❖ A computer file is defined as a medium used for saving and managing data

in the computer system.

❖ The data stored in the computer system is completely in digital format,
although there can be various types of files that help us to store the data.
❖ File systems are a crucial part of any operating system, providing a
structured way to store, organize, and manage data on storage devices such
as hard drives, SSDs, and USB drives.
❖ Essentially, a file system acts as a bridge between the operating system
and the physical storage hardware, allowing users and applications to
create, read, update, and delete files in an organized and efficient manner.

What is a File System?

A file system is a method an operating system uses to store, organize, and manage
files and directories on a storage device. Some common types of file systems
include:
• FAT (File Allocation Table): An older file system used by older
versions of Windows and other operating systems.
• NTFS (New Technology File System): A modern file system used by
Windows. It supports features such as file and folder permissions,
compression, and encryption.
• ext (Extended File System): A file system commonly used
on Linux and Unix-based operating systems.
• HFS (Hierarchical File System): A file system used by macOS.
• APFS (Apple File System): A new file system introduced by Apple for
their Macs and iOS devices.
A file is a collection of related information that is recorded on secondary storage.
Or file is a collection of logically related entities. From the user’s perspective, a file
is the smallest allotment of logical secondary storage.
The name of the file is divided into two parts as shown below:
• Name
• Extension, separated by a period.

Issues Handled By File System

We’ve seen a variety of data structures where the file could be kept. The file
system’s job is to keep the files organized in the best way possible.
A free space is created on the hard drive whenever a file is deleted from it.

To reallocate them to other files, many of these spaces may need to be recovered.
Choosing where to store the files on the hard disc is the main issue with files one
block may or may not be used to store a file.

It may be kept in the disk’s non-contiguous blocks. We must keep track of all the
blocks where the files are partially located.

Files Attributes And Their Operations

Attributes Types Operations

Name Doc Create

Type Exe Open

Size Jpg Read

Creation Data Xis Write

Author C Append

Last Modified Java Truncate

protection class Delete

File type Usual extension Function

Read to run machine

Executable exe, com, bin
language program

Compiled, machine
Object obj, o
language not linked
 File type Usual extension Function

Source code in various

Source Code C, java, pas, asm, a
languages

Commands to the command

Batch bat, sh
interpreter

Text txt, doc Textual data, documents

Various word processor

Word Processor wp, tex, rrf, doc
formats

Related files grouped into

Archive arc, zip, tar
one compressed file

For containing audio/video

Multimedia mpeg, mov, rm
information

It is the textual data and

Markup xml, html, tex
documents

It contains libraries of
Library lib, a ,so, dll
routines for programmers

It is a format for printing or

Print or View gif, pdf, jpg viewing an ASCII or binary
file.

File Directories

☺ The collection of files is a file directory. The directory contains information

about the files, including attributes, location, and ownership.
☺ Much of this information, especially that is concerned with storage, is
managed by the operating system.
☺ The directory is itself a file, accessible by various file management routines.
Below are information contained in a device directory.
• Name
 • Type
• Address
• Current length
• Maximum length
• Date last accessed
• Date last updated
• Owner id
• Protection information
The operation performed on the directory are:
• Search for a file
• Create a file
• Delete a file
• List a directory
• Rename a file
• Traverse the file system

Advantages of Maintaining Directories

• Efficiency: A file can be located more quickly.

• Naming: It becomes convenient for users as two users can have same
name for different files or may have different name for same file.
• Grouping: Logical grouping of files can be done by properties e.g. all
java programs, all games etc.
•

Single-Level Directory

In this, a single directory is maintained for all the users.

• Naming Problem: Users cannot have the same name for two files.
• Grouping Problem: Users cannot group files according to their needs.
 Two-Level Directory

In this separate directories for each user is maintained.

• Path Name: Due to two levels there is a path name for every file to
locate that file.
• Now, we can have the same file name for different users.
• Searching is efficient in this method.

