Time Machine backups of Multiple Macs to external drive with APFS: Should I use separate containers, or one container with different volumes?

Question

On an external drive, I have 3 APFS containers each with 1 volume in them respectively. The 3 containers and their respective volumes are used for Time Machine backups from 3 different Macs. The APFS containers use APFS (Case-sensitive, encrypted).

It occurred to me that my setup might not be optimal. According to a cursory search, with APFS it might be possible to have the 3 different Time Machine backup volumes share one APFS container, instead of having them reside in 3 separate containers containing 3 different volumes. This would potentially allow the Time Machine backup volumes to dynamically make use of the container space as needed (growing & shrinking).

Is this correct, and a more advisable solution?

Consider that Time Machine backups typically don't shrink, they rather grow with time, and ultimately there needs to be some limiting quota (which is why I originally opted for 3 separates) so there isn't too much cannibalization between different backups from different devices. Perhaps there is a way in Terminal to set a quota on each Time Machine volume?

Alternatively, if APFS containers can be resized on the drive without losing the data in the volumes in those containers, then that practically works quite okay as well in order to manage storage. Related question: Can APFS containers be resized while retaining their contents? (Case-sensitive, encrypted)

Lastly, I am not sure on which level encryption should happen in this regard. If I backup to a single APFS container, should the encryption happen at the level of the container, or on the level of the volumes/backup destinations?

Answer 1

Multiple Time Machine (TM) volumes in a single container:

• If you want strict control of size of TM volumes, then continue with multiple containers. But for flexibility it is allowed to have multiple TM volumes in a single container. I do this.

Regarding quotas on TM volumes:

• tmutil commands allow a quota to be specified at volume creation, but (according to man tmutil) do not provide a way of modifying the quota for an existing volume.
• When you use a volume for TM, it is is recreated by TM with the format required by TM. According to answers and comments on this Dec 2020 question Is it possible to limit the size of an APFS volume?, the TM volume does not inherit the quota set on the volume you specify. This needs to tested with current macOS.

Containers versus Volumes:

• I have multivolume backup disks (for mixed TM and Carbon Copy Cloner volumes) and have never wanted to use quotas or containers. I prefer to keep a watchful eye on volume growth.
• But I can see that you might want to carefully control backup size when used for multiple Macs. In that case, continue to use containers.

Encryption:

• With APFS, encryption is at the volume level.

And, of course, do any testing on a different disk.

Edit (after answer had been accepted)

You can set a quota on a TM volume. I repeated the steps in this 2 year old answer Limit Time Machine size in Big Sur with macOS 13.3. I can confirm that they do still work. This is the command I used:

diskutil apfs addvolume disk7 APFSX TM2 -quota 1000g -role T -passphrase testing 

Note the "APFSX" as well as the "role". They were new to me. The man page does not mention APFSX and does not have a description of all roles.

Add the new volume to TM and the volume is not recreated, thereby maintaining the quota (and encryption). The only caveat is that the quota can't be changed without recreating the volume.

With that addendum, I now recommend unreservedly creating multiple volumes in a single APFS container since you can control backup size with a quota on each volume.