Défragmentation

Goal of the defragmentation

General information on the defragmentation

When the files are very fragmented (i.e. divided into many small elements disseminated on the disc), the reading of these files becomes slow because the play-back heads must continuously move from one fragment to another: one can compare that with the reading of a book whose pages are arranged between the other books of a library, it will take much more time to reconstitute it than if it were “whole”. One should not lose sight of the fact that the reading rate of a disc is definitely lower than the reading in random access memory RAM and than this phenomenon can be determining for the speed of execution of the programs.

The defragmentation thus consists in moving the fragments of files so as to reconstitute homogeneous files and creating great zones of free space on the disc.

He is advised of défragmenter regularly the discs, especially with the primitive filesystems like the system FAT.

Principle of the physical reading on an hard drive and utility of the defragmentation

Perhaps the readers completely not-familiar of the physical processes at the base of the computers would like more on the utility of the defragmentation.

There exists indeed several type of memories for the computers:

the Random access memory
the Mémoire hiding place
the Hard drive, or main memory

If one must do a little anthropomorphism, imagine that one asks the computer to make a complex multiplication. One can then imagine that the memory hiding place represents the logical stages of this multiplication (one multiplies the figure of the units by the first number then one multiplies the figure of tens by the first number,… then one summons all these stages), the random access memory then contains the intermediate results of the multiplication (the stages described above), and the result will be registered on the hard drive, as a result in fine of this calculation.

In other words, the memory hiding place is a logical memory in the short run, the random access memory a total memory in the short run, and the main memory the long-term memory of the computer.

This being, that does occur physically on the hard drive?

In fact the Hard drive is composed of a certain number of magnetized discs, piled up the ones on the others, and that a magnetic head comes to address (i.e. this magnetic head comes to read or write on the magnetic disks). One can completely remember the old turn-discs, the old disc vinyl being now replaced by a magnetic disk and the sapphire head is then replaced by this magnetic head.

The disc is conditioned to receive information, i.e. its surface is fragmented in small separated elements, called blocks, which the head comes to address. At the time of the writing, nothing guarantees that the head uses blocks physically close on surface to the magnetized disc… It is this phenomenon which one calls fragmentation.

During the reading of such fragmented blocks (which for example recorded the stage of starting of an unspecified program), one includes/understands well that the play-back head will have to go to seek with the clean direction the various blocks scattered on the surface of the disc. In fact, this stage will require more time than the reading of the same number of blocks if they were adjacent.

Thus, the setting in compact sector (with the physical, or geographical direction on the surface of the disc) of relative informations to the same application saves time with the reading of information. Moreover, when information is gathered on the disc, the play-back head do not become exhausted with going to seek them anywhere!

One speaks then about defragmentation for this setting in compact sector of information on an hard drive.

Problems involved in the defragmentation

Presence of system files irremovable (or files which the defragmentor does not move in order to simplify the task), especially a file Swap, can block the defragmentation.

Some Filesystems show a greater susceptibility to fragmentation than of others, for example, a filesystem containing FAT splits up rather quickly. More evolved/moved filesystems, like NTFS or Ext2 fs resist fragmentation much more. These systems try to keep fragmentation with the lower part of a certain point so that the defragmentation is not necessary. This resistance to fragmentation functions well as long as the filesystem has a sufficiently high open disk space.

On systems without resistance to fragmentation, fragmentation is only developing when one is not interested in it, a periodic defragmentation is thus necessary to keep performances disc of point and to avoid the abusive accesses due to a less frequent defragmentation.

Utilities

The programs of defragmentation are often included in the utilities installed with the operating system (although Windows NT in did not include). They are accessible since the Démarrer menu - > Programmes - > Accessoires - > Outils systems - > Défragmenteur from disc since Windows XP.

One of the most known utilities is that included with MS-DOS 6.x and Windows 9x: Defrag which was a version “lite” of Norton Speed disk. Since Windows 2000, it is about a version “lite” of Diskeeper.

There exist many other software of defragmentation, commercial or not. In the field of the Free software, let us quote jkDefrag for Windows of Jeroen Kessel .

Site of JkDefrag
JkDefrag in French

Let us quote also PageDefrag of Mark Russinovich (repurchased by Microsoft), which is not strictly speaking a software of defragmentation, but which allows the starting of the system of défragmenter the system files, otherwise " indéfragmentables" because locked by the system.

Site of PageDefrag

External bonds

Tutorial on the method to be used to launch a defragmentation

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