6.3.22 Hierarchical Storage Management (HSM)  The OAIS Functions of Archival Storage embeds the notion of Hierarchical Storage Management (HSM) in the conceptual model. At the time OAIS was written the situation where large amounts of data could be affordably managed in other ways was not envisaged. The practical issue that underpins the need for HSM is the differing cost of storage media, e.g. where disc storage is expensive, but tape storage is much cheaper. In this situation HSM provides a virtual single store of information, while in reality the copies can be spread across a number of different carrier types according to use and access speeds.  However, the cost of hard disc has fallen at a greater rate than the cost of tape, to the point where there is an equivalency in price. Consequently the use of HSM becomes an implementation choice. Under these circumstances a storage system which contains all of the data on a hard disc array, all of which is also stored on a number of tapes, is a very affordable proposition, especially for digital storage systems up to 50 terabytes (and rising every year). For a smaller digital storage facility a fully functional HSM is consequently unnecessary and instead what is required is a much simpler system which manages and maintains copy location information, media age and versions and completely replicates the stored data on hard disc and on tape.  For medium to large digital storage systems the need for HSM storage systems remains and continues to be amongst the very expensive components of the digital storage systems.