This document discusses file systems and how they are implemented in operating systems. It introduces the need for file systems due to limitations in process address spaces. There are three main requirements for long-term information storage: storing large amounts of data, persistence of data, and concurrent access. Files fulfill these requirements through hierarchical directory structures and various allocation methods like contiguous, linked lists, and inode-based systems. The document provides examples of these allocation techniques and how they map files to disk blocks.

1. Rushdi Shams, Dept of CSE, KUET 1
Operating SystemsOperating Systems
File SystemsFile Systems
Version 1.0

2. Rushdi Shams, Dept of CSE, KUET 2
Introduction
• Processes can store a limited amount of information
within its own address space.
• For some applications this size is adequate but for
others (airline reservations, banking), the space is
too small

3. Rushdi Shams, Dept of CSE, KUET 3
Introduction
• Information stored must survive the termination of
the process using it. Typically, processes were
designed in a way such that after their usage on
variables, the values are lost when processes release
them.
• Inappropriate for applications like database systems

4. Rushdi Shams, Dept of CSE, KUET 4
Introduction
• Multiple processes must be able to access the
information concurrently. Truly speaking, while a
process operates on an information, it was not
accessible for others

5. Rushdi Shams, Dept of CSE, KUET 5
Introduction
• So, we have three essential requirements for long-
term information storage-
1. Store a large amount of information
2. The information must be stored for a long time
3. Multiple processes must be able to access the
information concurrently
• These requirements led us towards keeping the
information in files

6. Rushdi Shams, Dept of CSE, KUET 6
Introduction
• Information in files must be persistent- process
termination and creation must not affect them
• Only the owner can modify/delete the information if
she likes.

7. Rushdi Shams, Dept of CSE, KUET 7
File Naming
When a process creates a file, it gives a file name
When the process terminates, the file continues to
exist and can be accessed by other processes using its
name.
Some file systems distinguish between upper and
lower case letters, whereas others do not.
Are the file names cat.doc, cAt.doc, caT.doc and
CAT.doc same in windows? What about in UNIX?

8. Rushdi Shams, Dept of CSE, KUET 8
File Naming
Many OS support two-part file names- two parts
separated with a period (.).
The part after the period is called file extension.
Filename extensions can be considered a type of
metadata.
They are commonly used to infer information about
the way data might be stored in the file.

9. Rushdi Shams, Dept of CSE, KUET 9
File Structure
 Generally, there are three kinds of files-
1. Byte Sequence
2. Record Sequence
3. Tree Sequence

10. Rushdi Shams, Dept of CSE, KUET 10
File Types
 Generally, there are two
types of files-
1. ASCII files
2. Binary files
 ASCII files are easy to
understand if you use a text
editor whereas binary files
are completely glibberish if
you print them

11. Rushdi Shams, Dept of CSE, KUET 11
File Access Strategies
Sequential access
read all bytes/records from the beginning
cannot jump around, could rewind or back up
Random access
bytes/records read in any order
essential for data base systems
read can be …
 move file marker (seek), then read or …
 read and then move file marker

12. Rushdi Shams, Dept of CSE, KUET 12
File Access Strategies
There are also several file usage patterns.
Most files are small (e.g., .login and .c files), and most
file references are too small files.
Large files use up most of the disk space (e.g., mp3
files).
Large files account for most of the bytes transferred
between memory and disk.

13. Rushdi Shams, Dept of CSE, KUET 13
File Access Strategies
These usage patterns are bad news for file system
designers. 
To achieve high performance, a designer needs to
make sure that small files are accessed efficiently,
since there are many of them, and they are used
frequently.
A designer also needs to make sure that large files are
accessed efficiently, since they consume most of the
disk space, and account for most of the data
movement.

14. Rushdi Shams, Dept of CSE, KUET 14
Directories
 To keep track of files, file systems normally have
directories or folders
 In many systems directories themselves are files!
 Simply put, there are three types of directory
systems-
1. Single-level directory systems
2. Two-level directory systems
3. Hierarchical directory systems

15. Rushdi Shams, Dept of CSE, KUET 15
Single-level directory system
Simplest form of directory
system
One directory contains all the
files
Sometimes called root directory
Good choice when number of
user is only one
The directory in figure has two
owners- A and B having four
files
Problem- different users may
accidentally use same names for
their files

16. Rushdi Shams, Dept of CSE, KUET 16
Two-level directory system
Each user will have
separate directories
containing their owned
files
The problem with
naming is resolved.
Log in procedure is
required
May allow one user to
access others’ files

17. Rushdi Shams, Dept of CSE, KUET 17
Hierarchical Directory Systems
The problem of two-level
directory system is the
absence of grouping facility
You may group your works
like- games in one place,
movies in the other.
Hierarchical directory
systems have the facility of
two-level directory systems
plus the organization facility

