Very slow boot on Fusion and Big Sur

Finally, having spoken to people at Apple Support (here in Europe) who care and listened carefully at what I was saying in order to help them understand the problem, a Senior Apple engineer took the trouble to inform himself about what was going on with Big Sur and Fusion Drive iMacs, called me back, confirmed that there definitely is a problem that has nothing to do with hardware issues nor with any software that would not behave nicely, but indeed with the combination of Big Sur and Fusion Drives. 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 before any app becomes responsive, as described by so many users here and on the Developers forum.

What I am publishing here underneath, is what this Senior Apple engineer told me himself on the phone:

The solution: Wait for an update of Big Sur that will fix the problem. Don't waste time trying to run software that checks if all is OK. Don't waste time re-installing Big Sur. Don't waste time installing Big Sur from scratch (clean install). Nothing that has been tried to fix it in the last four months will fix it. It will have to be fixed from inside Big Sur with an update. Find and read the release notes of every update coming out from now to see if the problem was dealt with.

However: Do know that there is no guarantee that the problem can be fixed!...

This is the Apple response (finally) to our big problem.

I will post this in some other threads that deal with the same problem, as soon as I can.

Encouraged by the good results of reinstalling Big Sur 11.3 over my existing installation via an external drive, I decided to bite the bullet and do a complete clean install of 11.3:

The results are simply amazing:

Boot to login: 1min —> 20s

Login to desktop: 3min 20s —> 10s

Launching Outlook: 1 min 40s --> 20s

Launching stock apps like Finder, Safari and the App Store are nearly instantaneous, within one bounce.

Note that I did a clean install before with version 11.1 and that did not make any improvements. But now with version 11.3 it clearly has. I will now leave my machine overnight and check back tomorrow if things keep humming along.

For those interested, this is what I did:

- backed up my data and documents

- reboot in Recovery Mode (Command+R) and erase all internal partitions / volumes  with Disk Utility

- launch Terminal (from Utilities menu) and use the “diskutil resetFusion” command to rebuild my Fusion Disk

- install macOS Big Sur (it took a very long time)

- did not restore any Time Machine backups

- login and gave it some time to download iCloud stuff

- rebooted to verify performance

- installed some small productivity apps like Things, Agenda, Paste, …

- waited for Spotlight indexing to finish

- rebooted to verify performance

- connected my external USB disk and speakers

- waited for Spotlight indexing to finish

- rebooted to verify performance

- installed MS Office without launching it (it took a very long time)

- rebooted to verify performance

- launched and configured MS Office and OneDrive

- rebooted to verify performance

So far, I'm still at a responsive desktop in 30s (down from 4min20s). Fingers crossed.

At present this seems to be the best solution (boot from an external SSD drive). I purchased an external USB 3.0 case (about € 10) and I put a 512 GB SATA SSD (about 65 €) into it. Now my iMac is very very fast. Only some of remarks:

1. I tried 5 different cases and some of them were slow or unstable, with frequent crashes. If you want to boot from an external drive, you need a case that insures the maximum stability.
2. The fastest external cases are based on Thunderbolt, but they are quite expensive. With a good USB 3.0 case and a cheap SSD you can achieve more than 300 MB/s write and about 400 MB/s read that is good enough for a lot of purposes
3. If your iMac supports only USB 3.0 it's not useful to buy a NVME case with a high speed SSD: a SATA case is enough to saturate the USB 3.0 bandwidth.

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.

1. Backup to TimeMachine.

2. Reset Fusion Drive and reinstall Big Sur.

　See the following link

https://support.apple.com/en-us/HT207584

3. Restore data from TimeMachine with Migration Assistant tool.

OS startup time went from over 3 minutes to about the same as Catalina's.

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.

To rule out any software contribution to the increased boot time download and run Etrecheck. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Copy the report

and use the Additional Text button to include the report in your reply.

Then we can examine the report and see if we can determine what might be causing the problem.

