On October 23, 2012, Apple Inc. announced its new storage product called Fusion Drive that combines a small solid state drive (SSD) and a large hard drive. This integrated storage is managed by Apple's operating system, Mac OS X Mountain Lion, in a single logical space. This product can significantly accelerate data accesses in a cost-effective way for widely used Apple products, including iMac and Mac Mini.
The product's untold story is its close relationship with a research project conducted at The Ohio State University by former CSE PhD student Feng Chen (pictured above left) and his advisor Xiaodong Zhang (pictured above right), in collaboration with Intel Labs research scientist David Koufaty. The three researchers published and presented a paper entitled "Hystor: Making the Best Use of Solid State Drives In High Performance Storage Systems" in the 25th ACM International Conference on Supercomputing (ICS 2011) in May 2011. This work presents a hybrid storage system framework called Hystor with a small SSD and a large hard drive. The high performance and cost-effectiveness of the Hystor framework comes from three basic system components. First, instead of using the SSD as a hard drive cache, Hystor logically merges the SSD and the hard disk into a single block device managed by the operating system. Second, Hystor is driven by a set of algorithms that decide in which device (SSD or hard drive) the data should be stored and accessed. Finally, to provide sustained data processing performance, Hystor adaptively and timely migrates and retains data in the most suitable devices for users by storing smaller, more frequently accessed data in the SSD drive and larger, less used data in the hard drive.
The Hystor paper received the Best Paper Award in ICS 2011. Following the paper's publication, the Apple Fusion Drive group had detailed discussions with the authors of the paper. A senior software engineer of Apple made the following comment on the Hystor paper:
Hystor is a well-designed system, and its paper discussed several key systems trade-offs in details. The Apple software engineers had carefully and systematically evaluated Hystor. This work had a significant influence in the design of Apple's Fusion Drive. Some design elements and algorithms in Hystor have been directly used in Apple's Fusion Drive.
After receiving the above comment from Apple, Xiaodong Zhang, CSE Chair and Robert M Chritchfield Professor in Engineering expressed his excitement about the far-reaching effects of the Hystor research.
"I am glad to see another basic research project of ours has made a strong impact in advancing the general-purpose computing systems. In this case, millions of consumers can benefit from the fast data accesses of Apple's Fusion Drive without purchasing an expensive full SSD drive," Zhang said.
Feng Chen completed his PhD in CSE at Ohio State in 2010, where he received a Graduate Research Award. He joined the Intel Labs as a research scientist after his graduation.