More on Fusion Drive: How it works, and how to roll your own

ars technica by Lee Hutchinson

It's not Intel SRT, it's not file-based, and it works on OS X right now.

Two blog posts by Tumblr user Jollyjinx have shed some more light on the inner workings of Apple's Fusion Drive. Announced last week at Apple's event in San Jose, Fusion Drive marries a solid-state disk and a spinning hard disk drive together into a single volume, relying on the speed of the SSD to accelerate all writes and reads on the most often-used files and the size of the HDD to hold the much larger mass of less often-referenced files.

Based on Phil Schiller's remarks at the event, I speculated that Fusion Drive was a software-based, file-level automated tiering solution. A Fusion Drive-equipped Mac will come with a 128GB SSD and a much larger hard disk, from 1 to 3 terabytes. Floor reports from the event revealed that the two disks are visible as a single volume, with the total amount of space in the volume equal to the two drives' aggregated capacities. Schiller's comments indicated that Fusion Drive keeps track of what files and applications are being frequently read, physically moving (or "promoting," as it's commonly called in enterprise tiering solutions) those files and applications from the HDD to the faster SSD. At the same time, files and applications on the SSD which haven't been referenced in a while are moved back down ("demoted") to the HDD, to make room for more files to be promoted.

Many questions lingered, though, in the absence of any real technical info from Apple (and its Fusion Drive tech document provides very few hard details on the underlying functionality). Is Fusion Drive really a tiering technology, actually moving the data, or is it more of a caching solution? Does it rely on Intel's Smart Response Technology, which is available in Ivy Bridge chipsets like those in the new round of Fusion Drive-equipped Macs? Does it use the volume management features Apple introduced last year in Core Storage? Does it move whole files or just pieces of files? How does it keep track of what it's moving? Will it work on older Macs, or only newer Ivy Bridge Macs with Apple-provided SSDs? read more...