Very slow boot on Fusion and Big Sur

Mine is {iMac -2017 with Quad-core Intel Core i5 processor (3.4 Ghz.) L2 Cache per core) RAM – 8 GB} Fusion Drive – 1 TB). Earlier the machine was blazing on High Sierra – start-up time about 20 seconds.

During some weak moments, succumbed for upgrade due to constant pestering messages from Microsoft office 365 (updates available but you need to upgrade OS)

Thereafter, the quest began, scrambling to regain what I lost due to my own imprudence.

Apple service centre guy recommended the first step {Upgrade the RAM from meagre 8 to 32} promptly done spent fortune with no results whatsoever.

Second recommendation (delayed due to festive season) Replace 1 TB Fusion Drive by 1TB SSD Drive.

AND nothing works then you may do you tiny bit of contribution in overflowing Trillins dollar company by buying new machine (beg, borrow or steal) since that was the intent right from beginning ; under the garb of developing new software and removing non-existent bugs and glitches.

As ay other reasonable sane man, I also requested the tech guy, to relegate my machine to Catalina or High Sierra (10.13.6) {which was doing very good job). It took him two hours to do just that because apple had locked the ‘Fusion drive’ in the process of upgradation of Big Sur. He had cross several hurdles before he could bring it back with a bootable drive of Sierra and used My “Time-Machine SSD”.

AlAS! Even now the start-up (in sierra – with my old files and setup) is procrastinating and it has become a harrowing experience watching it coming to life. 

I have gone through all the suggestions given above thread, and as is apparent, none worked. Even the outside SSD has not worked eventually.

To quote from ‘Deep sea Diver’ “Those of us who have small SSD blades as part of the Fusion Drive system, will indeed see bootup times of in between 4 to 5 minutes”… If I have understood it correctly, the suggestion of Apple service Centre guy (replace Fusion By SSD) should work.

Need your expert opinion of this, before I embark upon this expedition..

Thanks in advance.

Thanks for this prompt response and for your words of wisdom Sir. I am in complete agreement with what you state in para 3. But in So far as parts 2, probably I did not male myself clear. Apple people would remove original ‘1-TB Fusion drive’ & install ‘1-TB SSD’ in its place.

So, should I hope, it will solve the problem.

I am going to wait a bit longer and hope Big Sur gets fixed with a future update. I can't go back to Apple Support with my 2015 as it would only cost me money. BTW: I have 24Gb of RAM, needless to say it doesn't make a difference. I do understand Big Sur is more taxing on the system, but this performance drop is of a different magnitude.

I'm not quite sure what to make from your system still running slow on previous macOS versions. Maybe a clean install will fix the issue?

Apple support or service people will tell you they will "replace the Fusion Drive with an SSD", but that is actually not exactly what they will do. Allow me to explain. The Fusion Drive is made up of two physical parts, the HDD and the SSD "blade", each of which are located in very different parts of the motherboard of the iMac. Upon disassembling the iMac the HDD is much easier to reach than the SSD blade (for which a lot of parts have to be disassembled to be able to reach it). As far as I understand the hardware of the Fusion Drive iMacs (you can check it in System Information), the SSD blade has a very fast connection to the motherboard: NVMExpress, which allows for speeds up to 8 GT/s - 8 GigaTransfers per second. The HDD part of the Fusion Drive is using a SATA connection, which "only" has a 6 Gigabit link speed.

Removing this SSD blade whilst replacing the HDD with an SSD would make your iMac faster, but not as fast as when they would leave the SSD blade in place whilst replacing the HDD with an SSD. Why? Because the "old" SSD blade (which has a much faster link speed than the new SSD that replaces the HDD) will still allow the new SSD to use it as a kind of extremely fast cache. In other words, the "old" Fusion Drive, made up of a "very fast" SSD blade plus a "rather slow" HDD, will be replaced by a "new" Fusion Drive, made up of the old, "very fast" SSD blade plus a new "fast" SSD (which only replaces the HDD, but not the old SSD blade). In still other words, the old, hybrid (SSD blade + HDD) Fusion Drive will be "replaced" by a new, uniquely SSD Fusion Drive (SSD blade + SSD drive), as the iMac will also "see" the combination of the SSD blade and the SSD drive as one "fused" drive.

