Safeguard

Finality

The copies of Sûreté are useful mainly for two things:

the most obvious first and is to make it possible to restore a computing system in an operating condition following an incident (loss of a memory support such as hard drive, magnetic band, etc, or of whole or part of the data which it contains).
the second, included in the most current first but certainly, is to facilitate the restoration of part of a computing system (a file, a group of files, an operating system, a data in a file, etc) following an accidental suppression or to a nondesired modification.

The most frequent technique is the recopy of the Donnée S on a support independent of the initial system (local Ordinateur, waiter, etc).

The operation reverses which consists in re-using Donnée S safeguarded is called a restoration. One speaks about “storage” for the data safeguarded on standby of a possible restoration.

Selection criteria

The choice of a technique of safeguard will be done while taking into account:

the storage capacity of the support (the volume of information)
the speed of safeguard,
the reliability of the support (in particular after a long period of storage),
the simplicity of classification,
facility to restore the data,
and of course the cost of the unit.

Also the possibility intervenes of selecting the data to be safeguarded. Finally for the great systems of safeguard, it is necessary to take account of physical criteria: physical volume of the memory supports, weight, sensitivity to the temperature, moisture, dust, the light.

Strategies of safeguard

One distinguishes the safeguard on Poste customer from the safeguard on waiter. One and other address to same nature of information (the computer data) and have same objective (to protect the information and to make it possible to find it if it were lost), but the methods of safeguard are different for several reasons:

the data on station customer are famous less important than the data managed on central systems;
the users are sensitized with the risk of loss of data than the professionals of data processing;
they have also less formation on the techniques of safeguard;
average the techniques are developed on station customer than on waiter, even if important progress were made these last years (falls of the cost ratio/volume of the supports of safeguard, simplification of the interfaces of safeguard, safeguards without intervention of the user, etc)

In fact the " backup" station customers remains marginal in the strategy of use of the computers. However the companies, by generalizing the use of the microcomputers and the resource sharing in network, felt a need for safety which supported the development of tools of safeguard on microcomputers, which gain the world of personal microprocessing gradually.

Safeguard on waiter

The safeguard falls under a more total step which consists in ensuring the continuity of activity of a computing system or, in the event of failure, its restarting as quickly as possible. This step is often formalized in a document which can bear various names, for example the " PRA" (recovery plan of activity) or the " PS" (plan of help), and which calls either upon automatisms (e.g. to give the alarm in the event of power cut or of loss of access to a storage unit) or with manual gestures (e.g. to replace defective magnetic bands). The tendency is with automation, famous surer in the emergencies than the manual operations.

In term of support, the waiters since required always supports with great storage capacity. The Magnetic band was the principal vector a long time, because of its great capacity, of its weak cost (compared to the other supports), of its capacity of re-use and its relating stability to time and wear. Then came the numerical cartridges (magnetic bands integrated in a standard plastic case DAT, DLT, SDLT, LTO), the hard drives and more recently the media optical, rewriteable or not, such as CD-R similar, DVD-R or formats.

Safeguard on system customer

During years 1975-95, the majority of the users of personal computers (PC) associated mainly the " term; backup" with the fact of making copies on diskettes. With the development of microcomputers equipped better, the personal users adopted more powerful supports: optical disks (CD-ROM or DVD), key USB. In the same way, the computers integrate increasingly evolved/moved functions of safeguard, for example:

of the tools integrated into the operating system such as the " points of restauration" that one can carry out before installing a new software and who will give the system in the state of before the installation if the user asks it;
of the software able to make a perfect image of the system at a time given (image called a " ghost ", word which means " fantôme" in English); this image will be stored on the computer itself or an external medium.

Safeguard on Internet

Today, copies of safety known as " in ligne" become popular and, with the vulgarizing of connections Internet to broad band and high banc, more and more of users resort to this service type of safeguard. They consist in being connected to an Internet site, called " hébergeur" , and to transfer its data to it. The advantages are multiple:

to minimize the risk of loss since the site is managed by a professional who does itself of the safeguards;
to reach its data starting from any computer connected to Internet;
often the cost of this service is moderate, sometimes even free for the small safeguards.

