Introduction to database management systems

Databases are stored in file formats, which contain records. At physical level, actual data is stored in electromagnetic format on some device. These storage devices can be broadly categorized into three types −

• Primary Storage: the memory storage that is directly accessible to CPU comes under this category. CPU's internal memory (registers), fast memory (cache) and main memory (RAM) are directly accessible to CPU, as they are all placed on motherboard or CPU chipset. This storage is typically very small, volatile and ultra-fast. Primary storage requires continuous power supply in order to maintain its state. In case of a power failure, all data is lost. 

• Secondary Storage: Secondary storage devices are used to store data for future use or as backup. Secondary storage includes memory devices that are not a part of the CPU chipset or motherboard, for example, magnetic disks, optical disks (DVD, CD, etc.), hard disks, flash drives, and magnetic tapes. 

• Tertiary Storage: Tertiary storage is used to store huge volumes of data. Since such storage devices are external to computer system, they are the slowest in speed. These storage devices are mostly used to take back up of an entire system. Optical disks and magnetic tapes are widely used as tertiary storage.