Fusion Drive is a type of

hybrid drive In computing, a hybrid drive (solid state hybrid drive – SSHD) is a logical or physical storage device that combines a faster storage medium such as solid-state drive (SSD) with a higher-capacity hard disk drive (HDD). The intent is adding s ...

technology created by

Apple Inc. Apple Inc. is an American multinational technology company headquartered in Cupertino, California, United States. Apple is the largest technology company by revenue (totaling in 2021) and, as of June 2022, is the world's biggest company ...

It combines a

hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magne ...

with a

NAND flash Flash memory is an electronic non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for the NOR and NAND logic gates. Both use ...

storage (

solid-state drive A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data persistently, typically using flash memory, and functioning as secondary storage in the hierarchy of computer storage. It is a ...

of 24 GB or more) and presents it as a single Core Storage managed

logical volume In computer storage, logical volume management or LVM provides a method of allocating space on mass-storage devices that is more flexible than conventional partitioning schemes to store volumes. In particular, a volume manager can concatenate ...

with the space of both drives combined. The operating system automatically manages the contents of the drive so the most frequently accessed files are stored on the faster flash storage, while infrequently used items move to or stay on the hard drive. For example, if

spreadsheet A spreadsheet is a computer application for computation, organization, analysis and storage of data in tabular form. Spreadsheets were developed as computerized analogs of paper accounting worksheets. The program operates on data entered in c ...

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.

Availability

The Fusion Drive was announced as part of an Apple event held on October 23, 2012, with the first supporting products being two desktops: the

iMac iMac is a family of all-in-one Mac desktop computers designed and built by Apple Inc. It has been the primary part of Apple's consumer desktop offerings since its debut in August 1998, and has evolved through seven distinct forms. In it ...

and

Mac Mini Mac Mini (stylized as Mac mini) is a small form factor desktop computer developed and marketed by Apple Inc. , it is positioned between the consumer all-in-one iMac and the professional Mac Studio and Mac Pro as one of four current Mac deskto ...

with

OS X Mountain Lion OS X Mountain Lion (version 10.8) is the ninth major release of macOS, Apple Inc.'s desktop and server operating system for Macintosh computers. OS X Mountain Lion was released on July 25, 2012, for purchase and download through Apple's Mac Ap ...

released in late 2012. Fusion Drive remains available in subsequent models of these computers, but was not expanded to other Apple devices: the latest MacBook and Mac Pro models use exclusively flash storage, and while this was an optional upgrade for the mid-2012 non-Retina MacBook Pro discontinued by Apple, it will replace the standard hard disk drive instead of complementing it in the fashion of Fusion Drive. As of November 2021, no Mac offers a fusion drive.

Design

Apple's Fusion Drive design incorporates proprietary features with limited documentation. 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 of which one has been used in the 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. # More frequently accessed data is stored on the SSD and the larger, less frequently accessed data stored on the HDD. # Data movement is based on access patterns: if data has been on the HDD and suddenly becomes frequently accessed, it will usually get moved to the SSD by the program controlling the Fusion Drive. During idle periods, data is adaptively migrated to the most suitable device to provide sustained data processing performance for users. Several experimental studies have been conducted to speculate about the internal mechanism of Fusion Drive. A number of speculations are available but not completely confirmed. # 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 HFS Plus or HFS+ (also known as Mac OS Extended or HFS Extended) is a journaling file system developed by Apple Inc. It replaced the Hierarchical File System (HFS) as the primary file system of Apple computers with the 1998 release of Mac OS ...

and

ZFS ZFS (previously: Zettabyte File System) is a file system with volume management capabilities. It began as part of the Sun Microsystems Solaris operating system in 2001. Large parts of Solaris – including ZFS – were published under an ope ...

. # Part of the SSD space is used as a write buffer for incoming writes. In the stable state, a minimum 4 GB 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 128 KB 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.

References

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External links

Fusion Drive
nbsp;– Apple Support Knowledge Base article
How to fix a split Fusion Drive
using Terminal – Apple Support Knowledge Base article
Fusion Drive vs SSD
- Geek Dashboard Computer-related introductions in 2012 Solid-state caching Hard disk drives Macintosh computers Solid-state computer storage

Availability

Design

See also

References

External links