Storage systems are systems for online data processing as well as for storage, archiving and backup copies. Depending on the requirements of the application, the amount of data to be stored, the required access time and the availability of the data, storage systems can operate as primary storage, secondary storage or tertiary storage.
Mass storage such as main memory, caches or flash memory belong to the primary storage systems, hard disks or hard disk arrays, compact discs and DVDs to the secondary, and tape storage, jukeboxes, tape drives and tape libraries to the tertiary.
Primary storage is extremely fast and is characterized by a short access time. Secondary storage offers storage capacities of several terabytes and is relatively inexpensive. However, access times are much longer than those of primary storage, amounting to several seconds for CD and DVD jukeboxes. The access times of tertiary storage such as tape drives, tape libraries and RAID systems are even longer. Their access times can be several minutes.