The first full backup is performed as a manual backup, and largely occurs in file-by-file copying from source to the backup store. It is more efficient than to HFS+, but differences could be less than 10%.
Time Machine
Should now be fully compatible with all locales, calendars, and with both 12- and 24-hour clocks.
A summary of the known benefits and current limitations of Time Machine backups to APFS, with links to more detailed accounts.
Does Big Sur’s Time Machine preserve sparse files and clones when backing up to APFS volumes?
It’s common to want or have to change either the source or destination disk for backups. How well does Time Machine to APFS cope with that?
How to bring a Time Machine backup to its knees: get it to back up a sparse file to an HFS+ disk. It’ll take forever and run you out of free space.
Given their very different structure, backups on APFS disks shouldn’t require routine maintenance. Checking and repair is performed using Disk Utility.
Until Big Sur started backing up to snapshots on APFS, there was little interest in being able to copy snapshots. Now we realise that we can’t.
Unlike HFS+ backups, those on APFS volumes look different depending on how you try to access them. What you see in the Finder is an illusion not available to other apps.
Analysis of the phases of backing up to APFS shows the many similarities with that to HFS+. Crucial differences arise from the use of snapshots as backups.
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