How can I tell if Fusion drive works properly?

Discussion in 'macOS' started by GregorMae, Nov 5, 2018.

  1. GregorMae macrumors newbie


    Nov 5, 2018

    Yesterday I bought the iMac 5K with 3TB Fusion drive. I've made two partitions - one for storage (2,5 TB) and one for the system (512 GB). Because I bought the iMac second hand I've clean installed the MojaveOS with the APFS file system. The other partition is Mac OS Extender (Journaled).

    Because of the "container" disk shown in the attached screenshot, that wasn't there before I've formatted the disk I'm now not sure if the Fusion drive is working properly.



    How can I check if everything is working optimally?

    Thanks in advance!

  2. chrfr macrumors 604

    Jul 11, 2009
    I would avoid partitioning the disk. There's no performance gain to be had by doing it, and you're likely to prevent the Fusion Drive from working as intended.
  3. GregorMae thread starter macrumors newbie


    Nov 5, 2018
    I don't understand why I wouldn't - I like to keep my data organised in this case on two separated partitions.
    • You can use Disk Utility to add a single macOS partition to the hard disk on Fusion Drive, and that partition will function as a separate volume, not as part of Fusion Drive. Disk Utility then dims the [​IMG] button to prevent additional partitions. If creating a Windows partition, use Boot Camp Assistant instead.
  4. Fishrrman macrumors P6


    Feb 20, 2009

    If you want to partition your drive, well... just do it.
    I do the same, and don't worry what others may think.

    Having said that, I remember a story about someone asking a German automobile engine designer about tachometers.
    The designer replied, "why do you need a tachometer? The engine is either running, or it isn't".

    Told you that to tell you this:
    If your newly-partitioned drive checks out ok using Disk Utility's "repair disk" feature, what else is there to know, really?
    The drive is either "running", or it isn't....! ;)
  5. Brian33 macrumors 6502a

    Apr 30, 2008
    USA (Virginia)
    You can verify that a Fusion drive is working properly by using Terminal commands. (At least with HFS+, I haven't tried with APFS-formatted volumes.)

    First you need to use "diskutil list" and figure out the system-assigned names for the physical drives that make up the "fusion drive"; one being the solid-state drive and one hard disk drive.

    Then you can use the Terminal command "iostat -d -w 1" to display the second-by-second disk activity. While that's going, copy or create a new file bigger than 4 GB. (On my system at least, the fusion set-up keeps about 4 GB of space free on the solid-state portion; we want to fill that up and then continue writing to the hdd.)

    During the file write, you should see lots of io activity, first on the solid-state device (until its "fusion buffer" is used up), and then on the associated hdd device.

    I've verified my own do-it-yourself Fusion drive this way.

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4 November 5, 2018