HSM (Hierarchical Storage Management) is policy-based management of file backup and archiving in a way that uses storage devices economically and without the user needing to be aware of when files are being retrieved from backup storage media. Although HSM can be implemented on a standalone system, it is more frequently used in the distributed network of an enterprise. The hierarchy represents different types of storage media, such as redundant array of independent disks systems, optical storage, or tape, each type representing a different level of cost and speed of retrieval when access is needed. For example, as a file ages in an archive, it can be automatically moved to a slower but less expensive form of storage. Using an HSM product, an administrator can establish and state guidelines for how often different kinds of files are to be copied to a backup storage device. Once the guideline has been set up, the HSM software manages everything automatically.
HSM adds to archiving and file protection for disaster recovery the capability to manage storage devices efficiently, especially in large-scale user environments where storage costs can mount rapidly. It also enables the automation of backup, archiving, and migration to the hierarchy of storage devices in a way that frees users from having to be aware of the storage policies. Older files can automatically be moved to less expensive storage. If needed, they appear to be immediately accessible and can be restored transparently from the backup storage medium. The apparently available files are known as stubs and point to the real location of the file in backup storage. The process of moving files from one storage medium to another is known as migration.
An administrator can set high and low thresholds for hard disk capacity that HSM software will use to decide when to migrate older or less-frequently used files to another medium. Certain file types, such as executable files (programs), can be excluded from those to be migrated.