How to set up a dual SSD Fusion Drive?

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5 points

How to set up a dual SSD Fusion Drive?

I own a 27-inch late 2012 iMac. The HDD in my 3 TB Fusion Drive has died, so I replaced it with a 2 TB SATA-SSD. Since the 128 GB M.2 Apple SSD is probably still a lot faster than the new SSD, I wanted the two drives to operate in "fusion mode" once again, rather than have two separate volumes.

Using diskutil, I unmounted the Fusion Drive, erased each of the physical disks, created a new CoreStorage Fusion Group (diskutil cs create "Macintosh HD" disk0 disk1) and then a CoreStorage Fusion volume (diskutil cs createVolume "Macintosh HD" JHFS+ "Macintosh HD" 100%). I then installed macOS High Sierra with no issues and migrated my data from a Time Machine backup. Everything appeared to be just fine until I arrived at installing Windows: Boot Camp would only let me use 110 GB for the new partition despite one TB of free disk space.

Another look at the drive configuration revealed that something has gone terribly wrong after all:

/dev/disk0 (internal, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *121.3 GB disk0

1: EFI EFI 209.7 MB disk0s1

2: Apple_CoreStorage Macintosh HD 24.5 GB disk0s2

3: Apple_Boot Recovery HD 650.0 MB disk0s3

4: Microsoft Basic Data BOOTCAMP 96.0 GB disk0s4

/dev/disk1 (internal, physical):

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *2.0 TB disk1

1: EFI EFI 209.7 MB disk1s1

2: Apple_CoreStorage Macintosh HD 2.0 TB disk1s2

3: Apple_Boot Boot OS X 134.2 MB disk1s3

/dev/disk2 (internal, virtual):

#: TYPE NAME SIZE IDENTIFIER

0: Apple_HFS Macintosh HD +2.0 TB disk2

Logical Volume on disk0s2, disk1s2

EAC5E1EA-BD4E-4295-992D-807629B3BECC

Unencrypted

The Boot Camp partition has been created on the Apple SSD while the SATA SSD appears to be the boot volume! Windows located on the Apple SSD is particularly unexpected since with the original configuration, Boot Camp could only access the HDD portion of the Fusion Drive.

Starting over from scratch, how can I create a Fusion Drive with the two SSDs that behaves like it is supposed to?

macOS and frequently used files are stored on (or dynamically moved to) the Apple SSD.
All other files and the Boot Camp partition go to the SATA SSD (just like they did before with the HDD).

Thank you!

iMac, macOS High Sierra (10.13.3)

Posted on Mar 14, 2018 3:44 PM

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Answer 1

Portico Author

Level 1

5 points

Mar 14, 2018 4:09 PM in response to dialabrain

Thanks Kappy and dialabrain!

Now I'm even more confused ...

About this Mac does NOT label my storage configuration as a "Fusion Drive":

However, Disk Utility does:

Also, I have followed the exact same steps to create my Fusion Drive as described in the article.

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Answer 2

Portico Author

Level 1

5 points

Mar 14, 2018 7:53 PM in response to dialabrain

Okay, I have once again fused the two SSD drives, this time exactly according to the instructions you gave me. Reinstalled High Sierra afterwards. However, "About this Mac" STILL doesn't label my volume as a Fusion Drive (like it is supposed to according to the fusing guide):

I really don't know what I do wrong.

On the bright side, Boot Camp does now seem to place the Windows partition on the SATA-SSD. While its size was limited to about 100 GB before, I now could (but won't) make it 1.9 TB.

Does the Fusion Drive label REALLY need to appear in "About my Mac", or does this maybe require an SSD/HDD combo? To anybody with a working dual SSD Fusion Drive configuration: What does "About my Mac" tell you about your drives. Is there a Fusion label or not? Do you see one or two volumes?

Thank you!

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Answer 3

Mar 14, 2018 9:48 PM in response to Portico

Portico wrote:

...the 128 GB M.2 Apple SSD is probably still a lot faster than the new SSD, I wanted the two drives to operate in "fusion mode" once again, rather than have two separate volumes....

Did you run any benchmark drive speed tests? I would think the two SSDs are about the same speed. The Fusion drive set up is optimized for one small faster drive to be combined with a larger slower drive. If you have equally fast small and large drives I don't quite understand why you would Fusion them together.

You could use a software RAID JBOD, where the drives don't have to be the same size, it just logically groups a bunch of drives together to appear as a single drive. Note: if you try to "stripe" RAID0 two drives of different size, you don't get the full capacity of both of the drives, it will only be twice the capacity of the smaller drive.

I think the Fusion scheme of moving data from the smaller fast drive to the larger "slower" drive in the background is just going to add unnecessary drive activity in the background and slow you down as compared to a RAID JBOD, where the data stays put, but you can read/write from both drives simultaneously. I have no personal experience with Fusion drives and do not understand them in anyway whatsoever, sounds like a gimmick to me.

