Computer data storage

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1 GB of SDRAM mounted in a personal computer. An example of primary storage.
40 GB PATA hard disk drive (HDD); when connected to a computer it serves as secondary storage.
160 GB SDLT tape cartridge, an example of off-line storage. When used within a robotic tape library, it is classified as tertiary storage instead.

Computer data storage is the name for a number of components of a computer. The main purpose of these components is to store data. The central processing unit is then able to fetch and change the data. In most computers, there is a memory hierarchy: Memory that is "closer" to the CPU is usually faster to access, but it is also smaller; data that is stored in this kind of memory usually needs electrical power to keep the data. Memory that is farther away is usually slower to access, but also bigger. Examples for this kind of data are classical storage media, such as hard drives or USB flash drives. Some media offer even more capacity, but accessing them is very slow. Examples for such media are tape drives. Modern CPUs have registers that can store data, they also often have several levels of cache. Finally, there is the main memory the CPU can access. These three are usually referred to as Primary data storage. Secondary storage is usually not directly accessible by the CPU, and data needs to be transferred to primary storage to be available. Secondary storage includes hard drives. Tertiary storage is mainly used for archival and backup; very often it is copied to secondary storage before use.