Which format should I choose for a Time Machine backup volume?

Question

I'm using Catalina on an formatted APFS SSD and am finding a correct way to backup the OS. In the plugged in external HDD, I create a backup volume with APFS format using Disk Utility. But when I choose it as the backup volume in Time Machine, Time Machine requires the volume have to be erased. I did it and then go to Disk Utility to check, the volume is now formatted as Mac OS Extended (Journaled).

Should I continue to use it? Will it occur any problems when I make a restore from backups on a Mac OS Extended (Journaled) to an APFS SSD? Thanks for any help.

Answer 1

While, in general, APFS is better suited for SSD disks and HFS+ (macOS Extended) is better suited for rotational media due to their respective statistical access patterns and ensuing consequences for seek time and flash wear, either format could in principle be used for backup purposes if Time Machine has enabled support for it in a given macOS release. In theory*, restoring from an HFS+ to an APFS volume (or vice versa) should present no problem, as the file copy is done at a higher abstraction layer in the call stack that is agnostic to the underlying storage format. Neither format has inherent limitations that would prevent it from being used as a TM backup and it's merely a question of application (i.e., Time Machine) support. Refer to Marc Wilson's insightful answer for details on which macOS version has enabled support for which formats.

* — The difference between theory and practice is that, in theory, there is no difference between theory and practice, and in practice, there is.

Answer 2

Time Machine in Catalina only supports HFS+ as a destination volume. The first macOS that supports APFS as a destination volume is Big Sur.

ref:

https://support.apple.com/guide/mac-help/types-of-disks-you-can-use-with-time-machine-mh15139/10.15/mac/10.15

vs:

https://support.apple.com/guide/mac-help/types-of-disks-you-can-use-with-time-machine-mh15139/11.0/mac/11.0