Hybrid drive

For vehicle drives, see Hybrid vehicle.
"Solid-state hard drive" and "SSHD" redirect here. For information about solid-state drives (SSDs), see Solid-state drive.

In computing, a hybrid drive is a logical or physical storage device that combines a fast storage medium such as NAND flash solid-state drive (SSD) with a hard disk drive (HDD), with the intent of adding some of the speed of flash storage 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.

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, or automatically by the operating system 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).


A high-level comparison of SSHD and dual-drive or FCM designs

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. Overall performance optimizations are managed in one of three ways;

  1. By the computer user, who manually places more frequently accessed data onto the faster drive.
  2. By the computer's operating system 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 bcache and dm-cache on Linux,[1] and Apple’s Fusion Drive and other Logical Volume Management based implementations[2] on OS X.[3]
  3. 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 SSD module) and HDD components, while managing performance optimizations via host software, device drivers, 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 SSHD[lower-alpha 1]) refers to products that incorporate a significant amount of NAND flash memory into a hard disk drive (HDD), resulting in a single, integrated device.[6] 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 drives (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[7] to be effective in delivering significantly improved performance over the standard HDD.

An example of an often confused dual-drive system being considered a 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.[8]


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 through device firmware, through device drivers or through software modules and device drivers.

SSHD products operate in two primary modes

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, device drivers, file systems, or a combination of these host-level components.[9]
Some of the specific features of SSHD drives, such as the host-hinted mode, require software support from the operating system. Microsoft added support for the host-hinted operation into Windows 8.1,[10] while patches for the Linux kernel are available since October 2014, pending their inclusion into the Linux kernel mainline.[11][12]


Hybrid-Drive technology has come a long way with modern implementations improving over the past decade beginning in 2007:


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.[18][19]

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.[19]

See also


  1. The initialism could be interpreted as "solid-state hard drive",[4] although it is more commonly interpreted as "solid-state hybrid drive".[5]


  1. Petros Koutoupis (2013-11-25). "Advanced Hard Drive Caching Techniques". linuxjournal.com. Retrieved 2013-12-02.
  2. "Hybrid-Drive". TarDisk.com. Retrieved 2016-05-31.
  3. "TarDisk Pear first look: Upgrade your MacBook flash storage in a few minutes". Macworld. Retrieved 2016-05-31.
  4. Dong Ngo (9 January 2013). "WD Black SSHD (1TB) Preview - CNET". CNET. CBS Interactive. Retrieved 22 May 2015.
  5. "How to Choose Between SSD, SSHD and HDD Storage for Your Laptop". seagate.com. Retrieved 22 May 2015.
  6. "How to Choose Between SSD, SSHD and HDD Storage for Your Laptop". seagate.com. Retrieved 22 May 2015.
  7. "SSDs vs. hard drives vs. hybrids: Which storage tech is right for you?". PCWorld. Retrieved 2016-05-31.
  8. +Peter Paul. "WD Black2 Dual Drive 120GBSSD + 1TB HDD Gets Discounted, $60 Off Its Price". The Best Laptops. Retrieved 22 May 2015.
  9. "SATA-IO FAQ: What else is new in SATA specification v3.2?" (PDF). SATA-IO. p. 2. Retrieved 2013-10-03.
  10. Andy Herron (2013). "Advancements in Storage and File Systems in Windows 8.1" (PDF). SNIA. Retrieved 2014-01-11.
  11. Michael Larabel (2014-10-29). "Linux Kernel Finally Being Optimized For SSHDs". Phoronix. Retrieved 2015-02-26.
  12. Jason B. Akers (2014-10-29). "Enable use of Solid State Hybrid Drives". LWN.net. Retrieved 2015-02-26.
  13. Perenson, Melissa. "Tested: New Hybrid Hard Drives from Samsung and Seagate". PCWorld. Retrieved 26 June 2013.
  14. "Seagate Point of View: Solid State Hybrid Drives – The Natural Evolution of Storage.". Seagate Technology, LLC. Retrieved 26 June 2013.
  15. "Apple - Press Info - iMac Available on November 30". www.apple.com. Retrieved 2016-06-01.
  16. Liszewski, Andrew. "This Leave-In SD Card Merges With Your MacBook's SSD to Increase Its Capacity". Gizmodo. Retrieved 2016-06-01.
  17. "Pure MacBook Storage | TarDisk". TarDisk.com. Retrieved 2016-06-01.
  18. Patrick Schmid and Achim Roos (2012-02-08). "Momentus XT 750 GB Review: A Second-Gen Hybrid Hard Drive". Retrieved 2013-11-07.
  19. 1 2 Anand Lal Shimpi (2011-12-13). "Seagate 2nd Generation Momentus XT (750 GB) Hybrid HDD Review (with 8 GB NAND cache)". Retrieved 2013-11-07.
This article is issued from Wikipedia - version of the 11/16/2016. The text is available under the Creative Commons Attribution/Share Alike but additional terms may apply for the media files.