Hybrid drives are storage devices that combine NAND flash solid-state drive (SSD) with hard disk drive (HDD) technology. That way, SSDs are utilized as another level of indirection within the data storage access paths, allowing improved speeds by utilizing fast SSDs as caches for slower hard drives (HDDs). Hybrid drives are bridging the gap between SSDs and HDDs — the costly speed of SSDs gets combined with the cheap storage capacity of traditional HDDs.
There are two main “hybrid” storage technologies that combine NAND flash memory, or SSDs, with HDD technology: dual-drive hybrid systems, and solid state hybrid drives.
Dual-drive hybrid systems
Dual-drive hybrid systems are combining the usage of separate SSD and HDD devices installed in the same computer, with overall performance optimization managed by the computer user, or by the computer's operating system software. Examples of this type of system are bcache and dm-cache on Linux, and Apple’s Fusion Drive.
A common implementation of the dual-drive system seen in the laptop computer category is through the use of flash cache modules (FCM). 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. Intel Smart Response Technology, which is implemented through a combination of certain Intel chipsets and Intel storage drivers, is the most common implementation of FCM hybrid systems today.
Solid-state hybrid drive
Solid-state hybrid drive (SSHD) refers to products that incorporate NAND flash memory into a hard drive, resulting in a single, integrated device. SSHD is a more precise term than the more general term hybrid drive, which has previously been used to describe SSHD devices and non-integrated combinations of solid-state drives (SSD) and hard disk drives (HDD).
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.
In the two forms of hybrid storage technologies (dual-drive hybrid systems and SSHDs), the goal is to combine HDD and NAND flash memory storage technologies 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, in the NAND flash memory or SSD 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 pinning mode)
- In this mode of operation, the SSHD enables an extended set of SATA commands defined in the Hybrid Information feature of the Serial ATA International Organization standards for the SATA interface. Using this feature, decisions about which data elements are placed in the NAND flash memory come from the host operating system, device drivers, host software or a combination of these host level components.
The figure below is a block diagram illustrating the primary design differences between a SSHD and a dual-drive or FCM design.
|This section needs additional citations for verification. (November 2013)|
In 2007, Seagate and Samsung 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 Microsoft Vista’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 models – a 500 GB and 1 TB, both with 8 GB of integrated NAND flash memory.
In September 2012, Toshiba announced its first SSHD, delivering SSD-like performance and responsiveness by combining 8 GB of Toshiba’s own NAND flash memory and innovative, self-learning algorithms with up to 1 TB of storage capacity.
In September 2012, WD (known as Western Digital) 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 April 2013, WD introduced 2.5-inch WD Black SSHD products, including a 5 mm–high SSHD with 500 GB of storage capacity and NAND flash memory size options of 8 GB, 16 GB and 24 GB.
SSHD benchmarks show the SSHDs do not offer SSD performance on random read/write and sequential read/write, however they do offer some improvement over HDDs for application startup and shutdown.
- Petros Koutoupis (2013-11-25). "Advanced Hard Drive Caching Techniques". linuxjournal.com. Retrieved 2013-12-02.
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- Perenson, Melissa. "Tested: New Hybrid Hard Drives from Samsung and Seagate". PCWorld. Retrieved 26 June 2013.
- "Seagate Point of View: Solid State Hybrid Drives – The Natural Evolution of Storage.". Seagate Technology, LLC. Retrieved 26 June 2013.
- Patrick Schmid and Achim Roos (2012-02-08). "Momentus XT 750 GB Review: A Second-Gen Hybrid Hard Drive". Retrieved 2013-11-07.
- Anand Lal Shimpi (2011-12-13). "Seagate 2nd Generation Momentus XT (750GB) Hybrid HDD Review". Retrieved 2013-11-07.