What Is Hybrid Hard Drive (HHD)? - Definition From

Hybrid hard drives vs. solid-state drives vs. hard drives

Hybrid hard drives bridge the gap between flash and fixed-disk magnetic storage. Solid-state drives (SSDs) are faster than traditional spinning disks, have no moving parts and have greater durability. They also have more storage capacity and better performance than traditional HDDs. Owing to advances in technology, the reliability of all three types of drives is comparable. As with any storage solution, the user requirements play a key role in deciding which technology is appropriate.

An HDD uses stacks of electromechanical spinning platters to store nonsequential data, written to sectors within specified blocks on each disk via an actuator arm. The HDD controller positions a hard disk, so data is read and written on the correct disk sector. The HDD mechanism and hard disk are packaged as an integrated unit, although both terms are used to describe it.

Engineers at IBM in the 1950s created the hard disk to provide random access to high capacities of data. Doing so at an affordable cost was one of IBM's key design criteria. Disk density has increased greatly from IBM's original 3.75 megabyte (MB) device.

HDDs today have reached 30 terabytes (TBs), with higher-capacity drives primarily targeting data center implementations. At the consumer level, several 22 TB drives represent the current upper limit of hard drive capacities.

SSD upper capacity limits reach 100 TB for data-center devices and 60 TBs for consumer devices. Most consumer SSD products average 2 TB to 8 TB.

An example of a typical SSHD is the Seagate FireCuda, with up to 2 TB of HDD storage and a cache with 8 GB of solid-state storage. The cache learns which data is used the most frequently and holds it in the flash memory to make it available for fast access.

HDDs connect to computing devices via a serial-attached Small Computer System Interface (SAS) or Serial Advanced Technology Attachment (SATA) interface. Similarly, SSDs -- also known as flash drives -- are based on SAS or SATA form factors and use those physical interfaces, but they have no internal mechanical movement.

SSDs consist of silicon chips designed as an integrated circuit to provide non-volatile memory for resilience. This differs from volatile cache storage on HDDs, which require an onboard capacitor or battery backup to protect data in the event of a system failure.

A hybrid hard drive adds a small dose of flash to the core disk architecture. The NAND cache buffer stores hot data and serves it to disk to accelerate application workloads. A typical HHD cache volume contains about 8 GB of flash and requires no special software driver.

A hybrid hard drive with embedded flash costs less than an SSD but can yield similar performance. Prices for HHDs are closer to those of regular hard disk drives as the technologies and manufacturing processes improve.