Are all DIY Fusion Drive Guides Wrong?

Discussion in 'Mac mini' started by Ridley, Nov 5, 2014.

  1. Ridley macrumors regular

    Mar 28, 2011
    I feel like there has to be a good explanation for this, and its why I'm posting here. I've been researching creating my own fusion drive in the mac mini. From around the internet the consensus seems to be the steps (terminal commands) are:

    diskutil list
    (see your disks and grab the disk identifiers)

    diskutil cs create "Fusion Drive" DiskIdentifier1 DiskIdentifier2
    (bond the two drives together)

    diskutil coreStorage createVolume SerialCode jfhs+ "Name of Fusion Drive" 100%
    (format 100% of the drive as HFS+)

    I question whether its right to say "100%"

    The reason being, according the wikipedia (and not completely confirmed) the fusion drive SSD maintains 4 GB of free space.

    "Part of the SSD space is used as a write buffer for incoming writes.[5][6] In the stable state, a minimum 4GB space is reserved for buffering writes.[2][5][6] A small spare area is set aside on the SSD for performance consistency."

    So it seems like if you use up 100% you aren't setting aside this incoming write space? Does this explain why some people claim to stop seeing a benefit from DIY fusion drive once their SSD fills up? Or am I mistaken? Thanks
  2. KrisLord macrumors 65816

    Sep 12, 2008
    Northumberland, UK
    Partitioning the drive (or creating volumes) doesn't dictate what's stored on it or how full individual drives get. If you set it at 96% then that 4% of it will be unavailable to the file system.
  3. Ridley thread starter macrumors regular

    Mar 28, 2011
    Ahh, ok that makes sense. Thanks for the response. If that 4 GB buffer exists I wonder if there is a way to manually change that around. From your experience do DIY fusion drives still do the same level of migration and promotion to and from the SSD? And in the setup does it matter in what order you list the volumes when bonding them together? Thanks
  4. KrisLord macrumors 65816

    Sep 12, 2008
    Northumberland, UK
    I've not seen any method mentioned of changing the buffer. It's a reasonable size and once it's partially filled OS X will move stuff to the mechanical disc to restore the buffer to the full size. As far as I can tell my DIY fusion drive works as Apple suggest a retail drive would have done.

    One thing worth mentioning is you can have 1 partition outside of the fusion setup. So I have about 50GB as a 2nd partition on the mechanical drive. I used this for dual booting between Mavericks and Yosemite when it was still in beta phase.

    Doubt drive order makes any difference.
  5. Ridley thread starter macrumors regular

    Mar 28, 2011
    That is a cool setup! I'll try and close down the thread, I think you answered it really well. Thanks
  6. gixxerfool macrumors 6502a


    Jun 7, 2008
    Sorry to jump in here, am I understanding correct the SSD doesnt need to occupy the primary slot? Thanks!

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