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I have read that APFS was optimized for SSDs, but that it's not an optimal choice[ for HDDs. However, I just got my iMac (Model 17,1; OS X Mojave, 10.14) back from repair, and both drives are formatted as APFS. I checked a diskutil output made before the repair work was done (i.e. as it came from the factory), and it seems that both drives were APFS when my iMac was originally built.

Is the APFS format on the HDD in a Fusion Drive a necessity (perhaps due the the need for the SSD using APFS)? If not, could performance/stability be enhanced by using HFS+ on the HDD?

RE: "Has it been reformatted as a Fusion Drive?"

The diskutil list output:

necromac:~ seamus $ diskutil list
/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.3 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                 Apple_APFS Container disk2         500.1 GB   disk0s2

/dev/disk1 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *3.0 TB     disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:                 Apple_APFS Container disk2         3.0 TB     disk1s2

/dev/disk2 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +3.5 TB     disk2
                                 Physical Stores disk0s2, disk1s2
   1:                APFS Volume Macintosh HD            139.3 GB   disk2s1
   2:                APFS Volume Preboot                 52.0 MB    disk2s2
   3:                APFS Volume Recovery                510.6 MB   disk2s3
   4:                APFS Volume VM                      2.1 GB     disk2s4

... and diskutil screenshots:

Screenshot Screenshot
First Second
Third empty
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  • I'd be more concerned about whether the hard drive is the original one. You can expect a mechanical drive to fail at some point after 5 years. When it fails as part of a Fusion drive, you lose the entire drive. If the repair included a new HDD, then I wouldn't worry about any performance issues.
    – benwiggy
    Commented Nov 23, 2022 at 9:16
  • @benwiggy: No - it's not the original one - it was replaced. But the reason for my question is that the "repaired" iMac (SSD replaced, HDD replaced) still has issues very similar to the ones it had prior to the repair - not quite as severe, but definitely dysfunctional. I'm just trying to get enough info to develop a theory.
    – Seamus
    Commented Nov 23, 2022 at 10:39
  • Has it been reformatted as a Fusion Drive? diskutil list would help. I ask because it is the Fusion Drive which is formatted APFS, not the individual disks.
    – Gilby
    Commented Nov 23, 2022 at 22:24
  • @Gilby: I've added some info to my question - does that clarify?
    – Seamus
    Commented Nov 24, 2022 at 0:11
  • Yes. Too often people have a disk repair and it is returned with macOS on the HDD only and wonder why it is so slow. Yours is clearly a Fusion Drive. and looks good from the partition and format point of view. Glad to see you have your answer below.
    – Gilby
    Commented Nov 24, 2022 at 2:30

1 Answer 1

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Fusion drives on Mojave and later use APFS to define a logical volume group, so APFS is required for Fusion to work in your case; there is no support for heterogenous filesystems. On High Sierra and earlier, logical volume groups use the Core Storage layer and HFS+ volumes. (Source: https://support.apple.com/en-us/HT207584)

It's true that the design of HFS+ optimizes for rotational storage and APFS optimizes for solid-state storage, so you can expect to take somewhat of a performance hit on your HDD. However, the whole point of Fusion is to allow macOS to dynamically migrate your most heavily used files (e.g., common system libraries, caches, and frequently accessed documents) onto the SSD, where they will experience an order of magnitude faster access times than they would on the HDD, APFS or not. So overall it's still a net win even if your HDD isn't formatted HFS+.

(If you were hell-bent on squeezing every last millisecond of performance out of your system AND you had a very specific use case, you could un-merge your Fusion drive and format the SSD as APFS and the HDD as HFS+ and use them as separate drives, but I can't imagine a real-world workflow where a manual access strategy could consistently outperform the Fusion hotfile algorithm. It'd be way more of a hassle in any case.)

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  • Thanks - I had searched unsuccessfully for a reference like the one you supplied! And FWIW, I'm not trying for maximum performance - just acceptable performance. I've sunk $$ in the (outsourced) repair job; the iMac was much-improved when it first arrived from repair, but within a few days fell back into the sluggish behavior I had before the repair. It's not as severe, but it's definitely unacceptable.
    – Seamus
    Commented Nov 23, 2022 at 10:52
  • Oh - meant to ask: Have you any experience replacing the Fusion Drive with a single HDD - or an aftermarket SSD in the HDD position? I ask because the replacement SSD the repair shop used was salvaged from another Mac. This appeared to be the only option because OWC states that their SSDs do not work with the Model 17,1.
    – Seamus
    Commented Nov 23, 2022 at 10:57
  • Apart from you needing to source a temperature sensor to replace the one embedded in the fusion drive, you'd have done better to just buy an SSD & forget the old spinny rust altogether. The ones you were looking at are too modern for older Macs, but all you needed was a standard old 'laptop size' 2.5" SSD for SATAIII. That would just slot in as a direct replacement. I have them in all the old Macs here, right back to a 2008 model. Anything like these - amazon.co.uk/…
    – Tetsujin
    Commented Nov 23, 2022 at 11:45
  • @Tetsujin: That may be the answer to the question I was going to post next! :)) If you could clarify/confirm a couple of things, I'll have a plan: 1) Can the existing SSD (PCI bus, I think) just simply be removed - or does it need to remain physically installed to make the system "happy"? 2) Will I be able to restore the "new, non-Fusion" system from a Time Machine backup I made when it was the "old Fusion system"? Here's the question.
    – Seamus
    Commented Nov 24, 2022 at 5:11
  • I've never actually taken an iMac apart - they look tricky ;) They should have 2 SATA connections; remove 2 drives, replace one with SSD. Literally any 2'5" SATAIII drive will work. You'll need to source the sensor separately. Once in & formatted, the system won't care whether it's a fusion or an SSD, it's just a "drive" & will operate like any other. That series of iMacs were all dog-slow because of the drives, so it will be night & day with an SSD in it.
    – Tetsujin
    Commented Nov 24, 2022 at 7:44

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