1. Subject Name: Operating Systems Subject Code:10CS53
Prepared By:Mrs.Jisha Jose,Mrs.Thanu Kurian,Mrs.Preethi Sheba Hepsiba
Department:Computer Science
Date:30/08/2014
11/13/2018

2. File Concept
Access Methods
Directory Structure
File-System Mounting
File Sharing
Protection

3. File Attributes Name – only information kept in human-readable form
Identifier – unique tag (number) identifies file within file system
Type – needed for systems that support different types
Location – pointer to file location on device
Size – current file size
Protection – controls who can do reading, writing, executing
Time, date, and user identification – data for protection, security, and usage monitoring
Information about files are kept in the directory structure, which is maintained on the disk

4. File Operations File is an abstract data type Create Write Read
Reposition within file
Delete
Truncate
Open(Fi) – search the directory structure on disk for entry Fi, and move the content of entry to memory
Close (Fi) – move the content of entry Fi in memory to directory structure on disk

5. Open Files
Several pieces of data are needed to manage open files: File pointer: pointer to last read/write location, per process that has the file open
File-open count: counter of number of times a file is open – to allow removal of data from open-file table when last processes closes it
Disk location of the file: cache of data access information
Access rights: per-process access mode information

6. Open File Locking Provided by some operating systems and file systems
Mediates access to a file
Mandatory or advisory:
Mandatory – access is denied depending on locks held and requested
Advisory – processes can find status of locks and decide what to do

7. File Types – Name, Extension

8. Access Methods Sequential Access read next write next reset
Direct Access
read n
write n
position to n
rewrite n
n = relative block number

9. Sequential-access File

10. Other access method-Index and Relative Files

11. A Typical File-system Organization

12. Directory Structure Operations Performed on Directory
Search for a file
Create a file
Delete a file
List a directory
Rename a file
Traverse the file system

13. Single-Level Directory
-All files are in the same directory
Advantages
Easy to support and understand
Simple
Disadvantages
When no of files increases,all files must have same names

14. Two-Level Directory Advantages Separate directory for each user
Can have the same file name for different user
Efficient searching
Disadvantage
Structure isolated one user from another

15. Tree-Structured Directories

16. Tree-Structured Directories (Cont.)
Advantages
Efficient searching
Allows users to create their own subdirectory
Users can access files of other users also by specifying pathnames
Disadvantages
User have to remember long path names
Path names 2 types
absolute path name-begins at root
relative path name-begins from current directory

17. Acyclic-Graph Directories

18. General Graph Directory

19. File System Mounting
A file system must be mounted before it can be accessed
A unmounted file system is mounted at a mount point (an empty directory)
For eg: on UNIX mount point is /home

20. (a) Existing (b) Unmounted Partition
- Triangles represent subtrees of directories

21. Mount Point

22. File Sharing Sharing of files on multi-user systems is desirable
Sharing may be done through a protection scheme
On distributed systems, files may be shared across a network
Network File System (NFS) is a common distributed file-sharing method

23. File Sharing – Multiple Users
File owner-controls the file,change attributes, grant access
Group-subset of users who can access the file
User IDs identify users, allowing permissions and protections to be per-user
Group IDs allow users to be in groups, permitting group access rights

24. Remote File Systems
Uses networking to allow file system access between systems Manually via programs like FTP
Distributed file systems
world wide web
1)Client-server model allows clients to mount remote file systems from servers Server can serve multiple clients
NFS is standard UNIX client-server file sharing protocol
Standard operating system file calls are translated into remote calls
2)Distributed Information Systems (distributed naming services) such as LDAP(lightweight directory access protocol) DNS, NIS(Network Information systems) implement unified access to information needed for remote computing

25. 3)Failure modes Failure in disk,network or hardware
If failure in client server system, client should delay its operations until server recovers
Store state information(knowledge of current activities and open files) in both client and server,to recover from failure

