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OS X Mountain Lion confirmed to support Fusion Drive on legacy Macs

A developer on Wednesday revealed that the Fusion Drive support built into OS X Mountain Lion is compatible with a makeshift hybrid storage device, proving the technology can be used with Macs which didn't come preconfigured to take advantage of the feature.

Fusion Drive

In a post to Tumblr, developer Patrick Stein unofficially confirmed that Mountain Lion provides Fusion Drive support for current Macs, as he was able to "build" a hybrid drive compatible with Apple's new storage technology.

When Apple announced its new Fusion Drive hybrid storage solution alongside the redesigned iMac earlier this month, it noted that OS X Mountain Lion would be able to operate the system without the need to update. This prompted some to question whether existing hybrid drives would work with current Macs not configured with Apple SSD and HDD combination, as the statement suggested support for the technology was already embedded in the operating system's code.

Using the Terminal version of disk utility, Stein was able to setup a solid state drive and a separate hard drive so that OS X recognized both as a single logical volume, which is basically what Fusion Drive promises. Just as Apple's Fusion Drive configures an SSD and an HDD into a single volume, Stein used an SATA-connected 128GB SSD and USB-attached 750GB HDD in his working solution.

In order to build a single logical volume, the developer used Core Storage, the OS X feature that links two separate storage units into one volume group, to join the SDD and HDD. Next, Stein created a 466GB HFS+ volume, otherwise known as Mac OS Extended, to facilitate file creation and transfer.

"Now in DiskUtility the individual disks no longer show, but the Logical Volume (LV) shows as one disk," Stein said. "Part One is finished, we’ve created a single Volume consisting of a SSD and a HDD."

To test whether the jury-rigged setup was being operated as a Fusion Drive, Stein created a number of directories with files equating to 140GB of data by using the mkdir and mkfile commands. During the process, data was funneled to the SSD until the 120GB mark, at which point the remaining files were written to the HDD in directories 11 to 13.


Point at which files begin to be written to HDD. | Source: Patrick Stein (jollyjinx) via Tumblr

Next, Stein used the dd command to force reads of the data located on the HDD, an activity that Fusion Drive uses to determine which files are heavily accessed and should thus be relocated to the faster SSD. After stopping the read process, Stein used iostat to monitor whether any files were transferred to the SSD.

Immediately following the dd process, the system began dumping data from the SSD to the HDD, stopping after about 14GB of copying. Stein then attempted to readout data from directories 11 to 13, and at first found they were still located on the HDD. However, after about an hour of reads the files were being accessed from the SSD, meaning Fusion Drive had transferred the data successfully.

HDD Access

First reads show no HDD to SSD file transfers (top). After an hour the SDD is being accessed (bottom),
confirming that Fusion Drive is active.

While the informal test does prove that Fusion Drive is active and usable on older Macs, the process of configuring the hybrid storage devices is definitely not plug-and-play. It remains to be seen if Apple will offer Mac owners an easy way to configure their own components without having to run Terminal and command line code.