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The storage of information meets three needs, nonexclusive from/to each other:

to store the information in sure place to answer a legal or conventional constraint (filing of the data),
to make information available (publication of the data)
to re-use information (data processing)

The choice of the method of storage is done according to several criteria:

the frequency of use of information;
the criticality of information;
the perenniality of information;
confidentiality of information;
the volume of information to be stored;
the time allocated with the process of storage;
and of course, its cost.

The watchword of the techniques of storage is: more of capacity, more quickly, more reliable, and less expensive. This is why the types of media are varied.

Typology of technologies of storage

Classically, one opposes the storage of mass, rather slow but allowing to manage large volumes of data on long run, and storage with rapid access, mainly used like working memory.

Storage of mass

The objective is here to store a great quantity of information on long run. One can distinguish several generations:

First generation: physical supports, with the Perforated card and the Paper tape. These supports are now obsolete.
Second generation: magnetic media, like the Magnetic band, her little sister the cassette, the Hard drive, the Diskette. The magnetic bands are used more only for the safeguard or the filing of the data (e.g. LTO). They remain nevertheless a privileged support of safeguard and filing of the data because of their very great capacity and their removable nature which make it possible to delocalize them easily. Thus, the bands or cartridges accommodate usually today more than 200 gigaoctets.
Third generation: optical supports, such as the Compact disk (CD, CD-R or CD-RW) and DVD (DVD-Romanian or DVD-RW).

Some speak today about the memory of the network Internet like support of fourth generation, because information published on Internet remains there without it being easy to control it (to modify it, to remove it or restrict its access).

Storage with rapid access, working memory

In parallel, of the more expensive but very fast memories were established on chip S to facilitate in-house data processing in the Ordinateur S:

the Random access memory (or memory RAM for Random Access Memory ), which saw only as long as the computer is lit and is erased when it is extinguished;
the Read-only memory (or ROMANIAN memory for Read-Only Memory ), which is engraved and (ex is never erased: NVRAM);
EPROM and EEPROM (rewriteable read-only memory), which evolved/moved in Mémoire flash, giving rise to recent key USB, direct competitors of old the Disquette S.

Problems of storage

How could one during time to store computer data of growing number on increasingly minimal supports?

Volumetry of the data

One of the critical points of the data-processing evolution is the inflation of the volume of information. Even if storage capacity, at constant price , follows the definite Loi of Moore for the Processeur S, volume to be stored increased in an exponential way, and storage constitutes problems of the companies in term of cost (it currently consumes most of the data-processing budget) and of management of the stored data (where to preserve them and how to re-use them).

To thwart this dependence with the limitations of space available of the physical supports, the algorithms of Data compression present, with shared storage, the largest hopes for tomorrow.

Moreover, " farms " waiters on PC which underlie the great Web sites of today manage the volumes of data incomparably more important than largest of the host computers of antan. Technologies of filing on tape followed this same modular approach: each bay can contain ten or a score of magnetic bands and the bays can gather, for example by 8 or 10, for a full capacity going until several tens of terabytes. The libraries of bands are at present regarded as the most accessible means to ensure the safeguard and the filing of all these data. (Source: 01Net.com)

Reliability of the supports

Because of their physical constitution, all the memory supports have one limited lifespan, involving a risk of loss of information. To free itself some, it is constantly necessary to check them and recopy the data on reliable supports. Today, a branch of the Information theory comes to produce remedy in the event of deterioration partial of the data: technique of the correct codes, used in particular in the installations RAID. The redundancy of information remains unfortunately the only rampart vis-a-vis the unreliability of the supports.

Data confidentiality

As regards the confidentiality, the Cryptographie proposes solutions of Chiffrement, which rest on various methods:

the Cryptography with public key (algorithm RSA inter alia);
the Cryptography with secret key (algorithms OF the, AES inter alia).

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