Not assembly

In Informatique, a not of assembly is a repertory from which are accessible the data being in the form of a Filesystem on a partition of Hard drive or a peripheral .

An Unix concept

The concept of point of assembly appears especially in the systems Unix: indeed, in a system Windows, the peripherals of data storage and the partitions are posted like readers independent in top of their clean Arborescence. Under Unix, on the other hand, they are included in the tree structure, because Unix treats also the partitions and peripherals of storage like file S.

If for example a system including/understanding is considered:

* a partition of hard drive where the system (Windows or Unix) is installed;

* a partition of hard drive where the data of the users are;

* a disk drive.

Under Windows, one will reach then these data in a separate way:

* C: : partition system of the hard drive;

* D: : partition user of the hard drive;

* A:: diskette (access-type: A:\chemin\fichier ).

Under Unix, the access will be done starting from the / root:

*/: first partition system of the hard drive;

* /home: partition user of the hard drive;

* /mnt/floppy: diskette (access-type: /mnt/floppy/chemin/fichier).

Several operations are possible there, as for all Filesystem. But two are imperative: the assembly (to be able to use it) and disassembling. Disassembling can be carried out only after all the operations in progress with the partition concerned are finished. This is valid as well for Unix as for Windows.

Example under Windows: One connects a key USB, one writes a large file above, the write time will be considerable and if one clicks on the icon to disconnect it, one will obtain an error message indicating that the operation is not possible for the moment and that it will have to be renewed later on.

If one cuts (physically) the access to the peripheral whereas one did not dismount, one will have:

* an inconsistency system when one reaches again this filing system (example: one physically ejects a CDROM and one injects another different, without informing the system of it),

* a very large risk of corruption of the filesystem associated (except if it is journalized), even risk with physical destruction partial of the media in the case of a Mémoire flash to confirm….

Obviously, if one stops the operating system properly, this one will take care to dismount all correctly that is assembled.

Assembly and disassembling under Unix

When the data are accessible starting from a point from assembly, it is said that the partition or the peripheral is assembled . In the sytèmes Unix, the point of assembly per defect is /mnt or /media. For example, a Disquette will be generally assembled in /mnt/fd0 and a Cdrom in /mnt/cdrom or /media/cdrom. The point of assembly per defect of the peripherals is specified in a file of configuration system: /etc/fstab (under Linux, /etc/vfstab under Solaris).

The Order Unix making it possible to assemble repertories is mount. The opposite order, which dismounts , is umount (and not unmount ).

Assembly

The order mount makes it possible to connect a Partition or a peripheral with a repertory, repertory by which the data present on the partition or the peripheral are accessible.

To assemble a peripheral or a partition with the order mount, it is necessary to indicate:

the type of the Filesystem by the option -t
the non-standard file representing the peripheral or the partition (generally /dev/*);
the repertory of assembly.

For example, the order below mounts the peripheral /dev/cdrom (cédérom) on /media/cdrom by indicating that the filing system is ISO 9660 mount - T iso9660 /dev/cdrom /media/cdrom

Certain indications can be omitted when they are specified in the file of configuration listing the points of assembly per defect (/etc/fstab under Linux, /etc/vfstab under Solaris…). One can omit the type of filesystem if the version of mount used is enough “intelligent”. On the other hand, even by indicating it, one will be able to never assemble a filesystem which the Unix core cannot manage (because it was not configured to use it for example).

When the assembly succeeded, an update is carried out in a system file counting the assemblies in progress (file /etc/mtab under Linux, /etc/mnttab under Solaris). The option -n of mount makes it possible to avoid this update in quite particular cases where the assembly would fail for this reason (if one works on a filing system chrooté in reading alone for example).

One can also under modern Unix assemble files which constitute a filesystem with them-only (loopback), thanks to the option -loop (under Linux, for Solaris there is no particular option, but it is necessary to place before by the order lofiadm). This is particularly useful in the case of images representing of the diskettes, CDROMs, DVDs. The orders dd and mkisofs can help to manufacture such files.

It is possible, under certain configurations, to go up (total or partial covering) over other already assembled systems.

In the same way, by using the system NFS (standard of filing system forwarding by the network), it can be possible to see a filesystem assembled by NFS on the system which one assembles by NFS (re-export), but it is rather rare.

Disassembling

To dismount a partition or a peripheral, the order umount should be used. For example: umount /media/cdrom

Disassembling goes only if the partition is not used, namely:

no file is being read or written on the partition;
no process has its repertory of work on the partition.

If disassembling is refused, one can use the order to fuse to know which processes use it. For example (if the disassembling of /media/cdrom is refused): to fuse /media/cdrom

When disassembling took place, the file /etc/mtab (Linux) or /etc/mnttab (Solaris) is updated.

to be supplemented.

Standard points of assembly under Unix

The hard drive is generally divided into several partitions which are assembled to the starting of the computer. A typical example: