Prepared By: Jerusalem Y.

File System 5/9/2019 1

 Fundamental concepts (data, metadata, operations,
organization, buffering, sequential vs. non sequential files)
 Content and structure of directories
 File system techniques (partitioning, mounting and un
mounting, virtual file systems)
 Memory-mapped files
 Special-purpose file systems
 Naming, searching, and access
 Backup strategies

File System 5/9/2019 2

 In computing, a file system is used to control how data is
stored and retrieved.
 Without a file system, information placed in a storage medium
would be one large body of data with no way to tell where one
piece of information stops and the next begins.
 By separating the data into pieces and giving each piece a
name, the information is easily isolated and identified.
 Taking its name from the way paper-based information
systems are named, each group of data is called a "file".
 The structure and logic rules used to manage the groups of
information and their names is called a "file system".

File System 5/9/2019 3

 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.
 Data is a set of values of qualitative or quantitative variables.
 Metadata is data (information) that provides information
about other data.
 In the 2010s, metadata typically refers to digital forms, but
traditional card catalogues contain metadata, with cards
holding information about books in a library (author, title,
subject, etc.)

File System 5/9/2019 4

Typical operations include the following:
 Create: A new file is defined and positioned within the
structure of files.
 Delete: A file is removed from the file structure and destroyed.

 Open: An existing file is declared to be “opened” by a

process, allowing the process to perform functions on the file.
 Close: The file is closed with respect to a process, so that the
process no longer may perform functions on the file, until the
process opens the file again.
 Read: A process reads all or a portion of the data in a file.

 Write: A process updates a file, either by adding new data that

expands the size of the file or by changing the values of
existing data items in the file.

File System 5/9/2019 5

 File organization refer to the logical structuring of the records
as determined by the way in which they are accessed.
 The basic operations that a user or application may perform on
a file are performed at the record level
◦ The file is viewed as having some structure that organizes
the records
 The physical organization of the file on secondary storage
depends on the blocking strategy.

File System 5/9/2019 6

 Important criteria include:
◦ Short access time
◦ Ease of update
◦ Economy of storage
◦ Simple maintenance
◦ Reliability
 For example, if a file is only to be processed in batch mode,
with all of the records accessed every time, then rapid access
for retrieval of a single record is of minimal concern.
 A file stored on CD-ROM will never be updated, and so ease
of update is not an issue.

File System 5/9/2019 7

 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.
 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 System 5/9/2019 8

 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.

File System 5/9/2019 9

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

1. Character special files − data is handled character by

character as in case of terminals or printers.
2. Block special files − data is handled in blocks as in the case
of disks and tapes.

File System 5/9/2019 10

 Four terms are in common use when discussing files structure:
• Field
• Record
• File
• Database
1. Field
 A field is the basic element of data.
 An individual field contains a single value, such as an
employee’s last name or a date.
 It is characterized by its length and data type (e.g., ASCII
string, decimal).
 Depending on the file design, fields may be fixed length or
variable length.
File System 5/9/2019 11
 In the latter case, the field often consists of two or three
subfields: the actual value to be stored, the name of the field,
and, in some cases, the length of the field.
 In other cases of variable-length fields, the length of the field
is indicated.
2. Record
 A record is a collection of related fields that can be treated as
a unit by some application program.
 For example, an employee record would contain such fields as
name, social security number, job classification, date of hire,
and so on.

File System 5/9/2019 12

3. File
 A file is a collection of similar records.
 The file is treated as a single entity by users and applications
and may be referenced by name.
 Files have file names and may be created and deleted.
 Access control restrictions usually apply at the file level.
 That is, in a shared system, users and programs are granted or
denied access to entire files.
 In some more sophisticated systems, such controls are
enforced at the record or even the field level.

File System 5/9/2019 13

4. Database
 A database is a collection of related data.
 The essential aspects of a database are that the relationships
that exist among elements of data are explicit and that the
database is designed for use by a number of different
applications.
 A database may contain all of the information related to an
organization or project, such as a business or a scientific study.

File System 5/9/2019 14

 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.

File System 5/9/2019 15

Information contained in a device directory are
 Name

 Type

 Address

 Current length

 Maximum length

 Date last accessed

 Date last updated

 Owner id

 Protection information

File System 5/9/2019 16

 Operation performed on directory are:
 Search for a file

 Create a file

 Delete a file

 List a directory

 Rename/update a file

1. Search: When a user or application references a file, the

directory must be searched to find the entry corresponding to
that file.
2. Create file: When a new file is created, an entry must be
added to the directory.

