Demystifying Apple's "Fusion Drive" Technology

One of the upgrades Apple made to its recently refreshed iMac line is the inclusion of Fusion Drive technology. As with hybrid solid state drives and Intel's SRT (Smart Response Technology), Fusion Drive leverages the performance of SSD NAND flash memory with the capacity of a mechanical hard drive to offer the best of both worlds. However, it turns out Fusion Drive is a different kind of technology, and one that Mac users can build on their own.

Let's first take a look at Apple's claims. According to Apple, Fusion Drive dramatically improves performance (compared to mechanical hard drives) during disk intensive tasks, everything from booting up to launching apps and importing photos.

Fusion Drive Benchmarks
Source: Apple

By moving frequently used items over to flash storage and less frequently used data over to the hard drive, Apple's Fusion Drive technology promises up to 3.5 times better performance in certain situations, including file copies. Fusion Drive reacts to your computing behavior, and adjusts itself accordingly. So, what's different about this technology compared to Intel's SRT?

The answer to that question came courtesy of blogger and Tumblr user Jollyjinx, who set out to see if it's possible to use a Fusion Drive on older Macs with an SSD and HDD. To answer that question, he had to make his own Fusion Drive, and that's what he did.


Fusion Drives are recognized in Mac systems as single volumes, and so that's what Jollyjinx created using a 120GB OCZ Vertex 2 SSD, USB 750GB HDD, and a command line. With his two drives now recognized as a single volume, he began reading and writing files and monitoring throughput performance using iostat.

The moment the user stopped moving files around and the system entered an idle state, "the system began shoveling data from the SSD to the HDD," about 14GB worth. Likewise, frequently accessed data was transferred to the SSD. After some time elapsed, the specific data he was testing began to show SSD-like performance, whereas previously it performed as pokey as a USB-attached HDD.

Intel SRT

This isn't the way Intel's SRT works, which is designed for smaller SSDs up to 64GB (Fusion Drive can work with larger SSDs). Intel's SRT scheme works its mojo at the OS level and caches/copies files (or mirrors data, if you will), whereas Fusion Drive actually moves/relocates the data and is basically an automatic, tiered storage solution.

Perhaps it's possible that a future Mac OS X update could block users from building their own Fusion Drive storage, but for now, it's entirely possible to roll your own.