18. Rushdi Shams, Dept of CSE, KUET 18
File System Layout
File systems are stored on disks
Disks are divided up into one or more partitions
Each partition has independent file systems
Sector 0 of the disk is called Master Boot Record
(MBR)
Computer uses MBR to boot itself
The end of MBR contains the partition table- that has
the start and end address of each partition

19. Rushdi Shams, Dept of CSE, KUET 19
File System Layout
One of the partitions in the partition table is marked
as active
When computer is booted, the BIOS reads in and
executes MBR
The first thing MBR program does is to locate the
active partition
The first block of this active partition is Boot Block.
MBR reads and executes it.
The program in Boot Block loads the OS contained in
that partition

20. Rushdi Shams, Dept of CSE, KUET 20
File System Layout
Every partition starts with a Boot Block
But only the partition having the OS will be marked
as active
Other partitions hold the Boot Block as you can
change your mind and copy another version of the OS
into one of them

21. Rushdi Shams, Dept of CSE, KUET 21
File System Layout

22. Rushdi Shams, Dept of CSE, KUET 22
File System Layout
Super Block contains all the key parameters about the file
system
It is read into memory when the computer is booted or the file
system is first touched
Typically holds a Magic Number to identify the file system type
(.exe or .bmp?), number of blocks, etc

23. Rushdi Shams, Dept of CSE, KUET 23
File System Layout
Free space management reveals the free blocks in the file
system
i-nodes is an array of data structures, one per file, simply telling
about the file
Root directory contains the top of the file system tree
Remainder of the disk contains all other directory and files for
that partition

24. Rushdi Shams, Dept of CSE, KUET 24
Implementing Files:
Contiguous Allocation
Simplest allocation scheme to store each file as a
contiguous run of disk blocks
Disk with 1 KB blocks, a 50 KB file would need 50
consecutive blocks; Disk with 2 KB blocks would
make it okay with 25 consecutive blocks

25. Rushdi Shams, Dept of CSE, KUET 25
Implementing Files:
Contiguous Allocation

26. Rushdi Shams, Dept of CSE, KUET 26
Implementing Files:
Contiguous Allocation
Advantages
Simple to implement 
You only need to know the disk address of the first
block and the distance of the file block from it
Read performance is excellent, as you need only two
seek operations (boot block and file block) and one
read operation (one contiguous stream of bytes)
Disadvantages
Disks become fragmented
Wastage of storage is possible

27. Rushdi Shams, Dept of CSE, KUET 27
Implementing Files:
Linked List Allocation
Every block will have two sections- a pointer to next
one (the first word of the block) and data (the rest of
the words)
Unlike contiguous allocation, every disk block can be
used in this method
No space is lost due to disk fragmentation

28. Rushdi Shams, Dept of CSE, KUET 28
Implementing Files:
Linked List Allocation
Reading a file sequentially is straight forward but
random access is slow.
To get block n, the OS has to start at the beginning
and read the n-1 blocks prior to it
Overhead due to pointer information

29. Rushdi Shams, Dept of CSE, KUET 29
Implementing Files:
Linked List Allocation

30. Rushdi Shams, Dept of CSE, KUET 30
Linked List Allocation:
using Table in Memory
The disadvantages are eliminated by taking the
pointer word from each block and putting it into a
table in memory
Such a table in main memory is called FAT (File
Allocation Table)

31. Rushdi Shams, Dept of CSE, KUET 31
Linked List Allocation:
using Table in Memory

32. Rushdi Shams, Dept of CSE, KUET 32
Linked List Allocation:
using Table in Memory
Entire block is available for data (no pointer
information)
Faster random access
The primary disadvantage-
the table must be in memory all the time to make it work
a 20 GB disk and with 1 KB block size, the table needs 20
million entries one for each of 20 million blocks
each entry is 4 bytes long
so, FAT will occupy 80 MB space in memory all the time

33. Rushdi Shams, Dept of CSE, KUET 33
i-nodes
It is a data structure that lists the attributes and disk
addresses of the file’s blocks
With i-nodes, it is possible to find all the blocks of a
file
i-nodes need only be in memory when the
corresponding file is open
If each i-node contains n-bytes and there are k-files
are opened at a time, that instant, kn-bytes are
reserved in memory

34. Rushdi Shams, Dept of CSE, KUET 34
i-nodes

35. Rushdi Shams, Dept of CSE, KUET 35
i-nodes
This array is usually far smaller than the space
occupied by the FAT
Table for holding the linked list is proportional to
disk itself.
If the disk has n blocks, the table needs n-entries
As the disks grow, the table grows linearly with them
i-node requires an array whose size is proportional to
the maximum number of files that may be open at
once (it does not matter if you have 1 petabyte HDD
)

36. Rushdi Shams, Dept of CSE, KUET 36
i-nodes
The disadvantage is if you fix the size of i-node and
your files have a tendency to grow larger (obviously
you cannot say your file will be always 6MB in size )