Basic Research in Hybrid Storage Systems Makes an Impact on Apple's Fusion Drive

Two Intel Labs researchers and one professor at the Ohio State University have designed and implemented 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 mainly 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 the 25th ACM International Conference on Supercomputing (ICS 2011) in May 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."

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Source: Ohio State University


Feng Chen, Intel Labs
David Koufaty, Intel Labs
Xiaodong Zhang, The Ohio State University