Tuesday, June 30, 2020

APFS and Time Machine in Big Sur

Howard Oakley:

APFS in macOS 11 changes volume roles substantially. The System volume within a boot Volume Group is now sealed using a tree of cryptographic hashes, as I have detailed here.


As 9to5Mac has already reported, Big Sur is the first version of macOS which can make Time Machine backups to APFS volumes without using a virtual HFS+ file system on a sparse bundle. However, to do so requires the destination APFS volume to be assigned the role of Backup, and allocation of storage space as a Physical Store.


APFS doesn’t support directory hard links, so can’t use the same mechanism when storing Time Machine backups. Instead, what appears to function as a form of virtual file system is created using new features in APFS. The volume assigned the role of Backup appears to be a regular APFS volume, and is protected from normal access, even by root. File data is kept as usual in the container’s Physical Store, to which file data is copied during each backup. […] This is synthesised into what is presented by the Finder as the customary hierarchy of files and folders, just as with HFS+ backups. However, matching unchanged folders have different volume numbers, as if they were stored on separate mounted volumes.

The updated APFS reference is here. I plan to keep my Time Machine backups using HFS+ because of APFS’s slow performance with spinning disks. Also, it’s not clear to me whether this synthesized display will cause problems accessing the backed up files using other apps or cloning the backup drive.


Update (2020-09-30): See also: Hacker News.

1 Comment RSS · Twitter

Could this account for the phenomenon (which I have noticed since I got my new iMac Pro running 11.0.1 - and almost immediately updated to 11.1… so everything fresh!) which causes the 'updating…' icon of the Time Machine Volume in the Finder sidebar to revolve for hours long after a TM machine backup has finished and/or when it is not running?


APFS slowness?

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