DIY Fusion RAM disk on a MacBook Pro

DIY Fusion RAM disk on a MacBook Pro

Summary: Back in the earliest days of Macintosh when storage meant floppy diskettes, RAM disks were all the rage. Now, a clever tip works a combo of new CoreStorage commands and Sparse Disk Images to create Fusion Drive RAM disk.


There's plenty of interest in the CoreStorage Fusion Drive technology used in Apple's recent MacBook Pro models. A number of blog posts have offered details on do-it-yourself Fusion Drives. As I've mentioned in previous posts, these projects aren't for the faint of heart when it comes to potential reliability. But some swear by them.

Here's a novel take in a blog post by Michael Logozar, who works by day building out large-scale messaging platforms for ISPs — meaning he's had some experience with tiered storage. He told me he was "intrigued" when he heard about the Fusion Drive and thought it could be applied to his music composition setup.

His brainchild is to combine the CoreStorage commands with Apple's built-in Disk Image virtualized storage containers.

This was all too risky for me, so I decided to try take it a step further. Borrowing a page from other virtualization technologies, I decided to try building a Fusion Drive out of disk images instead with the intent of benefiting from the speed of my internal SSD without having to rebuild my whole machine.

I did some re-arranging of my data to free up 128GB of space on my primary internal SSD and about 800GB of space on my internal HD and proceeded to build a Fusion Drive *without* reformatting my hard drives.

The post offers step-by-step instructions. However, there are steps where it's possible to make mistakes — serious ones.

Be very careful here to specify the correct disk identifiers or you could overwrite your main disks and cause severe data loss.

In the above example, it would be "disk5" and "disk6". Ideally you could run a "diskutil list" before attaching your disk images and one after to compare the difference to be certain.

Check Out: More on DIY Fusion Drives 

Topics: Apple, Laptops, Operating Systems, Storage

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  • all is nice, but

    Where is the "RAM" bit?

    It is no secret, that you can build an Fusion Drive from any SSD/HDD pieces, it is how things are supposed to work -- so what this guy did was nothing spectacular. In fact, the best thing he could do was to use the entire SSD and the entire HDD to make his Fusion Drive, but.. he probably wanted the "I did it myself" feeling.

    What would be interesting for Mac users is the ability to create arbitrary CoreStorage arrays, that if you add an SSD or two will "magically" behave like an Fusion Drive. Most people do not realize that OS X is just normal UNIX and you can create pretty complicated storage systems.

    My personal "quest" is to find an cheap two or more drive external "JBOD" enclosure with Thunderbolt interface that will let me have an external Fusion Drive for my MacBook.
    • iMac

      @danbi, Michael here -- the point of this exercise was I was using an iMac which already had an SSD+HD but data on it I did not want to destroy to play with Fusion Drive... and previous attempts at external thunderbolt enclosures had disappointing results. i.e. connecting at SATA 1 speeds instead of SATA 2 or 3.
      Michael Logozar