Custom Disk Utility configuration for Fusion Drive and APFS

[LazyMonkIE]

All,

If any of you have tried installing Catalina on an older iMac with Fusion Drive you will understand that the system becomes so slow (APFS is designed for SSD and doesn't play well with Fusion Drive) it is unusable.

I'm asking your collective knowledge and experience to tell me if the following two options are possible and if so, what steps I need to take to get the job done.

Option 1: Split the Fusion Drive and mimic
Split the fusion drive so I have 1 SSD and 1 HDD.
Use Disk Utility during fresh install from USB to manually create all volumes
- SDD as Macintosh HD
- HDD as Macintosh HD - Data
- Possibly a few other partitions needed but I don't know what they are and/or cannot see them

Option 2: Split the fusion Drive and configure Mount Point
Split the Fusion Drive so I have 1 SSD and 1 HDD
Install MacOS to SDD on APFS
Post MacOS installation, mount the HDD to Users

All responses very welcome. Just trying to give this old iMac a new lease of life. The latest OS flies when installed to the SSD. It just doesn't like the Fusion Drive configuration. I do not want to open the iMac and install instal a different SSD, I just want to make the current hardware useful for a while longer.

 

[hwojtek]

APFS works just as well with Fusion Drives, it's your superslow HDD that hinders the setup. All writes are committed to the SSD and only then moved to the HDD when not used for a certain amount of time, always leaving 4GB (IIRC) free on the SSD for fast writing.

 

[LazyMonkIE]

APFS works just as well with Fusion Drives, it's your superslow HDD that hinders the setup. All writes are committed to the SSD and only then moved to the HDD when not used for a certain amount of time, always leaving 4GB (IIRC) free on the SSD for fast writing.

You are most likely correct as this is all outside my skillset. All I know is that using Fusion Drive on High Sierra (installed with the option --converttoapfs NO) this machine is as good as new. If I install High Sierra with the conversion to APFS, just opening System Preferences takes about 2 minutes. Checking a box takes 20 seconds. Starting up the iMac takes 9 minutes.

I'm testing on High Sierra only because its the last MacOS where you have a choice to stick with hfs+ if you install vis the terminal.

 

[hwojtek]

I went with a Fusion Drive setup from when it was introduced in Mountain Lion and subsequently backwards-enginereed to allow DYI Fusion Drives on any hardware. For years I was using a 128 GB SSD and 2 TB HDD formatted in HFS+and it was quite OK for my needs (I ran it in a Mac Pro 2,1). When I switched to a Mac Pro 5,1, I also upgraded the FD to a 256GB SSD and 4TB HDD - it immediately felt faster. Most probably the faster and more modern HDD loaded the media files quicker, my only real-life test being iMovie and amateur editing.
I stayed away from APFS as long as I could, Catalina forced this upgrade and I do not 'feel' the setup to be any slower (perhaps some synthetic tests would prove otherwise). That said I will soon retire the FD for a NVMe stick, as my portables are just much "snappier".
This would not work in an iMac, though, so unfortunately whatever you do (apart from splitting the FD and setting it up as you laid out in the 1st post) you will need to do a surgery inside the iMac if you want to accelerate your storage. Tough luck, mate.

 

[SketchyClown]

I found with my iMac and its 2TB Fusion drive, it was a slug with HFS+ or APFS. Lots of beachballs and Spotlight indexing could easily bring it to its knees. So I would usually split the Fusion drive up and run the OS on the 128GB SSD and just use the rotator for storage.

When the old Seagate finally gave up the ghost I installed a 1TB SSD in place of it.

That's when I went back to a Fusion setup. Once I figured out how to set the APFS "main" and "secondary" flags in the Terminal to assign drive roles (I did screw it up the 1st time), it works very well. The main reason I went back to a Fusion setup is I simply enjoy the "single drive" experience.

Performance is 600MB/s write, 1850MB/s read speeds.

So in my experience, unless you decide to ditch the rotator and go full SSD, I would not bother using a Fusion setup with HFS+ or APFS.

 

[ignatius345]

This might be of interest: https://eclecticlight.co/2018/10/15/fusion-drives-in-apfs/

I had APFS running on an iMac with a Fusion Drive and honestly didn't notice a lot of difference from HFS+, just anecdotally speaking.

Eventually, though, I started seeing some incredibly high wear levels on the 128 GB SSD portion of the FD (more about this here) and had to open the machine up and install an SSD. I suppose I could've de-fused the HDD from the SSD, but that would've made the Mac too slow to use as far as I'm concerned. Even though a Fusion Drive is slower than an SSD, it's also a hell of a lot faster than a plain old HDD.

 

[SketchyClown]

Eventually, though, I started seeing some incredibly high wear levels on the 128 GB SSD portion of the FD

I have seen this too. When I got my iMac, which was gently used, sometime in early 2018, DriveDX reported that the Wear Level/SSD Lifetime was 91%on the 128GB. A 9% drop in a little over 2 years of use in the SSD/HDD Fusion.

Fast forward a little over 2 years it has now dropped to 73%, an 18% reduction in the same amount of time.

 

[ignatius345]

I have seen this too. When I got my iMac, which was gently used, sometime in early 2018, DriveDX reported that the Wear Level/SSD Lifetime was 91%on the 128GB. A 9% drop in a little over 2 years of use in the SSD/HDD Fusion.

Fast forward a little over 2 years it has now dropped to 73%, an 18% reduction in the same amount of time.

I had less than 10% lifetime left according to DriveDX! Brutal.

 

[El Gran Mago de Mixcoac]

Option 1: Split the Fusion Drive and mimic
Split the fusion drive so I have 1 SSD and 1 HDD.
Use Disk Utility during fresh install from USB to manually create all volumes
- SDD as Macintosh HD
- HDD as Macintosh HD - Data
- Possibly a few other partitions needed but I don't know what they are and/or cannot see them

I have a Macbook Pro Mid 2012, changed de HDD to SDD a few years ago and i wanted to give some new life to my HDD and bought a cady, I was trying to get a Fusion Drive to work with Patched Sur but it seems not to be possible so I'd tried setting up a RAID 0 but also didn't work so i was wondering how to do what you are saying here. I'm relatively new with all this RAID and FusionDrive matter so any advice you could tell me would help a lot