And, with either Fusion or RAID type drive combinations, you risk losing all data from both drives when one of them fails, so back them up twice. Because the backup drive will no doubt fail when you are halfway through the restore process, and then that second backup will come in real handy. That should occur in about 2 years and 11 months from now since you bought a drive with a 3 year warranty. :-)

Consider the RAID JBOD option instead of the Fusion option:

https://www.lifewire.com/use-disk-utility-to-create-a-jbod-raid-array-2260916

Create a disk set using Disk Utility on Mac - Apple Support

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Answer 4

Portico Author

Level 1

5 points

Mar 15, 2018 1:36 AM in response to Glen Doggett

Thanks Glen. I‘ve never checked myself but seen benchmarks where M.2 SSDs were twice or up to three times as fast as SATA SSDs. That said, I don‘t know if those M.2 benchmarks belonged to NVMe-based devices and if my 2012 iMac uses a SATA or PCIe connection, so I may be wrong after all. I’ll have to dig further into this, but for now, I‘d really like to get this dual SSD fusion thing to work properly.

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Answer 5

Portico Author

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5 points

Mar 15, 2018 1:38 AM in response to Kappy

Thank you, Kappy. My Disk Utility sidebar looks just like that. However, it has said „Fusion Drive" all along (see my first set of screenshots) even when my fused volume didn‘t behave like a properly configured Fusion Drive at all (putting the Boot Camp partition on the M.2 SSD and the like ...). Therefore, I‘d feel a lot better if „About this Mac“ would confirm what Disk Utility implies.

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Answer 6

Portico Author

Level 1

5 points

Mar 16, 2018 5:06 AM in response to Kappy

Hm, it also says „Fusion Drive“ and only shows one logical volume. In my case, both physical drives are listed and there is no mention of a Fusion Drive. However, Disk Utility DOES show a fused single volume! I‘m still lost: Do I have a properly configured Fusion Drive (since I followed the instructions in the fusion guide you provided) or not? It would be great to know before migrating my data once again ...

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Answer 7

Kappy

Level 10

361,741 points

Mar 18, 2018 3:33 AM in response to Portico

Yes, I think you're good to go. Mine may say fusion drive because I did not use this article at the time. I did not know about it. I relied on the kind assistance of the resident expert on diskutil, Loner T. I originally named mine as Fusion Drive but later renamed it to Macintosh HD. That may be why you see that in my image.

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Answer 8

Mar 18, 2018 3:33 AM in response to Portico

Portico wrote:...I‘ve never checked myself but seen benchmarks where M.2 SSDs were twice or up to three times as fast as SATA SSDs....

were those based on the M.2 being connected to the on-board SATA3 (6.0Gb/s) interface in the 2012 iMac? I think the interface for your flash drive is the same speed as the internal drive bay, so even though an M.2 flash drive might be able to perform faster in a different machine with a faster interface, I don't think the iMac M.2 will perform any better than an SSD on the same speed interface built-in to the motherboard. I can't find detailed specs that show the flash drive interface spec is any different from the on-board SATA3 interface that the main hard drive is connected to, I could be totally wrong about that, if someone can post a link to those detailed specs that would be great.

I understand you prefer the simplicity of rebuilding the same Fusion drive you had before upgrading from an RPM drive to an SSD drive. But the Fusion drive scheme was not intended to join two of the same kind of flash drives together that have roughly the same speed. Seems like a lot of effort to create a drive setup that is probably not an optimal use of the drives that you now have. The Fusion drive you are building now will move your data around from one SSD to another for no reason, you will see no real boost in performance and actually decrease the performance and lifetime of the drives.

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Answer 9

Portico Author

Level 1

5 points

Mar 18, 2018 3:31 AM in response to Glen Doggett

Thanks Glen, I've dug a little deeper and found this OWC replacement Blade SSD for 2012 27-inch iMacs:

OWC DIY Kit: 1.0TB Aura 6G Solid-State Drive for... at MacSales.com

According to the specs, peak data rates are 560 MB/s read and 460 MB/s write. That is about real life 6G SATA III performance, so the internal SSD interface isn't PCIe after all (which I think is kind of disappointing). That means you have been right from the start: Fusing two SSDs probably isn't a good idea, at least not with my 2012 iMac. Newer Macs probably feature faster PCIe connected Blade SSDs, so fusing might have become a clever option by now.

Despite all that, I will hold on to my current configuration for now. It feels faster and more responsive than ever in both macOS and Windows 10 and with a little luck (???) macOS won't move frequently used files around anyway, since (according to diskutil) my fused volume isn't a fusion drive at all - which is why this whole thread exists in the first place.

Kappy, dialabrain, Glen, thank you so much for your time and support! I've really learned a lot.

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Answer 10

Mar 18, 2018 9:49 AM in response to Portico

I think you are right, the newer models may have the faster PCIe interface for the blade SSDs, I am not sure what year they switched to those. I have an older 2010 model and it has an SATA expansion port for an optional SSD, but it is the even slower SATA 3Gb/s spec than the one you have in your 2012 model.

I have played around with RAID before, (not in the iMac, in an old G4) and did see some improvement with striped RAID0, but just did not like the complexity and risk of data loss, I think the SATA SSDs are plenty fast, and the external ports you have for optional large backup and bulk data don't have to be as fast. I would rather move my own data back and forth when I know I don't need to use something frequently.

If you were to split the Fusion, would that be Fission? Might be a good idea to have the OS and apps, the basic macOS installation on the blade SSD, and just use the other large drive for personal files, maybe your whole personal home directory, for example. Some folks who have split their Fusion drives prefer to do that. Whatever works best for you, sounds like you are now better informed about the specs of your particular machine.

PS - a useful site with detailed specs from all Mac models, (though this particular tidbit of info was not really apparent),

http://www.everymac.com

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