How can I confirm my fusion drive is working?

Question

My iMac suffered a hard drive crash a couple years ago and I was forced to reformat the drive and reinstall from scratch. Performance has been terrible ever since. It might be because the SSD never got configured to be the cache for the hard drive — i.e. there is no longer a fusion drive. (Or it might be because I also upgraded the OS (currently running Monterey), or it might be bloatware. Or maybe when I upgraded the RAM to 16G a few years ago I installed the wrong stuff.)

Machine is a 2017 27-inch iMac.

So right now I'm trying to narrow down possibilities. This morning I deleted a whole bunch of software I never use, including Parallels VM. Now I'm looking into the SSD issue.

tl;dr: how can I confirm that my SSD is being used in conjunction with my main drive?

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Edit: output of diskutil list:

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *28.0 GB    disk0
   1:                        EFI ⁨EFI⁩                     314.6 MB   disk0s1
   2:                 Apple_APFS ⁨⁩                        27.6 GB    disk0s2

/dev/disk1 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk1
   1:                        EFI ⁨EFI⁩                     209.7 MB   disk1s1
   2:                 Apple_APFS ⁨Container disk2⁩         999.9 GB   disk1s2

/dev/disk2 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +999.9 GB   disk2
                                 Physical Store disk1s2
   1:                APFS Volume ⁨Macintosh HD - Data⁩     869.9 GB   disk2s1
   2:                APFS Volume ⁨Preboot⁩                 308.5 MB   disk2s2
   3:                APFS Volume ⁨Recovery⁩                1.1 GB     disk2s3
   4:                APFS Volume ⁨VM⁩                      9.7 GB     disk2s4
   5:                APFS Volume ⁨Macintosh HD⁩            15.4 GB    disk2s5
   6:              APFS Snapshot ⁨com.apple.os.update-...⁩ 15.4 GB    disk2s5s1

/dev/disk3 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *5.0 TB     disk3
   1:                        EFI ⁨EFI⁩                     209.7 MB   disk3s1
   2:                  Apple_HFS ⁨Falk backups 3⁩          5.0 TB     disk3s2

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Things learned

There are two ways to find out if you have a fusion drive:

Run diskutil list and look for a "synthesized" drive with this notation: Physical Stores disk0s2, disk1s2 where the two drives match two of the drives listed above, which will be your SSD and conventional hard drive. If Physical Stores only lists one drive, then you don't have a fusion drive.

Alternatively, go to the menus  > About This Mac > Storage. There should be a drive shown there with the label "Fusion Drive".

(I believe for Ventura or later, you go to  > System Settings > General > Storage instead.)

See the accepted answer for the solution. This is a complete reinstall of the os, so do backups first. TimeMachine and an external drive make this seamless (but it still takes like half a day.)

Answer 1

Fusion drives consist of two physical volumes that are combined into one logical volume group. That doesn't appear to be the case here:

/dev/disk2 (synthesized):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      APFS Container Scheme -                      +999.9 GB   disk2
                                 Physical Store disk1s2

This says that your logical boot volume spans a single physical medium, disk1s2, which, from the earlier diskutil output, appears to be your 1 TB HDD. If the logical volume also spanned your 32 GB SSD (disk0), the last line would instead read Physical Stores disk0s2, disk1s2.

You can also use the diskutil apfs list command to see a hierarchical representation of your logical and physical volumes. It might make the picture more clear.

If you did not manually recreate the Fusion configuration using the diskutil resetFusion command or the appropriate series of low-level diskutil apfs XXXX commands, then it's not surprising to be running purely off the HDD and leaving the SSD unused. And that would certainly negatively affect your performance.

You may want to back up all of your data, erase your drive, boot into Recovery, and use the diskutil resetFusion command to start over, verifying your configuration with diskutil apfs list before reinstalling macOS and restoring your data.