<P> The secondary storage is often formatted according to a file system format, which provides the abstraction necessary to organize data into files and directories, providing also additional information (called metadata) describing the owner of a certain file, the access time, the access permissions, and other information . </P> <P> Most computer operating systems use the concept of virtual memory, allowing utilization of more primary storage capacity than is physically available in the system . As the primary memory fills up, the system moves the least - used chunks (pages) to secondary storage devices (to a swap file or page file), retrieving them later when they are needed . As more of these retrievals from slower secondary storage are necessary, the more the overall system performance is degraded . </P> <P> Tertiary storage or tertiary memory provides a third level of storage . Typically, it involves a robotic mechanism which will mount (insert) and dismount removable mass storage media into a storage device according to the system's demands; such data are often copied to secondary storage before use . It is primarily used for archiving rarely accessed information since it is much slower than secondary storage (e.g. 5--60 seconds vs. 1--10 milliseconds). This is primarily useful for extraordinarily large data stores, accessed without human operators . Typical examples include tape libraries and optical jukeboxes . </P> <P> When a computer needs to read information from the tertiary storage, it will first consult a catalog database to determine which tape or disc contains the information . Next, the computer will instruct a robotic arm to fetch the medium and place it in a drive . When the computer has finished reading the information, the robotic arm will return the medium to its place in the library . </P>

The two most common tertiary storage media are