Over the last few weeks I’ve been exploring the major new feature in Big Sur, in which Time Machine can now make backups to APFS volumes, what I’ve abbreviated to TMA. This article is a summary of the current benefits and limitations of TMA as its stands in macOS 11.2.3, at the end of which is a list of those detailed accounts. These should help you make a more informed decision as to whether to use TMA.

Major third-party backup utilities, including Carbon Copy Cloner, SuperDuper! and ChronoSync, have been able to back up to APFS for a lot longer, and now are well-proven. In their cases, the main benefits and limitations relate to those of APFS volumes.

The TMA backup process

• Significantly quicker, transferring a minimum of data, through use of sparse files, clones, and changed blocks. First full backups are less likely to be that much faster, but subsequent backups can be impressively quick.
• Timing and initiation are unchanged.
• Makes the same snapshots on the volume being backed up, which persist for 24-hours before being automatically removed.
• Uses the same method (FSEvents) to determine what to back up in most cases.

TMA access to backups

• Access and restore through the Time Machine app are essentially identical.
• Access and restore through the Finder are very similar.
• Access from third party apps and the command line are limited and complicated, normally involving the mounting of backup snapshots (from backup storage) and tortuous paths.

TMA backups

• Preserves sparse files and clones, and where possible saves changed storage blocks rather than atomic files. As a result, use of storage space is highly efficient.
• The file system of the backup store remains small and compact, as each backup is stored as a separate snapshot, rather than as millions of files and hard links in one huge file system.
• There’s no apparent need for routine maintenance, although longer-term experience is needed.
• Backups don’t rely on directory hard links.
• Automatic housekeeping supports both age-based and space-based thinning of backups.
• Whole backups can be deleted.
• Although both methods are indexed by Spotlight, TMA backups may require longer times to index.
• Individual files and folders can’t be deleted from existing backups.
• It’s currently difficult to change the source volume and maintain continuity of backups.
• Backups can’t be copied to another volume, as snapshots can’t be copied. If you want to make backups to a larger (or different) disk, those have to start from scratch.

The APFS volume itself

• APFS is a modern, robust high-performance file system, using copy-on-write to the file system metadata.
• Specific requirements of the volume include case-sensitive APFS and a backup role, and can’t be limited in its ability to fill its container. It can be awkward to accommodate with other volumes, etc.
• Only one tool is available (fsck_apfs) to check and repair file system problems. However, this is from the developers of the file system so is by definition state of the art.
• Hard disks can suffer fragmentation in both their file system metadata and file data. However, because backups are stored as read-only snapshots this isn’t believed to be as serious as it can become on regular data volumes. More experience is needed.

Miscellaneous

• Excellent access to backups is provided by BackupLoupe.
• It’s fully compatible with T2M2.
• Copious log entries allow precise diagnosis of problems.
• TMA is not (yet) supported on all networked storage.
• tmutil support is still limited.

Previous articles:
Evolution
Understanding backups
Initiating an auto backup
Backing up
How processes have changed
Backup structure and access
Maintenance and repair
Changing disks
How efficient are backups?
Getting started with Time Machine to APFS in Big Sur
The trouble with snapshots: how can you copy them?
How to check the integrity of your backups and iCloud files