Traditionally, due to the high cost of memory, creating more disk space often involves the users removing unused files, migrating these files to off-line storage or just simply buying more disks. These solutions can be costly in terms of time, money and productivity. A concept called Hierarchical Storage Management (HSM) was developed to solve these problems and is now widely implemented.

HSM is basically an automated process in which data is moved to some secondary storage and is recalled automatically when needed. HSM is responsible for pre-staging the data in the primary storage, to prepare it for transfer to a secondary storage. This is done to avoid degrading performance during peak hours. The secondary storage can take the form of a tape autoloader, optical juke box, a large tape drive or a combination of such devices. The advantage is their relatively low cost per Gigabyte. The transfer of files will begin as soon as a preset maximum volume is reached in the primary storage and will cease when a minimum volume remains. When the user accesses the file residing in the secondary storage, a recall agent will be instructed to move this file back as on-line. With HSM, the clients receive the benefits of virtually limitless disk storage without the high costs.