File System 5/9/2019 17

3. Delete file: When a file is deleted, an entry must be removed
from the directory.
4. List directory: All or a portion of the directory may be
requested. Generally, this request is made by a user and
results in a listing of all files owned by that user, plus some
of the attributes of each file (e.g., type, access control
information, usage information).
5. Update directory: Because some file attributes are stored in
the directory, a change in one of these attributes requires a
change in the corresponding directory entry.

File System 5/9/2019 18

 The simple list is not suited to support these operations.
 Consider the needs of a single user. The user may have many
types of files, including word-processing text files, graphic
files, spreadsheets, and so on.
 The user may like to have these organized by project, by type,
or in some other convenient way.
 If the directory is a simple sequential list, it provides no help
in organizing the files and forces the user to be careful not to
use the same name for two different types of files.
 The problem is much worse in a shared system. Unique
naming becomes a serious problem. Furthermore, it is difficult
to hide portions of the overall directory from users when there
is no inherent structure in the directory.

File System 5/9/2019 19

 A start in solving these problems would be to go to a two-level
scheme.
 In this case, there is one directory for each user, and a master
directory.
 The master directory has an entry for each user directory,
providing address and access control information.
 Each user directory is a simple list of the files of that user. This
arrangement means that names must be unique only within the
collection of files of a single user, and that the file system can
easily enforce access restriction on directories.
 However, it still provides users with no help in structuring
collections of files.
 A more powerful and flexible approach, and one that is almost
universally adopted, is the hierarchical, or tree-structure, approach.
File System 5/9/2019 20
File System 5/9/2019 21
 Efficiency: A file can be located more quickly.
 Naming: It become 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.
Problems
 Naming problem: Users cannot have same name for two
files.
 Grouping problem: Users cannot group files according to
their need.

File System 5/9/2019 22

 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
1. 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.

File System 5/9/2019 23

2. Direct/Random access
 Random access file organization provides, accessing the
records directly.
 Each record has its own address on the file with 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.
3. 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.

File System 5/9/2019 24

 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
1. 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.

File System 5/9/2019 25

2. 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.
3. 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.

File System 5/9/2019 26

◦ Name
 symbolic file-name, only information in human-
readable form
◦ Identifier
 Unique tag that identifies file within file system; non-
human readable name
◦ Type -
 for systems that support multiple types
◦ Location -
 pointer to a device and to file location on device
◦ Size -
 current file size, maximal possible size
◦ Protection -
 controls who can read, write, execute
◦ Time, Date and user identification
 data for protection, security and usage monitoring
◦ Information about files are kept in the directory structure,
maintained on disk
File System 5/9/2019 27
 Users need to be able to refer to a file by a symbolic name.
 Clearly, each file in the system must have a unique name in
order that file references be unambiguous.
 On the other hand, it is an unacceptable burden on users to
require that they provide unique names, especially in a shared
system.
 The use of a tree-structured directory minimizes the difficulty
in assigning unique names.
 Any file in the system can be located by following a path from
the root or master directory down various branches until the
file is reached.
 The series of directory names, terminating in the file name
itself, constitutes a pathname for the file.

File System 5/9/2019 28

File System 5/9/2019 29
 Although the pathname facilitates the selection of file names,
it would be awkward for a user to have to spell out the entire
pathname every time a reference is made to a file.
 Typically, an interactive user or a process has associated with
it a current directory, often referred to as the working
directory.
 Files are then referenced relative to the working directory.
 For example, if the working directory for user B is “Word,”
then the pathname Unit_A/ABC is sufficient to identify the
file in the lower left-hand corner of Figure above.
 When an interactive user logs on, or when a process is created,
the default for the working directory is the user home
directory.
 During execution, the user can navigate up or down in the tree
to change to a different working directory.

File System 5/9/2019 30

 A file extension or file name extension is the ending of a file
that helps identify the type of file in operating systems, such
as Microsoft Windows.
 In Microsoft Windows, the file name extension is a period that
is often followed by three characters, but may also be one,
two, or four characters long.
Examples
Picture files
 .bmp
 .gif
 .jpg
Music/Sound files
 .mp3
 .wav

File System 5/9/2019 31

Operating system files
 .dll

 .exe

Text and Word processing documents

 .doc

 .docx

 .rtf

 .txt

Web Page files

 .htm

 .html

File System 5/9/2019 32

1. Briefly discuss about file system techniques (partitioning,
mounting and un mounting, virtual file systems)
2. Briefly discuss about memory-mapped files
3. Briefly discuss about special-purpose file systems
4. Briefly discuss about backup strategies

File System 5/9/2019 33