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computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...
, 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 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 ...
(SSD) with a higher-capacity
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 mag ...
(HDD). The intent is adding some of the speed of SSDs to the cost-effective storage capacity of traditional HDDs. The purpose of the SSD in a hybrid drive is to act as a cache for the data stored on the HDD, improving the overall performance by keeping copies of the most frequently used data on the faster SSD drive. There are two main configurations for implementing hybrid drives: dual-drive hybrid systems and solid-state hybrid drives. In dual-drive hybrid systems, physically separate SSD and HDD devices are installed in the same computer, having the data placement optimization performed either manually by the
end user In product development, an end user (sometimes end-user) is a person who ultimately uses or is intended to ultimately use a product. The end user stands in contrast to users who support or maintain the product, such as sysops, system administrato ...
, or automatically by the
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
through the creation of a "hybrid" logical device. In solid-state hybrid drives, SSD and HDD functionalities are built into a single piece of hardware, where data placement optimization is performed either entirely by the device (self-optimized mode), or through placement "hints" supplied by the operating system (host-hinted mode).


Types

There are two main "hybrid" storage technologies that combine NAND flash memory or SSDs, with the HDD technology: dual-drive hybrid systems and solid-state hybrid drives.


Dual-drive hybrid systems

Dual-drive hybrid systems combine the usage of separate SSD and HDD devices installed in the same computer. Performance optimizations are managed in one of three ways: # By the computer user, who manually places more frequently accessed data onto the faster drive. # By the computer's
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
software, which combines SSD and HDD into a single ''hybrid volume'', providing an easier experience to the end-user. Examples of hybrid volumes implementations in operating systems are ZFS' "hybrid storage pools",
bcache bcache (abbreviated from ''block cache'') is a cache in the Linux kernel's block layer, which is used for accessing secondary storage devices. It allows one or more fast storage devices, such as flash-based solid-state drives (SSDs), to act as ...
and
dm-cache dm-cache is a component (more specifically, a target) of the Linux kernel's device mapper, which is a framework for mapping block devices onto higher-level virtual block devices. It allows one or more fast storage devices, such as flash-base ...
on
Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, whi ...
, Intel's Hystor and
Apple An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple trees are cultivated worldwide and are the most widely grown species in the genus '' Malus''. The tree originated in Central Asia, where its wild ancest ...
's
Fusion Drive Fusion Drive is a type of hybrid drive technology created by Apple Inc. It combines a hard disk drive with a NAND flash storage (solid-state drive of 24 GB or more) and presents it as a single Core Storage managed logical volume with the sp ...
, and other
Logical Volume Management In computer storage, logical volume management or LVM provides a method of allocating space on mass storage, mass-storage devices that is more flexible than conventional Partition (computing), partitioning schemes to store volumes. In particular ...
based implementations on
OS X macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac (computer), Mac computers. Within the market of ...
. # By chipsets external to the individual storage drives. An example is the use of ''flash cache modules'' (FCMs). FCMs combine the use of separate SSD (usually an
mSATA SATA (Serial AT Attachment) is a computer bus interface that connects host bus adapters to mass storage devices such as hard disk drives, optical drives, and solid-state drives. Serial ATA succeeded the earlier Parallel ATA (PATA) standard to ...
SSD module) and HDD components, while managing performance optimizations via host software,
device driver In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and o ...
s, or a combination of both. One example is Intel Smart Response Technology (SRT), which is implemented through a combination of certain Intel chipsets and Intel storage drivers, is the most common implementation of FCM hybrid systems today. What distinguished this dual-drive system from an SSHD system is that each drive maintains its ability to be addressed independently by the operating system if desired.


