7.6 Archival Storage The archival storage system sits underneath the repository, technically speaking, and incorporates a suite of sub-processes such as storage media selection, transfer of the Archival Information Package (AIP) to the storage system, data security and validity, backup and data restoration, and reproduction of AIP to new media. The basic principles of archival storage can be summarised as follows There should be multiple copies. The system should support a number of duplicate copies of the same item. Copies should be remote from the main or original system and from each other. The greater the physical distance between copies the safer in the event of disaster. There should be copies on different types of media. If all the copies are on a single type of carrier, such as hard disc, the risk of a single failure mechanism destroying all the copies is great. The risk is spread by having different types of carriers. IT professionals commonly use data tape as the second (and subsequent) copy. The major cost in the data storage systems is not the hardware, but the Hierarchical Storage Management (HSM) System. The OAIS Functions of Archival Storage embeds the notion of 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 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 a small to medium sized digital audio collection. For this type of system a fully functional HSM is unnecessary and instead what is required is a much simpler system which manages and maintains copy location information, media age and versions (Bradley, Lei and Blackall 2007).