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Apple's Policy on Apple File System (APFS) for High Sierra

Stork

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Apple Support Document:
Prepare for APFS in macOS High Sierra

"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 upgrade to macOS High Sierra, systems with all flash storage configurations are converted automatically. Systems with hard disk drives (HDD) and Fusion drives won't be converted to APFS. You can't opt-out of the transition to APFS."

Other World Computing has a "translation" of information in the Apple Support Document: Translating Apple’s New High Sierra & APFS Compatibility Document.


Update: I can confirm that TRIM is on by default in High Sierra using APFS with a Samsung 950 Pro M.2 NVMe SSD as reported by:

About this Mac > System Report > Hardware > NVMExpress > TRIM Support > Yes​

No need to use the Terminal command trimforce or other TRIM applications.

Update: aol found another excellent article on APFS is at ArsTechnica. Scroll down to the bottom of the page in the last paragraph, Compatibility with other OSes, there is another caution which IMO is a biggie! (My bold for emphasis.)
Apple says that it “plans to document and publish the APFS volume format specification” at some point, but, for now, the only operating system that can read and write to APFS volumes is High Sierra (you may be able to use Sierra, too, but its APFS implementation is incomplete and only intended for developer testing). You’ll want to hold off on converting all your external disks until you’re sure you’ll only be using them with High Sierra Macs.
Update November 05, 2017. CCC’s Bombich now “approves” the use of APFS > Evaluating the High Sierra Upgrade


References:
Carbon Copy Cloner's Blog: https://bombich.com/blog
ArsTechnica
Appleworld Today's What you need to know about APFS and macOS High Sierra compatibility
PikeRAlpha's Do I use APFS?
Macintouch.com
 
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So... systems rooted on SSDs will be un-bootable after upgrading to 10.13, without the latest Clover and EFI driver for the APFS filesystem... Better get those sorted before updating. See also this.
 
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RehabMan

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Apple Support Document:
Prepare for APFS in macOS High Sierra
"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 upgrade to macOS High Sierra, systems with all flash storage configurations are converted automatically. Systems with hard disk drives (HDD) and Fusion drives won't be converted to APFS. You can't opt-out of the transition to APFS."

Reference:
Macintouch.com
It is possible to opt-out of the APFS conversion by using 'startosinstall'.
Note:
Code:
SPEEDY-NUC:nuc.git rehabman$ /Applications/Install\ macOS\ High\ Sierra\ Beta.app/Contents/Resources/startosinstall --usage
Usage: startosinstall --volume <target volume path>

Arguments
--applicationpath, a path to copy of the OS installer application to start the install with.
--license, prints the user license agreement only.
--agreetolicense, agree to license the license you printed with --license.
--rebootdelay, how long to delay the reboot at the end of preparing. This delay is in seconds and has a maximum of 300 (5 minutes).
--pidtosignal, Specify a PID to which to send SIGUSR1 upon completion of the prepare phase. To bypass "rebootdelay" send SIGUSR1 back to startosinstall.
--converttoapfs, specify either YES or NO on if you wish to convert to APFS.
--installpackage, the path of a package to install after the OS installation is complete; this option can be specified multiple times.
--usage, prints this message.
--volume, path to the target volume.

Example: startosinstall --volume /Volumes/Target --converttoapfs YES
 
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Has performance improved since half a year ago? I remember APFS not even reaching HFS+ speeds on SSDs then. Of course there is more to a FS than just speed, but speed is a lot IMO.
 

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It is possible to opt-out of the APFS conversion by using 'startosinstall'.
Note:
Code:
SPEEDY-NUC:nuc.git rehabman$ /Applications/Install\ macOS\ High\ Sierra\ Beta.app/Contents/Resources/startosinstall --usage
Usage: startosinstall --volume <target volume path>

Arguments
--applicationpath, a path to copy of the OS installer application to start the install with.
--license, prints the user license agreement only.
--agreetolicense, agree to license the license you printed with --license.
--rebootdelay, how long to delay the reboot at the end of preparing. This delay is in seconds and has a maximum of 300 (5 minutes).
--pidtosignal, Specify a PID to which to send SIGUSR1 upon completion of the prepare phase. To bypass "rebootdelay" send SIGUSR1 back to startosinstall.
--converttoapfs, specify either YES or NO on if you wish to convert to APFS.
--installpackage, the path of a package to install after the OS installation is complete; this option can be specified multiple times.
--usage, prints this message.
--volume, path to the target volume.

Example: startosinstall --volume /Volumes/Target --converttoapfs YES
Good find, RehabMan, and thanks for how to use that option. I'll pass this onto tony & MacMan.
 
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Im guessing that if I install High Sierra on my Desktop using APFS Scheme it will change my Mavericks Boot Drive to APFS? I don't even think Mavericks will have support for APFS.
 

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Good find, RehabMan, and thanks for how to use that option. I'll pass this onto tony & MacMan.
I didn't "find" it, but I read a lot (it is documented by Apple).
I tested it and it works.
 

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Im guessing that if I install High Sierra on my Desktop using APFS Scheme it will change my Mavericks Boot Drive to APFS? I don't even think Mavericks will have support for APFS.
No. In my experience, it only converts the system partition to which you're installing 10.13.
But, I haven't tested what it does with a data-only partition on SSD, as I don't have any.
 
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No. In my experience, it only converts the system partition to which you're installing 10.13.
But, I haven't tested what it does with a data-only partition on SSD, as I don't have any.
I will have to do a clean install of the Latest Beta on my Desktop. Im still running the first Public Beta.
 

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Has performance improved since half a year ago? I remember APFS not even reaching HFS+ speeds on SSDs then. Of course there is more to a FS than just speed, but speed is a lot IMO.
I haven't done much perf testing on APFS. Mostly running HFS+J for now.
Better to evaluate perf after final release when they have had a chance to optimize it and remove excess debug code.
 
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