Solid-state hybrid drive

''Solid-state hybrid drive'' (also known by the
initialism An acronym is a word or name formed from the initial components of a longer name or phrase. Acronyms are usually formed from the initial letters of words, as in ''NATO'' (''North Atlantic Treaty Organization''), but sometimes use syllables, as ...
SSHD) refers to products that incorporate a significant amount of NAND flash memory into 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 mag ...
(HDD), resulting in a single, integrated device. The term SSHD is a more precise term than the more general ''hybrid drive'', which has previously been used to describe SSHD devices and non-integrated combinations of
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 ...
s (SSDs) and hard disk drives. The fundamental design principle behind SSHDs is to identify data elements that are most directly associated with performance (frequently accessed data, boot data, etc.) and store these data elements in the NAND flash memory. This has been shown to be effective in delivering significantly improved performance over the standard HDD. An example of an often confused dual-drive system being considered an SSHD is the use of laptops which combine separate SSD and HDD components into the same 2.5-inch HDD-size unit, while at the same time (unlike SSHDs) keeping these two components visible and accessible to the operating system as two distinct partitions. WD's Black2 drive is a typical example; the drive can either be used as a distinct SSD and HDD by partitioning it appropriately, or software can be used to automatically manage the SSD portion and present the drive to the user as a single large volume.


Operation

In the two forms of hybrid storage technologies (dual-drive hybrid systems and SSHDs), the goal is to combine HDD and a faster technology (often NAND flash memory) to provide a balance of improved performance and high-capacity storage availability. In general, this is achieved by placing "hot data", or data that is most directly associated with improved performance, on the "faster" part of the storage architecture. Making decisions about which data elements are prioritized for NAND flash memory is at the core of SSHD technology. Products offered by various vendors may achieve this with device
firmware In computing, firmware is a specific class of computer software that provides the low-level control for a device's specific hardware. Firmware, such as the BIOS of a personal computer, may contain basic functions of a device, and may provide h ...
, device drivers or software modules and device drivers.


Modes of operation

; Self-optimized mode :In this mode of operation, the SSHD works independently from the host operating system or host device drives to make all decisions related to identifying data that will be stored in NAND flash memory. This mode results in a storage product that appears and operates to a host system exactly as a traditional hard drive would. ; Host-optimized mode (or host-hinted mode) : In this mode of operation, the SSHD enables an extended set of SATA commands defined in the so-called Hybrid Information feature, introduced in version 3.2 of the Serial ATA International Organization (SATA-IO) standards for the SATA interface. Using these SATA commands, decisions about which data elements are placed in the NAND flash memory come from the host
operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...
, device drivers, file systems, or a combination of these host-level components. : Some of the specific features of SSHD drives, such as the host-hinted mode, require software support from within the operating system. Microsoft added support for the host-hinted operation into Windows 8.1, while patches for the
Linux kernel The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ...
are available since October 2014, pending their inclusion into the Linux kernel mainline.


