Storage systems can be divided according to their access times into those with the shortest, longer and very long access times. The former include online media with direct access. These include flash memory, solid-state drives and hard disks. These storage media are characterized by access times of nanoseconds and microseconds. The second group with longer access times is known as nearline storage. These include jukeboxes, JBOD systems, RAID systems, hard disk drives and tape drives that use removable media. Their access times can be milliseconds, seconds or minutes. And the third group includes external optical storage, DVDs or Blu-Ray discs.
As for nearline storage systems, they belong to the secondary storage group and form a compromise between online storage with its short access times and offline storage for archiving. Nearline storage systems can be used for both, although the access times may be sufficiently different. Corresponding storage systems are used to store data, most of which has not been accessed for several months. Nearline storage does not have the high availability or redundancy ofprimary storage. To ensure that the data can be accessed quickly, nearline storage is often implemented with hard disk drives.