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Tiered storage is a data storage method or system consisting of two or more storage media types, such as CDs, DVDs, hard drives, optical discs, hard disc drive arrays and magnetic tape drives. The media type used to store a category of data is determined by several factors, including media cost, data availability requirements, data recoverability, etc.
Tiered storage and “hierarchical storage management” (HSM) are sometimes referenced as interchangeable. However, HSM usually refers to an automatic system of data transfer to a number of media types depending on the frequency of use. For example, data stored on disk drives may be transferred automatically to magnetic tape if not used for a period of months.
Tiered storage may involve two tiers when the available media types are only hard drives and tapes; each of these tiers involves differences in four major attributes, that is, price, performance, capacity and function.
New technology and old technology may represent the available media types; this would dictate two tiers as each would involve differences in all four attributes. Similarly, high performance storage devices and slower, low performance devices may also dictate two tiers.
Tiered storage requirements are also determined by functional differences, such as the need for replication for security purposes and high-speed restoration of data by hard disk drive arrays. In this case data is stored for two different functions; thus, at least two tiers would be appropriate. Two very widely used tiers, significantly divided by access speed, are the tiers of magnetic disk and tapes; another common two-tier system is magnetic disk and optical disk.
Depending on company defined policy for data storage, some vendors’ products may blur the distinctions between tiered storage and HSM. They provide software to implement the policy by automatically assigning or transferring data storage depending on changing data, frequency of use and the available storage media.