History

Hybrid-drive technology has come a long way with modern implementations improving over the past decade beginning in 2007: * In 2007, Seagate and
Samsung The Samsung Group (or simply Samsung) ( ko, 삼성 ) is a South Korean multinational manufacturing conglomerate headquartered in Samsung Town, Seoul, South Korea. It comprises numerous affiliated businesses, most of them united under the ...
introduced the first hybrid drives with the Seagate Momentus PSD and Samsung SpinPoint MH80 products. Both models were 2.5-inch drives, featuring 128 MB or 256 MB NAND flash memory options. Seagate's Momentus PSD emphasized power efficiency for a better mobile experience and relied on
Windows Vista Windows Vista is a major release of the Windows NT operating system developed by Microsoft. It was the direct successor to Windows XP, which was released five years before, at the time being the longest time span between successive releases of ...
's ReadyDrive. The products were not widely adopted. * In May 2010, Seagate introduced a new hybrid product called the Momentus XT and used the term ''solid-state hybrid drive''. This product focused on delivering the combined benefits of hard drive capacity points with SSD-like performance. It shipped as a 500 GB HDD with 4 GB of integrated NAND flash memory. * In November 2011, Seagate introduced what they referred to as their second-generation SSHD, which increased the capacity to 750 GB and pushed the integrated NAND flash memory to 8 GB. * In March 2012, Seagate introduced their third-generation laptop SSHDs with two modelsa 500 GB and 1 TB, both with 8 GB of integrated NAND flash memory. * In September 2012,
Toshiba , commonly known as Toshiba and stylized as TOSHIBA, is a Japanese multinational conglomerate corporation headquartered in Minato, Tokyo, Japan. Its diversified products and services include power, industrial and social infrastructure systems, ...
announced its first SSHD, delivering SSD-like performance and responsiveness by combining 8 GB of Toshiba's own SLC NAND flash memory and innovative, self-learning algorithms with up to 1 TB of storage capacity. * In September 2012,
Western Digital Western Digital Corporation (WDC, commonly known as Western Digital or WD) is an American computer drive manufacturer and data storage company, headquartered in San Jose, California. It designs, manufactures and sells data technology produ ...
(WD) announced a hybrid technology platform pairing cost-effective MLC NAND flash memory with magnetic disks to deliver high-performance, large-capacity integrated storage systems. * In November 2012, Apple Inc. released the factory-configured dual-drive hybrid system named
Fusion Drive Fusion Drive is a type of hybrid drive technology created by Apple Inc. It combines a hard disk drive with a NAND flash storage (solid-state drive of 24 GB or more) and presents it as a single Core Storage managed logical volume with the sp ...
. * In October 2015, TarDisk introduced the plug-and-play dual-drive hybrid system "TarDisk Pear", with flash memory size options up to 256 GB. * In August 2021, Western Digital introduced OptiNAND™, a New Flash-Enhanced HDD Architecture. It uses a new iNAND read/write cache system for performance. This feature is for when power is lost during a write phase, to prevent data loss. The System-on-a-Chip (SoC) of an OptiNAND drive, in under a second, will use the rotational power generated by the already spinning disk platter inside the drive to power internal capacitors until the iNAND cached data transfers to non-volatile NAND.


Benchmarks

Late 2011 and early 2012 benchmarks using an SSHD consisting of a 750 GB HDD and 8 GB of NAND cache found that SSHDs did not offer SSD performance on random read/write and sequential read/write, but were faster than HDDs for application startup and shutdown. The 2011 benchmark included loading an image of a system that had been used heavily, running many applications, to bypass the performance advantage of a freshly-installed system; it found in real-world tests that performance was much closer to an SSD than to a mechanical HDD. Different benchmark tests found the SSHD to be between an HDD and SSD, but usually significantly slower than an SSD. In the case of uncached random access performance (multiple 4 KB random reads and writes) the SSHD was no faster than a comparable HDD; there is advantage only with data that is cached. The author concluded that the SSHD drive was the best non-SSD type of drive by a significant margin, and that the larger the solid-state cache, the better the performance.


See also

* ExpressCache *
Fusion Drive Fusion Drive is a type of hybrid drive technology created by Apple Inc. It combines a hard disk drive with a NAND flash storage (solid-state drive of 24 GB or more) and presents it as a single Core Storage managed logical volume with the sp ...
*
Hybrid array A hybrid array is a form of hierarchical storage management that combines hard disk drives (HDDs) with solid-state drives (SSDs) for I/O speed improvements. Hybrid storage arrays aim to mitigate the ever increasing price-performance gap between H ...
*
ReadyBoost ReadyBoost (codenamed EMD) is a disk caching software component developed by Microsoft for Windows Vista and included in later versions of Windows. ReadyBoost enables NAND memory mass storage CompactFlash, SD card, and USB flash drive devices t ...


Linux topics

*
bcache bcache (abbreviated from ''block cache'') is a cache in the Linux kernel's block layer, which is used for accessing secondary storage devices. It allows one or more fast storage devices, such as flash-based solid-state drives (SSDs), to act as ...
*
dm-cache dm-cache is a component (more specifically, a target) of the Linux kernel's device mapper, which is a framework for mapping block devices onto higher-level virtual block devices. It allows one or more fast storage devices, such as flash-base ...
* flashcache


Notes


References

{{Disk images Computer peripherals Computer storage devices Computer storage media Solid-state caching Hard disk drives Non-volatile memory