SSHDs

SSHDs, or solid-state hybrid drives, are storage devices that combine a traditional spinning hard disk with a small amount of NAND flash memory used as a fast cache. The aim is to deliver much of the responsiveness of an SSD at a lower cost per gigabyte by keeping frequently accessed data in flash.

The drive's controller tracks data access and automatically promotes hot data into the flash cache. Reads served from flash are faster, while writes are appended to the HDD portion, with data later flushed. The OS sees a single drive; caching is automatic and transparent to software.

Performance benefits depend on workload and cache hit rate. SSHDs can shorten boot times and improve the initiation of frequently used programs, but they do not equal a pure SSD for sustained random IO or long sequential transfers, because most data still resides on the HDD.

Form factors include 2.5-inch and 3.5-inch drives using standard SATA interfaces; capacities typically span several hundred gigabytes to a few terabytes. The flash cache is a small fraction of total capacity, and the drive presents as a single volume to the host.

Reliability is similar to that of conventional HDDs, since the mechanical portion dominates. The NAND cache has a limited write lifespan, but writes are usually small in volume. Some models lack full TRIM support, and cache behavior depends on firmware.

In the market, SSHDs are positioned as a cost-effective compromise between HDD capacity and SSD speed. They are less common in modern systems with large-capacity SSDs but may still be used in laptops and desktops seeking improved performance without paying for a full SSD.