How to reserve Time Machine space on an APFS drive

In macOS Big Sur, you can finally use APFS-formatted volumes as Time Machine destinations. They aren’t backwards compatible with macOS Catalina or earlier versions of macOS, and they require erasing an existing HFS+ (Mac OS Extended) disk to reformat—you can’t convert HFS+ to APFS for Time Machine and retain the drive’s data in place as you can when upgrading your macOS startup volume to High Sierra (SSDs), Mojave (Fusion Drives, HDDs), or later. (See “macOS Big Sur supports Time Machine on APFS-formatted drives, but there are a few catches,” for more details.)

However, what if you want to share that APFS drive, dividing space between Time Machine and other purposes? Apple has quite specific advice on how to proceed: add a volume, not a container.

A brief refresher: APFS is Apple’s modern, SSD-optimized replacement for the once-modern filesystem that’s been in use for many years. APFS is much more sophisticated than HFS+, allowing more flexibility for how data is structured and kept secure and separate, as well as offering particular features suited to SSD. The units of measure on an APFS formatted drive are containers, which are collections of volumes. In HFS+, drives were partitioned solely into volumes. One way to think about it: HFS+ was a carton of eggs, each egg a volume. APFS is a box that contains cartons of eggs.

Each APFS container has a fixed portion of the drive’s storage space allocated to it (as in partitioning) or can fill the entire drive. But volumes within a container share the container’s allocation dynamically. In most cases, adding volumes to an existing container makes more sense than adding containers, because volumes can grow or shrink without any work on your part. This allows the greatest flexibility.

However, a disk used for Time Machine backups is quite particular. As explained in the column referenced above, Big Sur adds a new “role,” or volume type, called “Backup.” But, as Apple additionally notes in the Big Sur manual, that Time Machine volume requires the entire disk. That’s a little confusing, isn’t it? (A few readers have written in wondering what it means, in fact.)

What Apple appears to be saying is an APFS Time Machine volume requires a single container that takes up the entire disk—you can’t add other containers, and that container has access to all the store space on the disk. Within that container lives a Time Machine volume. If you want to use the disk for other purposes, don’t add a container; instead, use Apple’s advice and add a volume within the existing container.

That is limiting, because the Time Machine backup could eventually swell to fill the entire available storage in the container (and disk), crowding out the other volume or volumes you create.

However, Time Machine automatically purges older files and attempts to keep itself to a reasonable size, while still offering significant points to revert to. You can also delete older backup snapshots and backed up files, following this advice from Macworld UK.

This Mac 911 article is in response to a question submitted by Macworld reader Ben.

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