For me it's crystal clear now that everything related to disk read/writes are the main issue (before reinstalling). For instance, I kept some files on the Desktop (no more than 15) and that was the part where the boot up process took over a minute to get those files to show up (even the thumbnail in the Download stack in the Dock remained blank for a minute), launching Finder windows took forever, copying/moving small files took way too long, etc. I guess that's also the reason why booting and launching apps was so slow.

And now, after reinstalling from scratch, it's all instantaneous. I kept my iMac on overnight, and performance is still the same. So Big Sur hasn't started reorganising files on/off the SSD blade of the Fusion Drive. All stock apps open instantaneous. My biggest apps (Office) are clearly on the slow hard disk part, but they still launch within 9-17 seconds, which is perfectly acceptable for these big apps on an old machine. Adding OneDrive and Paste as items that launch at startup only added a few seconds, the way it should be.

To reply to your questions:

1) I did not: I terms of big apps I had MS Office, iWorks, Pixelmator Pro and Xcode.

2) I only reinstalled MS Office so far (but these apps are also approaching 5GB)

3) My Music and Photos libraries are huge (200GB) but I keep them in the cloud with the options to "Optimise Mac Storage" enabled.

4) It's a 1TB with 24Gb SSD blade: it still has 960GB free (as most things are still in the cloud now, even OneDrive does not sync anything I haven't used yet). So I'm well over my SDD capacity.

The big question remains: when I install more apps and download more data, will Big Sur all of a sudden swap crucial OS files away from the SSD part or not. Only time will tell.

From my experiment I can almost certainly conclude that reinstalling from scratch is the only (possible) solution (if something did in fact change in the way 11.3 operates, which we still not know for 100% sure). Restoring your apps and data from a Time Machine backup could potentially be fine as well as the important thing is that the OS is kept on the SSD blade, but I'm not going to try that scenario ;-)

My SSD disk is disk0 and the HD is disk1. If you want to have more control in the creation of the Fusion Drive, after separating it with the command:

diskutil apfs deleteContainer <identifier>

and after formatting the two distinct disks, you can run the command:

diskutil apfs createContainer -main <ssd identifier> -secondary <hard drive identifier>

diskutil apfs addVolume <container identifier> APFS "Volume Name"

As ssd and hard drive identifiers you should use the identifiers of the free partitions (disk0s2 and disk1s2), not the one of the disks (disk0 and disk1).

A wrong identification of the SSD can be the cause of the low speed of your system.

BTW: there is a simple way to determine if the low speed is determined by the HW configuration or by your profile: after resetting the fusion drive and reinstalling the OS, you can try to boot the clean installed system (3 times and letting the time for the optimization).

I have now performed a clean install of Big Sur 11.3, first wiping my Fusion Drive, resetting it, installing 11.3 from a bootable thumb drive, and finally creating a new user account whilst leaving it empty. I rebooted the system a few times, each time leaving it idle for about 30 minutes. Boot times were totally normal and well under 30 seconds, from Apple logo to responsive desktop and apps.

Then I used Migration Assistant to copy over everything from my old account. This increased the boot time again, but instead of more than 3 minutes on previous installations of Big Sur (11.1 and 11.2), my 1TB Fusion Drive initially started up from Apple logo to responsive desktop and apps in around 2 minutes. An improvement, but not nearly good enough to call it OK.

After the first boot after the migration of my old user account, I allowed the system to run idle, for around 40 minutes. Then I rebooted and the boot time was noticeably better again (around 1m30), but still nowhere near to 30 seconds or less. I have let the system run idle for several hours yesterday evening, hoping that future boot times will improve even more.

