Editing Computer data storage (section)

== Related technologies ==

=== Redundancy ===
{{Main|Disk mirroring|RAID}}
{{See also|Replication (computing)#Disk storage replication|label 1=Storage replication}}

While a group of bits malfunction may be resolved by error detection and correction mechanisms (see above), storage device malfunction requires different solutions. The following solutions are commonly used and valid for most storage devices:

* '''Device [[Disk mirroring|mirroring]] (replication)''' – A common solution to the problem is constantly maintaining an identical copy of device content on another device (typically of the same type). The downside is that this doubles the storage, and both devices (copies) need to be updated simultaneously with some overhead and possibly some delays. The upside is the possible concurrent reading of the same data group by two independent processes, which increases performance. When one of the replicated devices is detected to be defective, the other copy is still operational and is being utilized to generate a new copy on another device (usually available operational in a pool of stand-by devices for this purpose).
* '''Redundant array of independent disks''' ('''[[RAID]]''') – This method generalizes the device mirroring above by allowing one device in a group of devices to fail and be replaced with the content restored (Device mirroring is RAID with ''n=2''). RAID groups of ''n=5'' or ''n=6'' are common. ''n>2'' saves storage, when compared with ''n=2'', at the cost of more processing during both regular operation (with often reduced performance) and defective device replacement.

Device mirroring and typical RAID are designed to handle a single device failure in the RAID group of devices. However, if a second failure occurs before the RAID group is completely repaired from the first failure, then data can be lost. The probability of a single failure is typically small. Thus the probability of two failures in the same RAID group in time proximity is much smaller (approximately the probability squared, i.e., multiplied by itself). If a database cannot tolerate even such a smaller probability of data loss, then the RAID group itself is replicated (mirrored). In many cases such mirroring is done geographically remotely, in a different storage array, to handle recovery from disasters (see disaster recovery above).

=== Network connectivity ===
A secondary or tertiary storage may connect to a computer utilizing [[computer network]]s. This concept does not pertain to the primary storage, which is shared between multiple processors to a lesser degree.

* [[Direct-attached storage]] (DAS) is a traditional mass storage, that does not use any network. This is still a most popular approach. This [[retronym]] was coined recently, together with NAS and SAN.
* [[Network-attached storage]] (NAS) is mass storage attached to a computer which another computer can access at file level over a [[local area network]], a private [[wide area network]], or in the case of [[online file storage]], over the [[Internet]]. NAS is commonly associated with the [[Network File System|NFS]] and [[CIFS/SMB]] protocols.
* [[Storage area network]] (SAN) is a specialized network, that provides other computers with storage capacity. The crucial difference between NAS and SAN, is that NAS presents and manages file systems to client computers, while SAN provides access at block-addressing (raw) level, leaving it to attaching systems to manage data or file systems within the provided capacity. SAN is commonly associated with [[Fibre Channel]] networks.

=== Robotic storage ===
Large quantities of individual magnetic tapes, and optical or magneto-optical discs may be stored in robotic tertiary storage devices. In tape storage field they are known as [[tape libraries]], and in optical storage field [[optical jukebox]]es, or optical disk libraries per analogy. The smallest forms of either technology containing just one drive device are referred to as [[Tape library#Autoloaders|autoloaders]] or [[Record changer|autochangers]].

Robotic-access storage devices may have a number of slots, each holding individual media, and usually one or more picking robots that traverse the slots and load media to built-in drives. The arrangement of the slots and picking devices affects performance. Important characteristics of such storage are possible expansion options: adding slots, modules, drives, robots. Tape libraries may have from 10 to more than 100,000 slots, and provide [[terabyte]]s or [[petabyte]]s of near-line information. Optical jukeboxes are somewhat smaller solutions, up to 1,000 slots.

Robotic storage is used for [[backup]]s, and for high-capacity archives in imaging, medical, and video industries. [[Hierarchical storage management]] is a most known archiving strategy of automatically ''migrating'' long-unused files from fast hard disk storage to libraries or jukeboxes. If the files are needed, they are ''retrieved'' back to disk.