I have the same problem iMac 2019 with 1TB Fusion. I've followed the comments about removing Avast etc and followed the Apple Support advice which included reinstalling Big Sur but has made no difference to the start up times. I just hope they fix this in 11.2.4.

Apropos, my posts dated (1:23 AM & 2:55 AM); I had only two options either to go in for a new Mac or for replacement of 'Fusion Drive' with 'SSD' as was suggested by one of my friend, to whom I used to regard as a true wizard (before this fiasco).

However, the second option did not found favour with our find here 'Deep Sky Diver' so I looked for help from apple support, who willingly extended all the help they could on two occasion, but all of it proved to be of no avail.

Hence, before taking the big leap, I decided to gamble on SSD (although even apple people were also skeptical about its efficacy) and EUREKA!!!!!! Upgraded system from High Sierra to Big Sur ; restored my files from time machine ; and now my Mac is starting up in just about 25 seconds. Yuppie!!

So mu sincere advise, if at all you want to do anything to salvage this grim situation today itself, you do not even have to wait, just get the SSD in place of 'Fusion drive' and that is it. 'Good Luck' to you all

Firstly,

In Disk Utility, there is only one SSD.

Secondly, the original 'Fusion Drive' was removed in my presence,

And

Lastly, the original apple's drive is now doing duty as 'External Time Machine' having been safely installed in an external HDD enclosure of 'Transcend'.

[Edited by Moderator]

The simple truth is that you are not able to understand the difference between what constitutes the "Fusion Drive" inside an iMac. It consists of 2 parts, not one.

I am wrong? Prove it, by using "System Information" and do what I told you: to check both items in the Hardware section.

Disk Utility shows you not one SSD but one drive: the fused drive, consisting of 2, physically separated parts, that are installed in 2 different places on the iMac's motherboard. Have you tried to see what "System Information" tells you? No? Then I would advise you to stop insulting me.

Can you please enlighten us all and tell us what your "original Fusion Drive" looked like when it was removed (replaced) in your presence? If it was only one part, the original HDD, then I am right in explaining you for the umpteenth time the difference between a "Fusion Drive" and the HDD part of a "Fusion Drive". A "Fusion Drive" is not even a physical drive, but a sort of "upgraded" RAID system, consisting of 1. an SSD blade (installed deep inside the iMac, which makes it a whole lot of work to physically remove it) and 2. a conventional HDD.

If you refuse to tell us what "System Information" is showing in the two sections of the Hardware (NVMExpress and SATA/SATA Express), then it is someone else who is living in a castle on a cloud...

What your original HDD is doing now, is of no importance to what I said above. Or actually it is, because you don't say anything about the SSD blade, which is the other integral part of the "Fusion Drive", being removed...

Until I see a screenshot of the two sections in System Information, described above, your Fusion Drive iMac is still a Fusion Drive iMac, as only the HDD part of it has been replaced by an SSD - as is usual. Removing the SSD blade would make no sense as it most probably would render your iMac's motherboard dysfunctional.

The only reason why I still reply to your insulting comments, is because you are not the only user here and your comments make their problem even more complicated than it already is. Replacing the original 1TB HDD with an expensive 1TB SSD is not a solution, it's a work-around, and a very expensive one. The solution for the problem with Big Sur booting in 4 to 5 minutes on recent 1TB Fusion Drive iMacs, is - as I was told by a senior Apple engineer working on the problem - to rewrite parts of the OS. I hope they will do it successfully.

Are you willing and able to answer my questions and show us screenshots of the "System Information" hardware sections (NVMExpress and SATA/SATA Express)? I hope you do.