Once the system is booted up, every single app starts very fast and runs very smoothly. For now, I do not intend to downgrade back (once again...) to Catalina. Neither do I have the time right now to install and fine-tune every app I have and need for my daily bread as a photographer, from scratch. I'm pretty sure now that doing a clean install of both Big Sur 11.3 and all the apps would give me totally normal boot times. Maybe one day I will have the time to do that. For now, I will let everything be and see if my system doesn't return to normal boot times by itself, after giving it some days or a week of normal use to figure out what part of the OS to put where (either SSD blade or HDD part of the Fusion Drive) to achieve normal boot times. I will report back when I have more news. For now, I thank all of you who have had the patience and the intelligence to approach this problem with logic, a lot of testing, and some very precise observations of what happens at what time and in what circumstances. The problem of the very slow boot times, however, is not solved, as they still exist when simply upgrading from a system running Mojave or Catalina to Big Sur (even in its current version of 11.3). We have to do a totally clean install of both the OS and all the apps we have, to be sure our Fusion Drive iMacs are able to start up and be responsive in less than 30 seconds. It's not perfect, but nothing and nobody is, and Apple engineers can be helped helping us by giving them feedback that is as precise as possible. Not here, but on the Apple feedback page.

I've given up on Big Sur until there's absolute certainty that a new release addresses Fusion Drive issues.

I've wasted weeks of my life dealing with this. Things would slowly devolve to being spinning balls waiting on anything to happen, lag on clicking on items in Finder or Apps, slow bus speeds of copying files, long boot times, and finder/files taking even longer to show up.

I've done many of the aforementioned processes. Reinstalled Big Sur over the existing, and it would work better for a bit, but then back to issues a day or two later.

While I had a backup of my computer already, I was concerned about a couple things (including a VMWare Virtual Machine) that may not have been backed up fully or as recent as I needed it. The ability to backup from this troubled Fusion drive to a USB, a WebDav, or a Thunderbolt Target Drive to another Mac was moving at a snails pace... it would have copied a few gigs in days!

I finally decided to just format the hard drive/resetFusion, and install Big Sur 11.3 clean from an external drive. Did not install many apps for a few days, and just used it. Let it sit on Desktop several times and reboot like it's been suggested too. Install a couple nominal apps and moved files from backup (not any system/library files at all). It all came right back eventually.

What is obvious is that something in Big Sur is affecting the Fusion partitions such that even acting as a Target Drive (not running OS, but simply a connected drive), is a problem to use it. Combinations of Safe Mode, Recovery mode, reinstall OS over existing, will at least make it usable for a while so you can pull files off it.

I formatted again, clean installed Catalina, including all my files and apps. I have NONE of these issues with Catalina. Days now... quick boot, no spinners, all is good.

I can't keep wasting my time on this and will stick with Catalina until I learn something new about a release that Apple acknowledges.

iMac (Retina 5K, 27-inch, 2017)

3.8 GHz Quad-Core Intel Core i5

40 GB Ram

Radeon Pro 580 8 GB

1TB Fusion

If you wanted performance, you would have purchased the iMac with an SSD, not that slow hybrid, rotational Fusion drive. If you were to perform a clean install of Big Sur on an external SSD, and change that to your default startup drive, you would see what I mean about the performance improvement. Once good source of SSD is OWC, or Crucial, both frequently recommended here.

Are you sure that the blade SSD of your 1TB Fusion Drive is only 24GB? I have a 2019 27inch iMac 5K with 1TB Fusion Drive and my blade (SSD) is 28 (not 24) GB. However, I do have the same problem: a boot time of around 2,5 to 3 minutes (from chime to the moment apps becomes responsive).

My guess is that the OP (jimdem582) has a larger SSD blade: 32 GB, as that would be the default size for it on a 2017 iMac 27inch with 1TB Fusion Drive.

Could you please check the size of your SSD blade again?

As jimdem582 reported: He managed to solve the problem with a clean install of Big Sur and a re-install (from scratch) of the user account(s), the apps, and copying back the data from a (non-Time Machine) backup. However, it could be (educated guess) that his 32 GB is just large enough to manage everything and anything that is needed to boot up normally and fast enough, but that a 28 GB (or 24 in your case?) one is not...