Operating System - File System
- File
A file is a named collection of related information that is
recorded on secondary storage such as magnetic disks, magnetic tapes and
optical disks. In general, a file is a sequence of bits, bytes, lines or
records whose meaning is defined by the files creator and user.
- File Structure
A File Structure should be according to a required format that
the operating system can understand.
· A
file has a certain defined structure according to its type.
· A
text file is a sequence of characters organized into lines.
· A
source file is a sequence of procedures and functions.
· An
object file is a sequence of bytes organized into blocks that are
understandable by the machine.
· When
operating system defines different file structures, it also contains the code
to support these file structure. Unix, MS-DOS support minimum number of file
structure.
File Type
File type refers to the ability of the operating system to
distinguish different types of file such as text files source files and binary
files etc. Many operating systems support many types of files. Operating system
like MS-DOS and UNIX have the following types of files −
Ordinary files
- These are the
files that contain user information.
- These may have
text, databases or executable program.
- The user can
apply various operations on such files like add, modify, delete or even
remove the entire file.
Directory files
- These files
contain list of file names and other information related to these files.
Special files
- These files
are also known as device files.
- These files
represent physical device like disks, terminals, printers, networks, tape
drive etc.
These files are of two types −
· Character
special files − data is handled character by character as in case of
terminals or printers.
· Block
special files − data is handled in blocks as in the case of disks and
tapes.
File Access Mechanisms
File access mechanism refers to the manner in which the records
of a file may be accessed. There are several ways to access files −
- Sequential
access
- Direct/Random
access
- Indexed
sequential access
Sequential access
A sequential access is that in which the records are accessed in
some sequence, i.e., the information in the file is processed in order, one
record after the other. This access method is the most primitive one. Example:
Compilers usually access files in this fashion.
Direct/Random access
· Random
access file organization provides, accessing the records directly.
· Each
record has its own address on the file with by the help of which it can be
directly accessed for reading or writing.
· The
records need not be in any sequence within the file and they need not be in
adjacent locations on the storage medium.
Indexed sequential access
- This mechanism
is built up on base of sequential access.
- An index is
created for each file which contains pointers to various blocks.
- Index is
searched sequentially and its pointer is used to access the file directly.
Space Allocation
Files are allocated disk spaces by operating system. Operating
systems deploy following three main ways to allocate disk space to files.
- Contiguous
Allocation
- Linked
Allocation
- Indexed
Allocation
Contiguous Allocation
- Each file
occupies a contiguous address space on disk.
- Assigned disk
address is in linear order.
- Easy to
implement.
- External
fragmentation is a major issue with this type of allocation technique.
Linked Allocation
- Each file
carries a list of links to disk blocks.
- Directory
contains link / pointer to first block of a file.
- No external
fragmentation
- Effectively
used in sequential access file.
- Inefficient in
case of direct access file.
Indexed Allocation
- Provides
solutions to problems of contiguous and linked allocation.
- A index block
is created having all pointers to files.
- Each file has
its own index block which stores the addresses of disk space occupied by
the file.
- Directory
contains the addresses of index blocks of files.
