Introduction to file system in OS

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Introduction to File System

A file can be "free formed", indexed or structured collection of related bytes having meaning only to the one who created it. Or in other words an entry in a directory is the file. The file may have attributes like name, creator, date, type, permissions etc.

A file system is a process that manages how and where data on a storage disk, typically a hard disk drive(HDD), is stored , accessed and managed. It is a logical disk component that manages a disk's internal operations as it relates to a computer and is abstract to a human user.
A file system typically manages operations such as storage management , file naming ,Directories /folders , metadata , access rule and privileges.
Commonly used file system include file allocation table 32(FAT), new technology file system(NTFS), and hierarchical file system (HFS).

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.
Some file structure are shown below:-

  • Simple Record Structure with lines of fixed or variable lengths.
  • Complex Structures like formatted document or reloadable load files.
  • No Definite Structure like sequence of words and bytes etc.
File access method 

The way that files are accessed and read into memory is determined by Access methods. Usually a single access method is supported by systems while there are OS's that support multiple access methods.

1. Sequential Access

  • Data is accessed one record right after another is an order.
  • Read command cause a pointer to be moved ahead by one.
  • Write command allocate space for the record and move the pointer to the new End Of File.
  • Such a method is reasonable for tape.

2. Direct Access

  • This method is useful for disks.
  • The file is viewed as a numbered sequence of blocks or records.
  • There are no restrictions on which blocks are read/written, it can be dobe in any order.
  • User now says "read n" rather than "read next".
  • "n" is a number relative to the beginning of file, not relative to an absolute physical disk location.

3. Indexed Sequential Access

  • It is built on top of Sequential access.
  • It uses an Index to control the pointer while accessing files.

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.

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