This document discusses file sharing and secondary storage management in operating systems. It covers several topics: File sharing allows multiple users to access files, but access rights and simultaneous access must be managed. Access rights include permissions levels from none to deletion. Simultaneous access requires enforcing mutual exclusion to prevent conflicts. Secondary storage management involves allocating blocks to files from available disk space. File allocation methods include contiguous, chained, and indexed allocation. Contiguous allocates all blocks at once while chained uses pointers between non-contiguous blocks. Indexed addresses problems with the other methods. Free space is managed using techniques like bit tables to track used/free blocks, chained free portions, or a free block list maintained on disk

1. SUJITHA.M
I-M.SC(CS&IT)
NADAR SARASWATHI COLLEGE OF ARTS
AND SCIENCE,THENI.
FILE SHARING, SECONDARY STORAGE MANAGEMENT
FILE ALLOCATION

2. FILE SHARING
 In a multiuser system, there is a requirement for
allowing files to be shared among a number of
users.
 There two issues may arise.
 They are:
Access Rights
Management of simultaneous Access

3. ACCESS RIGHTS
 The file system provides a flexible tool for allowing file
sharing among the users.
 It provides a number of options, so that the files can be
accessed and controlled easily by the users.
 So that the user has rights to access the file which
provides at a wide range.
 There are certain lists that access rights which is
assigned to a particular user for a particular file.

4.  NONE:
If the user does not know the existence of the file,
he would not be allowed to read the user directory that
is included in this file.
 KNOWLEDGE:
The user can determine the exist file and the owner
of the existing file.
 EXECUTION:
The user can load and execute the file but that file
cannot be copied .
 READING:
The user can read , copy and execute for any purpose.

5.  APPENDING:
The user can insert data to the file, but the process
of modification and deletion is restricted.
 UPDATING:
The user can modify, delete and add data to the file.
 CHANGING PROTECTION:
The user can change the access rights which is
granted to the other user.
 DELETION:
The user can delete the file from the file system.

6. ACCESS THAT ARE PROVIDED IN
DIFFERENT CLASSES
 SPECIFIC USER:
The individual users who are assigned by the
user ID.
 USERGROUPS:
A set of users who are not defined individually.
 ALL:
These are the public files which is assigned by
all users.

7. SIMULTANEOUS ACCESS
 When access allowed to append or update a file
more than one user, the operating system must
enforce discipline.
 A brute force approach is to allow a user to lock the
entire file when it is to be updated.
 Issues of mutual exclusion and deadlock must be
addressed in designing the shared access capability.

8. SECONDARY STORAGE MANAGEMENT
 On Secondary storage ,a file consists of a collection of
blocks .
 The Operating system or file management system is
responsible for allocating blocks to the files .
 There are two management issues:
 Secondary storage must be allocated to the files.
 It is necessary to keep track of the space which is
available for the allocation.

9. FILE ALLOCATION
 The file allocation can be done using a single disk.
 The several issues are:
 When a new file is created, it requires the maximum
space.
 The space is allocated to a file as one or more
contiguous units.
 When a sort of data structure or table is used to keep
the portions which is assigned to a file.
 Such table is called as “File Allocation Table”.

10. PORTION SIZE
 The major issue is the size of the portion which is
allocated to a file.
 It is used to hold the entire file which is allocated.
 The major alternatives are:
VARIABLE,LARGE CONTIGUOUS PORTION:
The variable size avoids waste and the file
allocation tables are small.
BLOCKS:
 The small fixed portion provides greater flexibility.
 They may require large tables or complex structures for
allocation, so blocks are created when they are needed.

11. FILE ALLOCATION METHODS
 The specific allocation methods are:
 Contiguous Allocation
 Chained Allocation
 Indexed Allocation
 A single contiguous set of blocks is allocated to a file at
the time of file creation is called as “ Contiguous
Allocation”.
 The opposite extreme from contiguous allocation is
called as “Chained Allocation”.
 “Indexed Allocation” is used to address many problems
of contiguous and chained allocation.

12. File Allocation Methods
Contiguous Chained Indexed
Preallocation ? Necessary Possible Possible
Fixed or variable
size portions ?
Variable Fixed blocks Fixed Variable
blocks
Portion size Large Small Small Medium
Allocation
frequency
Once Low to high High Low
Time to allocate Medium Long Short Medium
File allocation
table size
One entry One entry Large Medium

13. FREE SPACE MANAGEMENT
 The space should be managed when the files are
allocated.
 The “Disk Allocation Table” is used to know what
blocks are available in the disk.
 The techniques that are implemented are:
BIT TABLES:
 This method is uses vector containing one bit for each
block on the disk.
 Each entry of 0 corresponds to a free block and 1
corresponds to a block in use.

14. CHAINED FREE PORTIONS:
Each free portion may be chained together using a
pointer and length value.
INDEXING:
The indexing approach treats free space as a file.
FREE BLOCK LIST:
 Each block is assigned as a number sequentially.
 The list of numbers of all free blocks is maintained in a
reserved portion of the disk.
 24 or 32 bits are needed to store a single block of
number.

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