Looks like no one’s replied in a while. To start the conversation again, simply ask a new question.

Question:

Question: 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

Reply
Question marked as Helpful

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

Question marked as Helpful

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

There’s more to the conversation

Read all replies

Page content loaded

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

User uploaded file


However, Disk Utility does:

User uploaded file


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

Mar 14, 2018 4:09 PM

Reply Helpful

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

User uploaded file

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.

User uploaded file


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!

Mar 14, 2018 7:53 PM

Reply Helpful

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

Mar 14, 2018 9:48 PM

Reply Helpful

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

Mar 15, 2018 1:36 AM

Reply Helpful

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

Mar 15, 2018 1:38 AM

Reply Helpful

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

Mar 16, 2018 5:06 AM

Reply Helpful
User profile for user: Portico

Question: How to set up a dual SSD Fusion Drive?