Fusion Drives in iMacs: still worth it (or perhaps even MORE worth it)?

[Shamino (David C.)]

@jsrnephdoc

So, my bottom-line question: does a 1 TB NVMe stick in the processor direct slot + a 1 TB SATA drive in the drive bay, if fused together, create a 1 or 2 TB storage device?

First off, I think the question is moot if you have a 1TB SSD. Install macOS on the SSD and format the HDD as a separate secondary volume where you can manually store documents that you don’t want on the SSD.

With the SSD big enough to hold macOS, your apps and home directories, you really don’t need macOS to be automatically moving files between the two devices.

To answer your question, based on what I remember reading many years ago (when this was new tech):

• Fusion drives formatted for HFS+ have a capacity that is the sum of both devices. A file only exists on one device and macOS moves files between the two according to the Fusion Drive logic.

• Fusion drives formatted for APFS have a usable capacity that is the size of the HDD (maybe the largest drive, but it would be a pathological condition to set up a Fusion drive where the HDD is smaller). All files always exist on the HDD and the SSD acts as a cache.

  So even though the fusion drive will appear (to disk utilities) to have a capacity that is the sum of both devices, once you start writing data, the practical capacity will be that of the HDD.

That being said, take a look at the following article about Fusion drives: Fusion Drives in APFS – The Eclectic Light Company

When a fusion drive is formatted for HFS+, what you get is:

• Two storage volumes (one on the SSD and one on the HDD) are bound together as a CoreStorage logical volume, which is presented to the system as a third (virtual) storage device.
• This virtual storage device is then partitioned and formatted for HFS+.

When the fusion drive is formatted for APFS, you get something a bit more complicated:

• The SSD has a single volume, formatted as an “APFS Physical Store Disk” and tagged as “Main”

• The HDD has a single volume, also formatted as an “APFS Physical Store Disk”, but tagged as “Secondary, Designated Aux Use”

• A single APFS container is created, containing both of the physical stores

• Within that container, all of the usual APFS volumes are created.

  Since this article was written based on macOS Mojave (the first version where APFS fusion drives were supported), these contain your root, Preboot, Recovery and VM volumes.

  I suspect that if you do this today using Big Sur, that APFS container will have System, Data, Preboot, Recovery and VM volumes - like what you would find on a non-Fusion drive.

@jsrnephdoc

Has anyone here done it?

I’ve never done it, but …

With a genuine Apple Fusion drive (not sure if it will work if you’ve replaced your internal drives with aftermarket parts), the diskutil resetFusion command is supposed to do all the work for you (wiping both devices in the process). Here’s the text of the manual page from Big Sur (type man diskutil and scroll down a bit to find it):

@diskutil man page

resetFusion

For Fusion Drive machines (two internal disk device hardware configurations), reset the disk devices in the machine to the factory-like state of one empty Fusion volume.

This command will only run on a machine that contains exactly two internal disk devices: one solid-state device (SSD) and one rotational device (HDD), or, alternatively, two solid-state devices. This command must be able to make a positive identification thereof. If these requirements are met, you are prompted, and if you confirm, the erase and reset begins.

Both internal disk devices are (re)-partitioned with GPT maps, and then they are turned into (used to create) an APFS Fusion Drive Container with one APFS Volume.

All internal-disk data is lost. This includes any “extra” partitions (e.g. for Boot Camp or other “user” purposes). No system software is installed and no user data is restored. After running this command, you should (re)-install macOS on the machine (on the newly-created APFS Volume); otherwise, the machine will not be usable (bootable).

You generally must be booted from the Internet Recovery System (CMD-OPT-R) or from an externally-connected macOS boot disk (e.g. a USB drive), because you cannot erase a disk that hosts the currently-running macOS.

Externally-connected disk(s) are not affected. Ownership of the affected disks is required.

If that won’t work for your system and you need to do it manually, here are some articles that may help:

• Creating your own Fusion Drive (OWC, 2012)
• How to make your own Fusion Drive | Macworld (MacWorld, 2012)
• https://www.econtechnologies.com/chronosync/tn-cs-apfs-fusion-drive.html (2019)

The articles from 2012 only describe making HFS+ Fusion drives, because that’s all there was at the time. Note also that an APFS Fusion drive requires macOS 10.14 (Mojave) or later.

A summary of the procedure for HFS+:

• Create a CoreStorage logical volume group consisting of the two drives you want to fuse
• Create a logical volume on that group.

A summary of the procedure for APFS:

• Create an APFS container spanning both devices, specifying the SSD as main and the HDD as secondary. This container will cause a new virtual device to be created
• Add an APFS volume to the container’s virtual device

Allegedly, that’s all there is to creating a Fusion drive - a CoreStorage volume group or APFS container that spans multiple devices.

What I don’t know is if macOS will, upon seeing an SSD grouped with an HDD will automatically start implementing Fusion Drive logic (keeping most-commonly-accessed files on the SSD and everything else on the HDD) or if you’ll just end up with a dumb spanned volume where performance will vary based on where files happen to end up being stored.

I think we need information from someone who has actually tried this in order to answer that question.

But ultimately, I think this should be just an academic exercise for you. With a 1TB SSD, I see no reason why making a Fusion drive would be better than simply using it and your HDD as separate storage devices.