HSM – Hierarchical Storage Management. Storage systems and media are viewed as being in a hierarchy, ranging from fast and expensive, to slow and inexpensive. The hierarchy starts with storage-class memory (SCM) with access at DRAM-class speed. The next-fastest and expensive media is solid state storage, SSDs, followed by hard disk drives (HDDs) which have slower than SSD access and are less expensive than SSDs to buy – although deduplication and QLC flash can bring the total lifetime cost of SSDs into the HDD area. 

Memory and storage hierarchy.

Finally there are tape storage systems, even slower to access than disk drives, and with the lowest cost per terabyte and the viability to store data for many years. Optical disk storage, using Blu-ray technology, is positioned as an alternative to tape. DNA storage is being developed with the hope that it can provide an archival storage tier cheaper, longer-lasting and more capacious than tape.

Broadly speaking memory (DRAM) is accessed in nanoseconds, SSDs in microseconds, disks in milliseconds, and tape in minutes. Each tier of storage is less expensive than the preceding tier. HSM software moves data, as it ages, from a fast tier of storage to a slower and less expensive tier. See also the Archive entry.