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Hybrid drives are storage devices that combine NAND flash solid-state drive (SSD) with hard disk drive (HDD) technology, with the intent of 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, by keeping copies of the most frequently used data on the SSD for improved overall performance.
Solid-state hybrid drive (SSHD) refers to products that incorporate a significant amount of NAND flash memory into a hard drive, resulting in a single, integrated device.[3] 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[citation needed] to be effective in delivering significantly improved performance over the standard HDD.
Operation
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.
