iMac's Fusion drive not fully compatible with BigSur?

[Jefke Peeters]

I have since 1,5 year an 2019 27" iMac. He came with an 2TB Fusion drive.
Apple gave the promise that all the often used apps will be automatically stored on the SSD part of the Fusion drive.
This was the truth with Mojave and Catalina.
Cold boot took less than 10seconds to the login screen. After entering password, 2 seconds left and the machine was ready.

Very often used apps opened like on an SSD. Less used apps took longer to open. So this was indeed like a Fusion drive must do.

With BigSur, starting up from a cold Boot takes +/- 5 minutes before the iMac is able to work on.
Apps do start random from the SSD part or the "classic" part of the Fusion.

I also own a 13" MBP late 2013. 4GB ram and 2,5Ghz i5 processor, but SSD harddrive. It opens much faster his apps and boots within one minute.

I did a clean install of Catalina on the iMac and upgraded a week later to BigSur 11.1.
It works stable and fast, but not with the right things on the SSD part.

Is this solvable on this Fusion drive?
tnx
Gert

 

[BrianBaughn]

The best solution would be to replace the Fusion drive with an SSD…and a blade SSD might be better than a 2.5" (someone correct me here if wrong!).

Apple definitely knew in 2019 that their new APFS file system would run horribly on those Fusion drives…as it does on any mechanical drive. I guess that's not as reckless as knowing your state's power distribution is going to fail in a few days when the temps drop but not telling anyone!

Someone may correct me here, also, but I'm thinking the 2TB Fusion drive has a 128GB SSD and the rest is mechanical, spinner HD space.

You could possibly split the two drives, install macOS on the SSD, format the mechanical portion HFS+ and then use several "symbolic links" to change your user folder and other system folders that end up taking a lot of space over to the mechanical drive. This is similar to what I've done on my Hackintosh with a measly 500GB SSD system drive.

 

[decafjava]

My iMac, late 2015 model with fusion runs quite well actually. A bit slower on bootup but nothing too bad however I usually just sleep it.

 

[ignatius345]

The best solution would be to replace the Fusion drive with an SSD…and a blade SSD might be better than a 2.5" (someone correct me here if wrong!)

Part of my Fusion Drive (the blade SSD) died last year* so I opened the machine up and replaced the HDD with a SATA SSD. It's an involved process and carries a risk of destroying the machine. I wouldn't recommend unless you're in a "nothing left to lose" situation. Mine was a 2014 iMac years out of AppleCare. A 2019 iMac is a lot newer and I think it's hard to justify that kind of surgery for what seems to me like a software issue -- at least until all other options are exhausted.

Apps do start random from the SSD part or the "classic" part of the Fusion.
[...]
I did a clean install of Catalina on the iMac and upgraded a week later to BigSur 11.1.
It works stable and fast, but not with the right things on the SSD part.

I'm curious how you'd know which things are stored on the SSD. If it's just general slowness, that might be something else. @BrianBaughn mentions the APFS formatting and from what I've read that could easily have a lot to do with it. If it was me, I think I'd back everything up, reformat the Fusion Drive completely in the old HFS format it would have come with, and install whatever version of the OS your iMac was last running quickly with. Worth a try, anyway.

Fusion Drives really promised a lot and are amazing in theory -- but in practice (see also below) they really come with some serious compromises. Especially now that Apple has moved every other Mac to SSD and tuned the OS accordingly.

Another thing you could do if all else fails is invest in a fast external Thunderbolt SSD and install MacOS on that. It's bound to be a hell of a lot faster than a compromised Fusion Drive and a hell of lot easier than cracking the machine open.


* the iMac's SSD had just been hammered by so many read/write cycles that it was down under 10% lifetime left. It's a known weakness, apparently, as data is always being shuttled to/from the SSD. But replacing the blade SSD itself is even more involved and requires sourcing special adapters and things, so I bypassed it and used the SATA connector instead.

 

[Jefke Peeters]

When I bought this iMac, I had the intention to swap the Fusion by an SSD when his warranty is over. But I have to wait till November 2021. I think I have to be very careful not to destroy the front glass when opening the machine. The rest will be easy.

Another option is to switch over to a M1 iMac when it's released, but I'm a little bit afraid about the fast dying SSD's in M1 MacBooks today... .

Fusion Drives really promised a lot and are amazing in theory -- but in practice (see also below) they really come with some serious compromises.

I was always very satisfied about this Fusion, till I did the upgrade to BigSur.
I owned a Macbook with an SSD of 128GB, running the same apps as on the iMac. Today and with Catalina I still use the same apps, so space enough on the SSD part of the Fusion. Apps start very quickly, so I thought this apps were on the SSD and the bigger files (movies, music, photos) were stored on the 'normal' part of the Fusion.

If it's the truth BigSur isn't made anymore for this Fusion, the startup behavior is declared.

Normally the machine goes into standby and booting-up time from standby is fast enough. So a Thunderbold SSD isn't necessary yet.

 

[hagar]

I had similar performance issues with Big Sur on my Late 2015 iMac. The problem now seems to be resolved on version 11.3, but only after doing a clean install. I'm now back to normal boot times and app launch times.

 

[Jefke Peeters]

I'm running a few days on BigSur 11.3.1 now on my 2019 iMac with Fusion drive.
I think they solved some problems in this version. The cold booting time is as fast as with Catalina. A little slower compared to the same iMac with SSD, but it's usable again.

Since BigSur 11.2, I had some errors on "high energy consuming" USB devices. This was a USB-C to HDMI/USB-A converter. This device had no issues on other, not BigSur, computers. Also this error is solved with 11.3.1