Fusion Drive is a data storage technology developed by Apple Inc. Announced as part of an Apple event held on October 23, 2012, the technology combines a hard drive with 128 GB of NAND flash storage (solid-state drive) and presents it as a single logical volume with the space of both drives combined. The operating system automatically manages the contents of the drive so the most frequently accessed files, applications, documents, photos and other data are stored on the faster flash storage, while infrequently used items move to or stay on the hard drive. For example, if spreadsheet software is used often, the software will be moved to the flash storage for faster user access. In software, this logical volume speeds up performance of the computer by performing both caching for faster writes and auto tiering for faster reads.
The Fusion Drive was initially announced to be available for those purchasing either late 2012 iMacs or Mac Minis, which are supported by OS X Mountain Lion, and were made available as a storage option on built-to-order (BTO) machines, in the following configurations:
- 128 GB flash storage + 1 TB HDD — Mac Mini (late 2012), and iMac (late 2012)
- 128 GB flash storage + 3 TB HDD — iMac (late 2012), 27" model only
System design insights 
|This section relies on references to primary sources. (April 2013)|
Apple's Fusion Drive is a proprietary software with limited information about its internal designs. It has been reported that the design of Fusion Drive has been influenced by a research project called Hystor. According to the paper, this hybrid storage system unifies a high-speed SSD and a large-capacity hard drive with several design considerations, which may be used in Fusion Drive.
- The SSD and the hard drive are logically merged into a single block device managed by the operating system, which is independent of file systems and requires no changes to applications.
- A portion of SSD space is used as a write-back buffer to absorb incoming write traffic, which hides perceivable latencies and boosts write performance.
- A set of algorithms and techniques are designed to decide in which device (the SSD or the hard drive) data should be stored and accessed. A simplified view of the rules is to store the smaller, more frequently accessed data in the SSD and the larger, less frequently used data in the hard drive.
- Data movement is based on runtime access patterns. During idle periods, data is adaptively migrated to the most suitable devices to provide sustained data processing performance for users.
- Fusion Drive is a block-level solution based on Apple's Core Storage, a logical volume manager managing multiple physical devices. The capacity of a Fusion Drive is confirmed to be the sum of two devices. Fusion Drive is file system agnostic and effective for both HFS Plus and ZFS.
- The SSD space is used as a write buffer for incoming writes. In the stable state, a minimum 4GB space is reserved for buffering writes. A small spare area is set aside on the SSD for performance consistency.
- Data is promoted to the SSD based on its access frequency. The frequency is detected at the block level  and below file system memory cache. Data migration happens in 128KB chunks during idle or light I/O periods.
- Operating system and other critical documents are always cached on the SSD. Applications are likely to be handled similarly. A regular file can reside on both devices.
See also 
- Hutchinson, Lee (October 23, 2012). "Apple Fusion Drive—wait, what? How does this work?". Ars Technica. Condé Nast. Retrieved October 25, 2012.
- Shimpi, Anand Lal (October 24, 2012). "Understanding Apple's Fusion Drive". AnandTech. Retrieved October 25, 2012.
- "Computer Science Research at Ohio State Makes Impact in Apple's Hybrid Storage Product". www.cse.ohio-state.edu. 2013-04-08.
- Feng Chen, David Koufaty, Xiaodong Zhang, "Hystor: making the best usage of Solid State Drives in high performance storage systems", Proceedings of 25th ACM International Conference on Supercomputing (ICS), 2011. http://www.cse.ohio-state.edu/hpcs/WWW/HTML/publications/abs11-6.html
- "Achieving fusion—with a service training doc, Ars tears open Apple’s Fusion Drive". www.arstechnica.com. 2012-11-05.
- "A Month with Apple's Fusion Drive". www.anandtech.com. 2013-01-18.
- "Fusion Drive - loose ends". jollyjinx.tumblr.com/. 2012-10-31.[unreliable source?]
- "More on BYO Fusion drive". jollyjinx.tumblr.com/. 2012-10-31.[unreliable source?]
- "Fusion Drive last words". jollyjinx.tumblr.com/. 2012-11-04.[unreliable source?]