Although Apple started using the new APFS file system by default a couple of years ago in Mac OS 10.14 Mojave, the Time Machine backup system has continued to use the old HFS+ file system. The reason is that APFS does not support hard-linked folders, a technology Apple introduced just for Time Machine backups.
In Big Sur, Apple has updated Time Machine so that you can now use Time Machine with APFS-formatted hard drives (and SSDs). This has been accomplished through new additions to APFS that appear to be some form of virtual volumes (Howard Oakley at Eclectic Light Company has a more detailed explanation if you are curious).
There is no requirement to switch to APFS - and if you already have a Time Machine backup with a lot of history doing so would be a bad idea. There is no way to convert an HFS+ Time Machine volume to APFS. The only way to change the format is to reformat the hard drive and start over from scratch.
If you have a new backup disk, however, you can choose to format it as APFS and activate Time Machine, but then you should also know that it can only be used with Macs running Big Sur or later and not Mojave or Catalina. You can not see the backup volume on older systems at all, you can not even manually go in and extract individual files from the backup.
The advantage of moving to APFS is that Time Machine is significantly faster. Ars Technica has tested the speed and on a locally connected hard drive, Big Sur with APFS is almost three times faster with the initial backup and four times faster on subsequent incremental backups. The difference is much smaller for network drives.
Some features that previously existed, such as the ability to delete individual older backups, appear to have disappeared. You can not use the terminal command tmutil delete to delete selected copies, for example to free up space on the backup disk without getting rid of the oldest copy. This will hopefully be added in later updates, but as it is today, Time Machine is more complete with HFS+ formatted hard drives.
Our tip is to continue using HFS+ for Time Machine hard drives for the time being, especially if you already have one with a backup history. APFS really has no advantages over HFS+ on old-fashioned hard drives, but it is on SSDs that the modern file system can be useful. Although the prices of SSDs have fallen sharply in recent years, it is still much cheaper per gigabyte to purchase a "regular" hard drive - and you will hopefully not need to use the backup very often.
For more details about using Time Machine read: How to use Time Machine to back up a Mac and How to restore Mac files from a Time Machine backup.
For alternative backup solutions read: Best Mac Backup Software.
This article originally appeared on Macworld Sweden. Translation by Karen Haslam.