk1s8s1

All the APFS volumes and snapshots exist in the same container partition and therefore share the space allotted to the container.

The OP's question indicates the assumption that each version of macOS would reside in a single volume. This is not true. In this example, Mojave uses 4 volumes, Catalina uses 5 volumes and Big Sur uses 6 volumes. The Preboot (disk1s2), Recovery (disk1s3) and VM (disk1s4) volumes are shared by all three macOS versions.

Below are the result of enter commands to get the volume uuid for the MyMojave, MyCatalina and MyBigSur - Data volumes. Each UUID is generated when a volume is created and therefore all such UUIDs are unique.

dma@dmas-Mac-mini ~ % diskutil info MyMojave | grep "Volume UUID"
   Volume UUID:               7C48B3D0-263E-4C25-9268-FD4001759334
dma@dmas-Mac-mini ~ % diskutil info MyCatalina | grep "Volume UUID"
   Volume UUID:               DCC8D3B0-3308-4FCC-B057-D4CE495A91FF
dma@dmas-Mac-mini ~ % diskutil info "MyBigSur - Data" | grep "Volume UUID"
   Volume UUID:               383CF355-F467-48CE-9124-B24149322EA7

Normally, macOS boots from the Preboot volume. Although all three macOS version share the same Preboot volume, they do not share the same preboot software. The preboot software for each macOS version is stored in a folder which has the same name as the UUID for a volume uniquely used by each macOS version. For this example, the boot software for MyBigSur is be stored in the folder named 383CF355-F467-48CE-9124-B24149322EA7 on the Preboot volume. Note that the name of the folder is the same as the volume UUID for the "MyBigSur - Data" volume given above.

Basically, the same setup is used to store the macOS Recovery software for each version of macOS, except the Recovery (disk1s3) volume is used.

The VM (disk1s4) volume is used by all three versions of macOS for virtual memory.

Mojave has an additional volume which is the root volume. For this example, this root volume is named MyMojave and has the identifier disk1s1.

When booted to Catalina⁩, the root volume is mounted read-only. For this example, this root volume is named MyCatalina⁩ and has the identifier disk1s6. Catalina has an additional volume which is both read and writeable. For this example, this volume is named MyCatalina - Data and has the identifier disk1s5.

When booted to Big Sur, the volume is named MyBigSur(disk1s8) is not even mounted. Instead, a snapshot volume (disk1s8s1) is mount as root. Big Sur has an additional volume which is both read and writeable. For this example, this volume is named MyBigSur - Data and has the identifier disk1s5.

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  • thanks for the info. So basically, once we get a pure APFS machine (such as the new M1 Macbook), then we can just ignore doing any partition, and one year later, when macOS 2021 comes out, install it onto the same container, and when macOS 2022 comes out, do that same, and that Macbook can then triple boot (3 macOSes on the same machine)? Nov 12, 2020 at 22:54
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    I am not sure how you define pure APFS. There probably will always to the EFI partition which is FAT32 formatted. When Apple introduced the first Intel based Macs, there was JHFS+ partitions. Eventually, Core Storage storage partitions were added. The lastest addition has been APFS. I can not predict where Apple will be by 2022. Nov 12, 2020 at 23:05
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    If you are asking if the next release after Big Sur can just be added to an existing APFS container, the answer is probably yes. In other words, Apple probably to taking macOS a certain direction with each new release. Therefore, one can assume APFS has been designed to accommodate any planned future releases. Nov 12, 2020 at 23:17
  • yup just the overall principles... so for example, we could have High Sierra, Mojave, Catalina all in the same AFPS partition and container... just different volumes, and they can have flexible sizes. The same with having Mojave, Catalina, and Big Sur. Nov 13, 2020 at 14:58

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