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Operating Systems: File Systems
File systems are an integral part of
any operating systems with the capacity
for long term storage. There are two distinct parts of a file system, the
mechanism for storing files and the directory structure into which they are
organised. In mordern operating systems where it is possibe for several user to
access the same files simultaneously it has also become necessary for such
features as access control and different forms of file protection to be
implemented.
A file is a collection of binary data. A file could represent a program, a
document or in some cases part of the file system itself. In modern computing
it is quite common for their to be several different storage devices attached
to the same computer. A common data structure such as a file system allows the
computer to access many different storage devices in the same way, for example,
when you look at the contents of a hard drive or a cd you view it through the
same interface even though they are completely different mediums with data
mapped on them in completely different ways. Files can have very different data
structures within them but can all be accessed by the same methods built into
the file system. The arrangment of data within the file is then decided by the
program creating it. The file systems also stores a number of attributes for
the files within it.
All files have a name by which they can be accessed by the user. In most modern
file systems the name consists of of three parts, its unique name, a period and
an extension. For example the file 'bob.jpg' is uniquely identified by the
first word 'bob', the extension jpg indicates that it is a jpeg image file. The
file extension allows the operating system to decide what to do with the file
if someone tries to open it. The operating system maintains a list of file
extension associations. Should a user try to access 'bob.jpg' then it would
most likely be opened in whatever the systems default image viewer is.
The system also stores the location of a file. In some file systems files can
only be stored as one contigious block. This has simplifies storage and access
to the file as the system then only needs to know where the file begins on the
disk and how large it is. It does however lead to complications if the file is
to be extended or removed as there may not be enough space available to fit the
larger version of the file. Most modern file systems overcome this problem by
using linked file allocation. This allows the file to be stored in any number
of segments. The file system then has to store where every block of the file is
and how large they are. This greatly simplifies file space allocation but is
slower than contigious allocation as it is possible for the file to be spread
out all over the disk. Modern oparating systems overome this flaw by providing
a disk defragmenter. This is a utility that rearranges all the files on the
disk so that thay are all in contigious blocks.
Information about the files protection is also integrated into the file system.
Protection can range from the simple systems implemented in the FAT system of
early windows where files could be marked as read-only or hidden to the more
secure systems implemented in NTFS where the file system administrator can set
up separate read and write access rights for different users or user groups.
Although file protection adds a great deal of complexity and potential
difficulties it is essential in an enviroment where many different computers or
user can have access to the same drives via a network or time shared system
such as raptor.
Published: April 24, 2009
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