Can I back up my APFS startup disk to an HFS+ formatted destination, then later restore that HFS+ volume to an APFS formatted destination? We anticipate that many people will have this question or concern, so I performed many tests to both verify the compatibility of these various filesystem variants, and also to establish a baseline expectation of boot performance relative to each filesystem. My tests show that Carbon Copy Cloner 5 is capable of producing bootable backups to and from each of these filesystem variants, and that the process of cloning has no measurable impact on boot performance.
Table 1: Bootability compatibility results and boot time performance for several filesystem variants. Source filesystem is listed on the left, destination filesystems are across the top. Boot time performance of the source filesystem, where relevant, is listed in parentheses for reference.
Mac mini 2011, 8GB RAM. Startup disk is a Corsair Force 3 in a two-slot RocketStor Thunderbolt dock running a clean installation of macOS High Sierra 10.13 (17A362a). All tests performed with CCC 5.0.1 using default settings.
Source and destination hardware
The primary test disk is a 250GB Samsung SSD 850 EVO in a two-slot RocketStor Thunderbolt dock. This device was used as the source and destination to rule out any performance differences due to hardware. Source HFS+ volumes are volumes on a Western Digital My Book, 2TB, attached via Firewire 800 (these tests require an HDD because the macOS Installer would otherwise convert the filesystem to APFS). Boot times for this source device cannot be compared to the SSD source device, therefore are omitted in Table 1. HFS+ Encrypted could not be tested because the macOS Installer refused to install High Sierra onto that volume.
Each source was erased in Disk Utility prior to testing, then I performed a clean install of macOS High Sierra 10.13 (17A362a) using the macOS Installer. I then erased a partition on the destination SSD using the specified filesystem variant, then used CCC to clone the source to the destination. For FileVault-protected destinations, FileVault was enabled while booted from the non-FileVault destination variant. I then repeated the cloning procedure to verify that it had no negative impact on the destination volume's FileVault configuration and bootability. In every case, macOS indicated that conversion would take many hours, so these tests were performed while conversion was underway.
Boot time measurement
To measure boot time, I held down the Option key to bring up the Startup Manager, waited for all devices to load, then used the arrow keys to make a startup disk selection. I started measuring time upon pressing the Return key, and stopped measuring time when the login prompt appeared. For encrypted volumes, measurement began upon pressing the Return key at the unlock UI and stopped when the Desktop picture replaced the gray startup background. Due to the slight difference in measurement technique for encrypted vs. non-encrypted volumes, I would caution against making comparisons between encrypted and non-encrypted filesystem performance based on the values presented in Table 1.
These tests demonstrate that CCC 5 can produce bootable clones of macOS High Sierra to and from HFS+ and APFS. The cloning procedure has no measurable impact on boot-time performance. These tests also demonstrate that boot time performance between HFS+ and APFS are comparable, with only one notable exception. The pre-encrypted APFS volume (both as a source and destination) encountered a very noticeable stall during startup when the progress indicator had filled to approximately 25%. I would not conclude that these numbers reflect a degradation of performance of the filesystem when it is pre-encrypted, rather it seems there is a flaw in the startup process that is specific to this filesystem variant. Regardless, if boot-time performance is important to your workflow, we recommend that you establish macOS on a non-encrypted volume initially, then enable FileVault via the Security & Privacy Preference Pane.
Startup delay on APFS Encrypted volumes
macOS High Sierra encounters a brief startup delay if the startup disk was erased as "APFS Encrypted" in Disk Utility prior to installation. This delay occurs when the startup progress indicator fills to approximately 25% and lasts for 20 seconds (possibly dependent on disk performance). This delay was reproduced on a clean installation of macOS High Sierra (17A362a). The same delay could not be reproduced on a clean installation of the same OS on a non-encrypted APFS volume upon which FileVault was subsequently enabled.
Finder doesn't show all mounted volumes
Periodically while testing the "Golden Master" release of macOS 10.13.0 we have seen incidents in which Finder does not show all volumes that are currently mounted. These volumes do not appear on the Desktop, nor in the sidebar. Restarting appears to be an effective workaround to the problem.
Disk Utility fails to erase an APFS volume unless it is first unmounted
This is an issue we've seen inconsistently, and it seems to occur more consistently when the affected APFS volume has an installation of macOS on it. If you're having trouble erasing an APFS volume in Disk Utility, choose "Show all devices" from the View menu first, unmount the APFS volume, then try erasing the parent APFS container device. Watch a video demonstration of the procedure here.
Indefinite shutdown stall
On a clean install of macOS 10.13 (17A362a), we occasionally encounter an indefinite stall when restarting the system. The system logs out, the screen turns black and just the mouse cursor remains. Sometimes the stall lasts for only a minute or so, sometimes it carries on overnight. Hold down the power button on your Mac to work around this one.
Kernel panic while writing to an APFS Encrypted volume on RAID hardware
We have received one report that macOS consistently encounters a kernel panic while writing to an APFS Encrypted RAID 0 device:
Kernel Extensions in backtrace:
We do not yet know how widespread this issue is. The user submitted the panic report to Apple, we're looking forward to a fix in an upcoming OS update.
- Best practices for updating your Mac's OS
- Everything you need to know about Carbon Copy Cloner and APFS
- Video: Downgrading from High Sierra to Sierra using a CCC bootable backup
- Preparing your backup volume for an installation of macOS
- Video: Working with Disk Utility on macOS High Sierra to prepare your CCC backup disk
- Testing your CCC backup
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