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Cover image for Why Fusion Drives & Windows Don't Mix

Why Fusion Drives & Windows Don't Mix

yellowbrickcode profile image Sarah Williams Originally published at yellowbrickcode.co.uk ・3 min read

My machine at work is a late 2015 model iMac, bought in August 2016. It's got a decent spec. The main bits, 3.3GHz i7 processor, 16GB RAM and 1TB Fusion Drive. That should definitely cover my requirements for development work, shouldn't it?

I spend 99.9% of my time in Windows. Due to my machine having 16GB RAM (which is what I want Windows to have to help out with memory hungry apps like Chrome, Slack and Visual Studio), it's an easy decision to install Windows in a Bootcamp partition rather than on a VM where I would have to share resources with OSX.

Fairly quickly after getting my iMac set up, I noticed that it was a bit slow in Windows. For quite a while I just put this down to me being spoiled by the SSD in my Macbook Pro that I use at home. As the months have gone on and the requirements for my work have gotten more intense, it's started to get to the point where I often find it unusable. I tend not to shut my machine down, instead just putting it to sleep. After a week of this, it takes around 25 minutes to close everything, reboot and open everything back up again. Even it it's only been on for one day, it can still take 12 to 20 minutes.

I booted into OSX recently for the first time in around 11 months and noticed it was running much quicker, close to the performance of OSX on my own Macbook Pro. I decided it was time to research and finally I found the reason why I regularly want to throw my machine out of the window I sit next to.

When you install Windows on a Bootcamp partition on a Fusion Drive, Boot Camp Assistant creates the Windows partition on the disk drive, not the SSD.

And here it is straight from Apple - https://support.apple.com/en-gb/HT201456 - see the 8th FAQ down (at the time of writing).

In a way, I'm happy because I at least know that it's not just me being impatient! I'm also a bit annoyed with myself though. Firstly for not doing any research into how exactly a Fusion Drive works before we went into the Apple Store to buy the machine, and also for not just registering that the Fusion Drive is an Apple concept so probably doesn't work elsewhere.

So hopefully someone finds this because they're doing what I should have done, research!

My personal opinion, if you're thinking of installing Windows in a Bootcamp partition on a Fusion Drive for full time dev work (or anything moderately intensive), don't do it. Go for the SSD. It runs like a dream on my Macbook Pro! Alternatively, if you really want to/don't have a choice and go for the Fusion Drive, the new model 21" iMac (and the 27" iMac) can have up to 32GB RAM now. I have tried Windows in a Parallels VM on the Fusion Drive and it does run better than it does in Bootcamp. So if you do go for a Fusion Drive, also go for 32GB of RAM and run Windows in a VM rather that on a Bootcamp partition.

This was originally posted on my blog in June 2017

Discussion

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Midpoint Nerd

You can use diskutil cs in the Mac's command line to break (or later rejoin) the Fusion core storage volume and use the SSD however you like.

Read the man page for diskutil for all the details.

Doing so likely puts both your existing installations at risk of deletion, always make backups.

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Sarah Williams Author

That's useful to know, thanks!

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martyh1

I know this is an old thread. I've been struggling with what I'm about 99% sure is a failed SSD drive in the fusion drive setup (did lots of debugging using diskutil both cmd line and gui, recovery, etc). Have spent lots of time trying to get around it. I use Window about 99% of the time also. With the SSD failed, I saved the Bootcamp partition but nothing from the Mac OS partition. Plan was to somehow use that img again. Preference is to not have to open my iMac but it's looking more like I'm going to have to (if I want bootcamp). I'm at the point where I can't even load a new bootcamp on a fresh Mojave installation. The mojave installation didn't seem to care about the failed SSD. It just installed to the HDD. Bootcamp/Windows goes through most of installation but then won't boot after it installs. It bluescreens with an AppleSSD.sys failure. Go figure, it somehow is trying to use the failed SSD. Ugh. Will NEVER go the route of a fusion drive again. I'm still not able to get bootcamp working even from all fresh installs at this point. Supposedly you can install bootcamp on an external drive, so I might try that next but I'm fully expecting it to STILL complain about the SSD for whatever reason.

In case anyone is interested, this late 2015 27in iMac uses a 24GB SSD for the fusion drive setup (1TB model). If you want REALLY big speed increases, you can not only switch out the internal SATA drive for an SSD, you can also switch out the PCIe SSD drive, and that drive can run much faster than the SATA 3 since it uses the PCIe interface. I might actually do that, but, again, am not looking forward to the difficulty in taking the iMac apart (I've watched some youtube videos).

update: I was able to install and boot the Mac OS from an external drive. But the real solution came when I simply renamed the applessd.sys driver that Windows (in bootcamp) was complaining about (and the backup of that file also). It wasn't exactly simple, I guess, since it's an NTFS partition. Had to go through a bit to mount it read/write on Mac. But I save my bootcamp partition and was booting again on windows with a fresh Mojave installation. Too bad I couldn't save the Mac OS partition though. Lost quite a bit there.

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Ben Halpern

Does Apple say anything about why it creates the Windows partition on the disk drive, not the SSD?

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Sarah Williams Author

I haven't found any official reasons, but from my research a 1TB Fusion Drive is actually just a 1TB hard drive plus 24GB of SSD. I would guess that it's just down to the SSD being so small.

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Ben Halpern

Ah, that makes sense.

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Perry Donham

Great observation, Sarah. I have to ask, if you spend 99.9% of your time at work in Windows, why did you opt for a Mac?

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Sarah Williams Author

Very good question Perry :) It was a company decision really, for 2 reasons. 1 - the iMac's all look nice on the desks, and 2 - they hold their resale value better than PC's.

For my own machine at home, I went with the Macbook Pro because I found they last longer and again they hold their value better.

I personally would choose a PC at work normally though as then I could use Docker too. That's another thing that doesn't work with Windows in Bootcamp unfortunately!