Transcript Presentation - lsp4you.com

SOCSAMS e-learning
FILE SYSTEM
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Introduction
File System is a mechanism which is provided by the
operating system to store and retrieve files on the disk.
This
system
is
primarily
responsible
for
the
management and organization of various files in a
system.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Concept
A file is a collection of related data stored as a named
unit on the secondary storage. It can store different
types of data, like text, graphics, database, executable
code, sound, videos, etc and on the basis of the data, a
file can be categorized as a data file, graphic file,
database file, executable file, sound file, video file,
etc.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Attributes
A file in a system is identified by its name. The file name helps
a user to locate a specific file in the system.
Different operating systems follow different file naming
conventions.
File attributes help the file system to manage a file within the
system. Files attributes may vary in different operating system.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Attributes
• Name
• Size
• Type
• Identifier
• Location
• Date and Time
• Protection
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Attributes
• Name
• Size
Helps to identify and locate a
file in the system.
• Type
• Identifier
• Location
• Date and Time
• Protection
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Attributes
• Name
• Size
• Type
Stores information about the
current size of the file.
• Identifier
• Location
• Date and Time
• Protection
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Attributes
• Name
• Size
• Type
• Identifier
Helps the operating system to
recognize and use the
recommended program to open
a particular file type.
• Location
• Date and Time
• Protection
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Attributes
• Name
• Size
• Type
• Identifier
• Location
A unique tag, usually a number
that helps the file system to
recognize the file within the file
system.
• Date and Time
• Protection
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Attributes
• Name
• Size
• Type
• Identifier
• Location
• Date and Time
A pointer that stores location
information of the device and
location of the file on that
device.
• Protection
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Attributes
• Name
• Size
• Type
• Identifier
• Location
Stores information such as date
and time of creation, last
modification and last use. This
is useful for protection,
security and monitoring.
• Date and Time
• Protection
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Attributes
• Name
• Size
• Type
• Identifier
• Location
• Date and Time
Stores information about the
access permissions (read, write,
execute) of different users.
• Protection
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Operations
File operations are the functions that can be performed on a file.
Operating system handles the file operations through the use of
system calls. Various operations that can be performed on a file
are:
• Create a file
• Open a file
• Write to a file
• Read a file
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Operations
• Seek file
• Close file
• Delete file
• Append file
• Rename file
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Operations
File operations are the functions that can be performed on a file.
Operating system handles the file operations through the use of
system calls. Various operations that can be performed on a file
are:
• Create a file
• Open a file
• Write to a file
“create” system call is used to create a file. When
this system call is used, the operating system searches
for the free space in the file system and allocates it to
the file. In addition, the OS makes a directory entry
to record the name, location and other information
about the file.
• Read a file
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Operations
File operations are the functions that can be performed on a file.
Operating system handles the file operations through the use of
system calls. Various operations that can be performed on a file
are:
• Create a file
• Open a file
• Write to a file
“open” system call accepts the file name and the
access-mode as parameters to open a file and returns
a pointer to the entry in the open-file table. The OS
searches the directory entry for the file name and also
checks the access permission.
• Read a file
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Operations
File operations are the functions that can be performed on a file.
Operating system handles the file operations through the use of
system calls. Various operations that can be performed on a file
are:
• Create a file
• Open a file
• Write to a file
• Read a file
“write” system call accepts the file name and the
data to be written to the file as parameters. The OS
searches the directory entry to locate the file and
writes data to the specified position in the file and
also updates the write pointer to the location where
next write operation is to take place.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Operations
File operations are the functions that can be performed on a file.
Operating system handles the file operations through the use of
system calls. Various operations that can be performed on a file
are:
• Create a file
• Open a file
• Write to a file
“read” system call accepts the file name, amount of
data to be read and a read pointer to point to the
position from where the data is to be read as
parameters. The current position pointer will be
updated after the read operation.
• Read a file
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Operations
• Seek file
• Close file
• Delete file
“seek” system call is used to position the pointer to a
specific position in a file. Once the pointer is
positioned, data can be read from and written to that
position.
• Append file
• Rename file
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Operations
• Seek file
• Close file
• Delete file
• Append file
“close” system call is used to close a file, when all the
operations on that file are completed. The OS erase
the file entry from the open-file table to make space
for the new file entries.
• Rename file
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Operations
• Seek file
• Close file
• Delete file
• Append file
“delete” system call is used to delete a file, when a
file is not required. The OS will erase its
corresponding directory entry also.
• Rename file
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Operations
• Seek file
• Close file
• Delete file
• Append file
• Rename file
“append” system call is used to add data at the end
of an existing file. The system call works similar to
the “write” system call, except that it positions the
pointer to the end of the file and then performs the
write operation.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Operations
• Seek file
• Close file
• Delete file
• Append file
• Rename file
“rename” system call is used to change the name of
an existing file. The OS changes the directory entry
also.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Types
The most common technique to implement file type is by
providing extension to a file.
The file name is divided into two parts separated by a period,
where the first part is the name and the second part after the
period is the file extension.
A file extension is generally one to three characters long. The
extension indicates the type of the file and the operations (read,
write, execute) that can be performed on that file.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File Types
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Access Methods
The information stored in the file can be accessed as follows:
•
Sequential Access
•
Direct Access
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Access Methods
The information stored in the file can be accessed as follows:
•
Sequential Access
•
Direct Access
Information from the file is accessed in order, one record after the other.
Compilers, multimedia applications, sound files, etc. are the most
common examples of programs using sequential access.
In case of a read operation, the record at the location pointed by the file
pointer is read and the file pointer is then advanced to the next record.
In case of write operation, the record is written to the end of the file and
pointer is advanced to end of new record.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Access Methods
The information stored in the file can be accessed as follows:
•
Sequential Access
•
Direct Access
Under direct access, a file is viewed as a sequence of blocks (or
records) which are numbered. The records of a file can be read or
written in any order using this number. The block number is relative to
the beginning of the file. This relative number internally has an actual
absolute disk address and this mapped by the file system. In direct
access, the system calls for read and write operations are using the
block number as parameter.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Directory Structure
To manage the data stored on a disk, the disk is divided into one
more partitions also known as volumes and each partition
contains information about the files in it. This information is
stored in directory.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Operations Performed on Directory
Search for a file
 Create a file
 Delete a file
 List a directory
 Rename a file
 Traverse the file system
It is possible to have more than one operating
system on a single disk, where a user can boot
any of the operating system.

Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Organize the Directory (Logically) to Obtain



Efficiency – locating a file quickly
Naming – convenient to users
 Two users can have same name for different
files
Grouping – logical grouping of files by
properties, (e.g., all Java programs, all games,
…)
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Single-Level Directory

A single directory for all users
Naming problem – No two files can have the same name.
Grouping problem – Only one directory is available for all users.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Two-Level Directory

Separate directory for each user (MFD, UFD)
n Path name
n Can have the same file name for different user
n Efficient searching
n No grouping capability
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Tree-Structured Directories
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Protection
The information stored in a system requires to be
protected from the physical damage and unauthorized
access.
A file system can be damaged due to various reasons
such as, a system breakdown, theft, fire, lightning or
any other nature calamities.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Protection
When the physical damage is irreversible, the data can
be lost permanently.
Physical damage to a system is unavoidable, we must
take proper measures to safeguard and protect the data.
Protection can be provided by storing a copy of
information on the disk to the disk itself, or to some
other removable storage media.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Protection
Apart, from protecting the file from physical damage,
the file in a system also needs a protection mechanism
to control improper access.
In a multi-user system, one user can access files of other
users, and the system is prone to improper access. A
protection mechanism is necessary in such a situation.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Types of Access
Different access rights that can be assigned to a particular user for
a particular file are:
 Read – Allow only reading the file.
 Write – Allow write or rewriting the file.
 Execute – Allow running the program or application.
 Append – Allow writing new information at the end of file.
 Copy – Allow creating a new copy of the file.
 Rename – Allow renaming a file.
 Edit – Allow adding or deleting information from the file.
 Delete – Allow deleting the file and releasing the space.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
Access Control
Access control is to protect the file from improper accesses.
 Password
 Access Control List (ACL)
To resolve the problems associated with ACL, the users can be
categorized as:

Owner

Group

Universe
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File System Structure
File system helps to organize, manage and retrieve data on the
disk. The file system resides permanently on the disk.
Two key issues in the design of file system:
 Defining a file and attributes, operations that can be
performed on a file and the directory structure.
 Creating data structures and algorithms for mapping the
logical file system into the secondary storage device.
Dept. of Computer Applications, MES College Marampally
SOCSAMS e-learning
File System Layers
Dept. of Computer Applications, MES College Marampally