Fusion drive on iMac model 17,1, Mojave 10.14 - APFS or HFS+ format for HDD?

Question

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
	empty

Answer 1

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.)