CS File Management Ref

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Computer Systems


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Content

Understand

What is a file and a filesystem?

File Attributes File Operations

What is a directory?

The contents here are for Aricent Group internal training purposes only and do not carry any commercial value 2Restricted Aricent Group 2011

Filesystem implementation

Overview of UNIX and DOS


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Files

The computer manipulates and stores information.

The information is stored either in main memory or secondary storage

i.e. disks in the form of files.

Consider an analogy of attendance register used by professor in the college fortaking attendance. Each professor takes attendance in his individual attendance

register for his session.

The attendance register is a file which contains information about the attendance


o eac s u en .

In the analogy, it is clear that any organized information stored persistently is a

file.

A file is a logical unit to store any organized information e.g. telephone

directory, to store a video etc.


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The OS allows users to manage files with filesystem.

A Filesystem is a logical arrangement of files.

To uniquely identify an attendance register, each attendance register is given aname to access it easily.

Similarly each file is given a name for a user to uniquely identify it.

Since each professor is having his own attendance register for his session for aparticular batch of students, the attendance registers are organized in a specificorder when submitted in a office to access it efficiently.

The attendance registers from all professors taking session for a batch i.e.

File System


B.Tech.2n year, Electronics branch are kept together under Electronics branchcategory ,which is same as files related to a particular computer application arekept in a directory.

The directory stores all files related in a logical way e.g. files belonging to a specificcomputer application.

For the entire batch of B.Tech ,all files are kept under B.Tech.category which isagain a directory on top of branch wise category (directory).

In the analogy it clearly states files and directories are logically arranged in ahierarchical order.


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File System

Courses

B.Tech. M.Tech. MBA

Electronics IT IT Marketing Finance


2nd year 2nd year Ist year 1st year2nd year 1st year


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File Operations

The files are used for storing the information.

The file information is stored in inode for UNIX and in file control block (FCB)

for MS-DOS.



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The files are associated with different operations for storing and retrieval of

information from a file.

Consider the same analogy of attendance register ,the attendance register must be

opened for taking attendance of student.

The professor writes as well as reads from register.

The professor close the register.

He deletes the name of the student from his attendance re ister if an student et his

File Operations


admission cancelled.

He adds the name of the new student ,who takes new admission in his class.

Files are also associated with similar types of operation.

User performs various operations e.g.open,read,write,close.


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File related system calls

The system calls related to files are as follows:

Create

The file is created with no data.

Open

The file is opened before the file is accessed.

Read


.

Write

Date are written to the specific position in a file .

Close

The file is not needed so it is closed, which releases the space occupied by the file.

What are the other operations which user can perform on a file and the

related system calls?


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Directories

File system organize files in directories.

The directories are arranged in a hierarchical order.

The topmost directory is called as a root directory.

/

B C D

Root Directory


FE G IH J

K L

MFile


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Directories

When files are organized hierarchically, some way is needed for

specifying file names.

There are two methods:

Absolute path name

It consist of the path from the root directory to the file.

For example: The path /B/E/K/M means that root directory contains a


, ,

subdirectory K,which contains file M.

Relative path name

The pathnames are taken relative to the working directory.

A user can designate one directory as the current working directory.

For example: If the current working directory is K then to access file M ,where

the absolute pathname is /B/E/K/M ,the relative pathname is M.

If the current working directory is E,the relative pathname is K/M


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File System

File system organizes the files on disk.

There are different filesystem in different OS.

FAT: old windows and MS-DOS standard

NFS : windows current standard

FFS : UNIX standard



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File System Layout

MBR

Entire disk

Partition Table Disk Partition

Restricted Aricent Group 2011The contents here are for Aricent Group internal training purposes only and do not carry any commercial value 13Restricted Aricent Group 2011

Boot Block Super Block Free Space

Management

I-nodes Root

Directory

Files and

Directories


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File System Implementation

File Systems are stored on disks.

Disks are divided up into multiple partition with independent file systems on

each partition.

Master Boot Record (MBR)

It is used to boot the computer and is sector 0 on the disk.

When the computer is booted, the BIOS reads in and executes the MBR.


The MBR program reads the first block called the boot block . Partition table

It has the starting and ending address of each partition.

Boot Block

The program (bootstrap loader) in the boot block loads the OS contained in the

partition. In UNIX it is known as boot block and in Windows it is called partition

boot sector.


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File System Implementation

Superblock

It has information about the filesystem which is read into memory when computer is

booted .

The information is as follows:

Block size

How much data a block can hold.


For example: The attendance register page is a block and the size of a page in attendance

register is same as block size.

Free blocks

Used blocks

In UNIX it is called as superblock and in windows the information of superblock isstored in master file table.

What other information a superblock has for filesystem so that files are managed

effectively and efficiently?


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MS-DOS

The files and directories are arranged in a hierarchical directory system.

Disk block is some multiple of 512 bytes.

It has a 32-byte directory entry which contains the

filename,attributes,creation date and time, starting block and file size.


File name

Extension Attributes

Time

Reserved

Date First Block

Number Size

8 3 1 10 2 2 2 4


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MS-DOS

It use FAT (File Allocation Table) for allocating blocks on disk.

MS-DOS keeps track of file blocks via FAT in main memory.

The directory entry contains the number of the first file block.

Each file is a linked list of disk blocks.

The disk blocks may or may not be contiguous.



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UNIX

The files and directories are arranged in a hierarchical directory system.

A UNIX directory entry contains one entry for each file in that directory.

A directory entry contains only two fields the file name(14 bytes) and the

inode number for that file(2 bytes).

Bytes(2) Bytes(14)


i-node number File name

Directory entry


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References

Chapter 10,11,OS Principles ,Silberschatz,Galvin,Gagne

Chapter 6,Modern Operating Systems,Tanenbaum



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Aricent Group makes no representations or warranties with respect to

contents of these slides and the same are being provided as is. The

content/materials in the slides are of a general nature and are not intended to address the specific circumstances of any particular

individual or entity. The material may provide links to internet sites (for

the convenience of users) over which Aricent Group has no control and

Disclaimer


for which Aricent Group assumes no responsibility for the availability orcontent of these external sites. While the attempt has been to

acknowledge sources of materials wherever traceable to an individual or

an institution; any materials not specifically acknowledged is purely

unintentional


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