File sharing

describe about file sharing system

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. USER GROUPS: 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.

15. THANKYOU !!!