File Systems: Difference between revisions

A filesystem provides a generalized structure over persistent storage, allowing the low-level structure of the devices (e.g., disk, tape, flash memory storage) to be abstracted away. Generally speaking, the goal of a filesystem is allow logical groups of data to be organized into ''files'', which can be manipulated as a unit. In order to do this, the filesystem must provide some sort of index of the locations of files in the actual secondary storage. The fundamental operations of any filesystem are:

* Tracking the available storage space

* Tracking which block or blocks of data belong to which files

* Creating new files

* Reading data from existing files into memory

* Updating the data in the files

* Deleting existing files

(Perceptive readers will note that the last four operations - Create, Read, Update, and Delete, or CRUD - are also applicable to many other data structures, and are fundamental to databases as well as filesystems.)

Additionally, there are other features which go along with a practical filesystem:

* Assigning human-readable names to files, and renaming files after creation

* Allowing files to be divided among non-contiguous blocks in storage, and tracking the parts of files even when they are ''fragmented'' across the medium

* Providing some form of hierarchical structure, allowing the files to be divided into ''directories'' or ''folders''

* Buffering reading and writing to reduce the number of actual operation on the physical medium

* Caching frequently accessed files or parts of files to speed up access

* Allowing files to be marked as 'read-only' to prevent unintentional corruption of critical data

* Providing a mechanism for preventing unauthorized access to a user's files

Additional features may be found on some filesystems as well, such as automatic encryption, or journalling of read/write activity.

=== Indexing Methods ===

There are several methods of indexing the contents of files, with the most commonly used being ''i-nodes'' and ''File Allocation Tables''.

==== Inodes ====

==== FAT ====

The File Allocation Table ([[FAT]]) is the primary indexing mechanism for MS-DOS and it's descendants. There are several variants on FAT, but the general design is to have a table (actually a pair of tables, one serving as a backup for the first in case it is corrupted) which holds a list of blocks of a given size, which map to the whole capacity of the disk.