I'm running BigSur on a iMac (late 2015) with 1TB Fusion Drive (24GB SSD), with FileVault enabled. Now my iMac boots in about 52 seconds from the login screen. When booting it from an external USB SSD, it takes about 35 seconds (but it takes much more time before reaching the login screen, then the two booting times are quite similar). In order to reduce the booting time, I've found that this is very effective: after a huge update (MacOs, Office, Final Cut, Photoshop, ...), booting the system without using it for 20-25 minutes. Maybe you should repeat this for 2 - 3 times. This allows the OS to optimize the content of the SSD cache, based on utilization statistics and then reducing the booting time.

I think that between Catalina and BigSur the performances of Fusion Drives decreased because in Big Sur there are many executables (from Mac Os and applications) that are "Universal" and this mean that they contain executables for two different CPU architectures. They use much more disk space and for that reason the SSD cache of the fusion drive is far less effective. The potential solution could be a different way to distribute the executable: targeting Intel iMacs with a dedicated set of executables (containing only Intel instructions) may reduce the impact on Fusion Drives.

Resolved... for you perhaps, but not for many of us who have followed that procedure already months ago without any difference in startup times between 3 to 4 minutes before any app becomes responsive.

I installed the 11.3 and now the results are surprising: 10 seconds from the power on to the logon screen and then 19 seconds from the password typed to the desktop full rendered. And at that point the applications can be started quite fast. I can say that now it's better than with Catalina. I've a Late 2015 iMac, 1 TB Fusion drive with only 24 GB SSD.

Dear Fabio,

Thank you, once again, for this very useful and positive news. Can I ask you, also once again, for a favour. Many of us would like to give it another go, but cannot waste anymore time in case the result would be negative and we would have to downgrade back to Catalina. So, please, could you detail, step by step, how you managed to get to this result? I asked you before for some more details, but there are more questions remaining and it seems it's very important for us to be able to copy exactly every step of the way.

I know you did a clean install and allowed Big Sur to do its work for 20 to 30 minutes after that install, without starting any application. But what I would like to know also is how you actually restored all your apps and your data. Two of my main questions are: Did you, at the end of the installation procedure make a new account in Big Sur without restoring anything at first, or did you simply follow the instructions of the Setup Assistant and immediately copied over your apps, data, and settings from your Catalina account? Second question: What method did you use to restore your apps, data, and settings (a Time Machine backup, a clone from your Fusion Drive made by a third-party application...)?

Again, it would be so useful for so many of us if you could detail, step by step, numbering each step, how you got Big Sur booting normally and running normally on your iMac with 1 TB Fusion Drive with a mere 24 GB on the SSD blade, from installing Big Sur to using it right now. Thank you very much in advance!

After updating from Catalina to BigSur I experienced the issue and I re-installed BigSur from scratch (and also the all the applications). The results were not good at that time, then I started booting from external SSD drive, forgetting the Fusion Drive. Some weeks ago I started trying again and I found that booting the system several times and allowing it to run for 20 - 30 minutes without using it, improved the booting time as documented before (52 seconds from the password typed to the desktop rendered).

Today I installed the 11.3 in the following way: I downloaded BigSur on another Mac. Then I created an install media on an external SSD. The I booted from the external SSD and I asked to install the new Big Sur 11.3 to the Mac HD. After the installation I let the system idle for one hour. Then I rebooted, it obtaining the new results (19 seconds from the password to the desktop). It's the first time I install Mac Os in that way: all the other times I followed the standard way. I don't know if the success is because the 11.3 has changed something or because installing from the external SSD the system the OS is installed directly in the SSD portion of the Fusion Drive.

Today I just updated to 11.3, I did not reinstalled the system. I did it as described in my previous post (booting from an install media I prepared on a external SSD). Please notice that this installation preserves you Data partition.

Yes, you're correct.

In addition to your step #3: after installing huge applications, please boot the system 2 -3 times, letting it idle for 30 minutes. As far as I understand, the magic of the Fusion Drive requires the system to be able to re-arrange data. I think this will continue to be valid in the future.

Now the boot time is 10 seconds with the Apple until the login prompt and then 19 seconds to the desktop. Filevault is on.