Thanks to sharp-eyed Rocket Yard commenter Chris pointing us to an Apple support document, we now have a bit more information on macOS High Sierra, Apple File System, and how it all works on Macs and on volumes that are formatted with the legacy HFS+ file system.
[NOTE: At the request of several readers, we’ve added some “translations” to the Apple document listed below to make it more understandable.]
Chris pointed us to this Apple tech document, titled Prepare for APFS in macOS High Sierra. Probably the most important information from Apple is this quote:
Apple File System (APFS) is the default file system in macOS High Sierra for Mac computers with all-flash storage. APFS features strong encryption, space sharing, snapshots, fast directory sizing, and improved file system fundamentals.
When you install macOS High Sierra on the built-in solid-state drive (SSD) of a Mac, that drive is automatically converted to APFS. Fusion Drives and hard disk drives (HDDs) aren’t converted. You can’t opt out of the transition to APFS. (Rocket Yard emphasis)
So basically, if you install APFS on any Mac that uses a solid-state drive as a built-in boot drive, it will automatically convert to APFS and you will not have a choice. Those Macs that use Fusion Drives (like the iMac, for example) or good old hard disk drives will not be converted, period. [UPDATE 09/25: Apple software engineering chief Craig Federighi has confirmed APFS will indeed be coming to Fusion Drives in a later update.]
Here is the rest of the technical note verbatim, with our translation of Apple’s writing in italics:
Devices formatted as Mac OS Extended (HFS+) can be read from and written to by devices formatted as APFS.
Translation: You can read and write to disk drives that were formatted as HFS+ from a Mac with a boot drive formatted as APFS.
Devices formatted as APFS can be read from and written to by:
- Other devices formatted as APFS
- Devices formatted as Mac OS Extended, if using macOS High Sierra
For example, a USB storage device formatted as APFS can be read by a Mac using High Sierra, but not by a Mac using Sierra or earlier.
Translation: Drives of any type (USB flash drives, SSDs, HDDs) formatted as APFS can be be read from and written to from a Mac using High Sierra, even if it is not formatted as APFS.
APFS and FileVault
FileVault volumes are converted from Mac OS Extended to APFS, just like unencrypted volumes.
Translation: If your Mac drive contains a FileVault encrypted volume and is converted to APFS, the FileVault volume is converted to APFS as well.
APFS and Boot Camp
Boot Camp doesn’t read from or write to APFS-formatted volumes, but is compatible with High Sierra.
Translation: Boot Camp works on High Sierra, and you can still create and use a Windows volume. However, the Windows volume can’t write to or read from an APFS formatted volume (for example, an external HDD formatted as APFS).
APFS and file sharing
- Volumes formatted as APFS can’t offer share points over the network using AFP.
- APFS supports SMB and NFS, with the option to enforce only SMB-encrypted share points.
Translation: If your Mac has been formatted as APFS, you can’t share a folder using AFP — the Apple Filing Protocol. That folder can be shared using SMB (Server Message Block) or NFS (Network File System). By default, File Sharing (set up in System Preferences > Sharing) is now done using the SMB protocol.
APFS and Time Machine
- You don’t need to change any Time Machine settings to back up APFS-formatted disks.
- Any Time Machine share points must be shared over SMB instead of AFP.
Translation: You should be able to connect your Time Machine backup drive to your APFS-formatted Mac and things should run as expected. The Backup Drive can be formatted as APFS or HFS+. If you share your Time Machine drive over a network, it will need to be shared over SMB rather than AFP (Apple Filing Protocol). Apple has been moving away from AFP for the past few iterations of macOS.