The major disadvantage is to leave its data to provision of a third which can with leisure consult them, to modify them, duplicate them, publish them or make trade of it; and even to make them inalienable (case of the bankruptcies, repurchases of sites by competitors, or commercial disagreement with the shelterer). Obviously, from the contractual provisions come to control these risks but they cannot prevent the shelterer from technically acting in a malevolent way. One of the parades to the abusive consultation consists with to quantify the data.

Another disadvantage comes from the limits imposed on the storage or the recovery of the data: to control the use of its discs and its Band-width, a shelterer can limit by contract his customer to a volume of storage or data consulted beyond of which it blocks the access to the data.

Most current methods of safeguard

The simplest method is the complete or total safeguard (also called " full backup") ; it consists in copying all the data to be safeguarded that those are recent, old, modified or not.

This method is also most reliable but it is long and very expensive in term of disk space, which prevents to use it in practice for all the safeguards to be carried out. In order to gain in speed and times of safeguard, there exist methods which proceed to the safeguard of the only data modified and/or added between two total safeguards. One counts two of them:

the differential safeguard
the incremental safeguard

The restoration of a disc with one of these methods proves longer and more tiresome since in addition to the restoration of the differential safeguard or the incremental safeguards, one must also restore the last complete safeguard. The files removed meanwhile will be also restored.

In order to include/understand the difference between the two methods, we will take the example of a protection plan according to the following cycle:

a safeguard supplements at the D-day (Sunday evening for example)
a safeguard of the files modified or new of the J+1 day at the day J+6 (of Monday evening at the Saturday evening included)
a complete safeguard at the day J+7 (next Sunday evening)

Mechanism

To be able to differentiate these various methods of safeguard/filing (supplements, incremental, differential), the mechanism installed is the use of a marker of filing. Each file has this marker of filing, which is positioned with " vrai" when or modified a file is created. One can include/understand this position like " I have just been modified or created: I am ready to be thus filed I position my marker with vrai". This marker is also called attribute of filing (or bit of filing).

Complete safeguard

At the time of a complete safeguard, one will give to " 0" the attribute of the file to memorize the fact that the file was recorded. When one works with the date, one memorizes the date of the last safeguard in order to be able to differentiate the files which were safeguarded the different ones (date of last modification).

Technical detail

At the time of a complete safeguard, all the files are safeguarded, independently of the position of the marker (true or false). Once the filed file, this one is seen allotting the position of its marker (or his bit) to " faux" (or with " 0").

Safeguard differential

The differential safeguard carries out a copy of the files created or modified since the last complete safeguard, whatever the intermediate safeguards. In other words, the complete safeguard of the D-day is used as reference to identify the files created, modified or added and thus to safeguard only the latter of the J+1 day at the J+6 day.

The restoration made starting from this type of safeguard requires the recopy on disc of the last complete safeguard and the differential safeguard most recent.

With our example, if the restoration goes on a complete disc which was safeguarded the J+2 day, one must then recopy on disc the complete safeguard of the D-day and the differential safeguard of the J+2 day in order to have the last version of the data.

However when it is about the restoration of a file or a repertory which was safeguarded the J+2 day only the last safeguard, here the differential, is useful.

Technical detail

At the time of a differential safeguard, all the files whose marker is with " vrai" are safeguarded. Once the filed file, this one keeps the position of its marker such as it had it before the safeguard.

Incremental safeguard

This method consists in safeguarding the files created or modified since the last safeguard whatever its type (supplements, differential or incremental).

Example: a complete safeguard is carried out the D-day. the day J+1, the incremental safeguard is carried out by reference to the D-day. the day J+2, the incremental safeguard is carried out by reference to the J+1 day. And so on.

If the restoration goes on a complete disc which was safeguarded the J+4 day, one must then recopy on disc the safeguard of the D-day and the incremental safeguards of the days J+1, J+2, J+3 and J+4 in order to obtain the last version of the totality of the data.

However when it is about the restoration of a file or a repertory which was safeguarded the J+3 day, only the last safeguard, here the incremental one, is useful.

Technical detail

At the time of an incremental safeguard, all the files whose marker is with " vrai" are safeguarded. Once the filed file, this one is seen allotted the position of its marker to " faux".

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