Tree-Structured Directory

The directory is maintained in the form of a tree. Searching is efficient and also
there is grouping capability. We have absolute or relative path name for a file.
 File Allocation Methods

There are several types of file allocation methods. These are mentioned below.
• Continuous Allocation
• Linked Allocation(Non-contiguous allocation)
• Indexed Allocation
Continuous Allocation ;_

A single continuous set of blocks is allocated to a file at the time of file creation.
Thus, this is a pre-allocation strategy, using variable size portions.
The file allocation table needs just a single entry for each file, showing the starting
block and the length of the file.
This method is best from the point of view of the individual sequential file. Multiple
blocks can be read in at a time to improve I/O performance for sequential
processing.
 It is also easy to retrieve a single block. For example, if a file starts at block b, and
the ith block of the file is wanted, its location on secondary storage is simply b+i-
1.
Disadvantages of Continuous Allocation

• External fragmentation will occur, making it difficult to find contiguous

blocks of space of sufficient length. A compaction algorithm will be
necessary to free up additional space on the disk.
• Also, with pre-allocation, it is necessary to declare the size of the file at
the time of creation.
•
Linked Allocation(Non-Contiguous Allocation)

❖ Allocation is on an individual block basis. Each block contains a pointer to

the next block in the chain. Again the file table needs just a single entry for
each file, showing the starting block and the length of the file.
❖ Although pre-allocation is possible, it is more common simply to allocate
blocks as needed.
❖ Any free block can be added to the chain. The blocks need not be continuous.
An increase in file size is always possible if a free disk block is available.
❖ There is no external fragmentation because only one block at a time is
needed but there can be internal fragmentation but it exists only in the last
disk block of the file.
❖
Disadvantage Linked Allocation(Non-contiguous allocation)

• Internal fragmentation exists in the last disk block of the file.

• There is an overhead of maintaining the pointer in every disk block.
• If the pointer of any disk block is lost, the file will be truncated.
• It supports only the sequential access of files.

Indexed Allocation:-

It addresses many of the problems of contiguous and chained allocation. In this

case, the file allocation table contains a separate one-level index for each file: The
index has one entry for each block allocated to the file. The allocation may be on
the basis of fixed-size blocks or variable-sized blocks.
Allocation by blocks eliminates external fragmentation, whereas allocation by
variable-size blocks improves locality. This allocation technique supports both
sequential and direct access to the file and thus is the most popular form of file
allocation.
 ❖ Just as the space that is allocated to files must be managed, so the space that
is not currently allocated to any file must be managed.
❖ To perform any of the file allocation techniques, it is necessary to know what
blocks on the disk are available.
❖ Thus we need a disk allocation table in addition to a file allocation table.
The following are the approaches used for free space management.
• Bit Tables: This method uses a vector containing one bit for each block
on the disk. Each entry for a 0 corresponds to a free block and each 1
corresponds to a block in use.

For example 00011010111100110001

In this vector every bit corresponds to a particular block and 0 implies

that that particular block is free and 1 implies that the block is already
occupied. A bit table has the advantage that it is relatively easy to find
one or a contiguous group of free blocks. Thus, a bit table works well
with any of the file allocation methods. Another advantage is that it is as
small as possible.

• Free Block List: In this method, each block is assigned a number

sequentially and the list of the numbers of all free blocks is maintained
in a reserved block of the disk.

Advantages of File System

• Organization: A file system allows files to be organized into directories
and subdirectories, making it easier to manage and locate files.
 • Data Protection: File systems often include features such as file and
folder permissions, backup and restore, and error detection and
correction, to protect data from loss or corruption.
• Improved Performance: A well-designed file system can improve the
performance of reading and writing data by organizing it efficiently on
disk.
Disadvantages of File System
• Compatibility Issues: Different file systems may not be compatible
with each other, making it difficult to transfer data between different
operating systems.
• Disk Space Overhead: File systems may use some disk space to store
metadata and other overhead information, reducing the amount of
space available for user data.
• Vulnerability: File systems can be vulnerable to data
corruption, malware, and other security threats, which can compromise
the stability and security of the system.