26. Protection Information should be safe from
Physical damage(reliablility)
Improper access(protection)
File owner/creator should be able to control: what can be done by whom
Controlled access-limiting the type of file access
Types of access
Read
Write
Execute
Append
Delete
List

27. Access Control
Access Control list- specifies user name and type of access allowed for each user Each file and directory associated with an access control list Classification of users Owner-creator of file Group-set of users sharing the file Universe-all other users

28. Other protection methods
Password for each file
Disadvantage
User has to remember many passwords
If only one password for all files,once its is discovered, all files become accessible
So password is associated with subdirectory(directory protection)

29. File-System Structure
File system resides on secondary storage (disks) -provides user interface to storage, mapping logical to physical
Provides efficient and convenient access to disk by allowing data to be stored, located retrieved easily
Disk provides in-place rewrite and random access I/O transfers performed in blocks of sectors (usually 512 bytes)
File system is composed of different levels
I/O control consists of device drivers
Basic file system issues commands to device drivers to read and write physical blocks
File organisation module knows about files-type of allocation,location of file etc
Logical file system manages the metadata

30. Layered File System

31. File-System Implementation
Structures and operations used to implement file system operations
Boot control block contains info needed by system to boot OS from that volume
Needed if volume contains OS, usually first block of volume
Volume control block (superblock, master file table) contains volume details
Total no of blocks, no. of free blocks, block size, free block pointers or array
Directory structure organizes the files
Per-file File Control Block (FCB) contains many details about the file
permissions, size, dates

32. A Typical File Control Block

33. Partitions and Mounting
A disk can be sliced into multiple partitions
Partition can be raw(contains no file system) or cooked(contains file system)
Root partition contains the OS, other partitions can hold other OSes, other file systems, or be raw
Mounted at boot time
Other partitions can mount automatically or manually

34. Virtual File System
VFS is an abstraction layer on top of the concrete file system
It allows client applications to access different types of file systems in a uniform way

35. Schematic View of Virtual File System

36. Directory Implementation
Linear list of file names with pointer to the data blocks
Simple to program
Time-consuming to execute
Linear search time
Could keep ordered alphabetically via linked list or use B+ tree
Hash Table – linear list with hash data structure
Decreases directory search time
Collisions – situations where two file names hash to the same location
Only good if entries are fixed size

37. Allocation Methods - Contiguous
An allocation method refers to how disk blocks are allocated for files:
Contiguous allocation – each file occupies set of contiguous blocks
Best performance in most cases
Simple – only starting location (block no) and length (number of blocks) are required
Problems include finding space for file, knowing file size, external fragmentation, need for compaction

38. Contiguous Allocation of Disk Space

39. Allocation Methods - Linked
Linked allocation – each file a linked list of blocks
File ends at nil pointer
No external fragmentation
Each block contains pointer to next block
No compaction, external fragmentation

40. Linked Allocation

41. Variation of linked allocation-using FAT
A section of disk contains the FAT
FAT has one entry for each disk block
Directory entry contains block number of the first block of the file.
Entry indexed by that block number contains block number of next block in file.
This continues until last block which contains a special eof value

42. Allocation Methods - Indexed
Indexed allocation
Each file has its own index block(s) of pointers to its data blocks
Logical view
index table

43. Example of Indexed Allocation

44. Size of index block
Linked scheme-for large files we can link together several index blocks
Multilevel index-first level index block points to second level index block, which points to the file blocks
Combined scheme-Say,first 12 pointers point to direct blocks( containing addres of blocks containing data files),next three points to indirect blocks

45. Free Space Management
File system maintains free-space list to track available blocks/clusters
Bit vector or bit map (n blocks)
For eg:consider a disk where blocks 2,3,4,5,8,9,10,11,12,13,17,18,25,26 and 27 are free.The free space bit map would be ….

46. Linked list
Another method is to link together all the free disk blocks keeping a pointer to the first free block in a special location in disk

47. Grouping
Store addresses of n free blocks in the first free block.The last block contains addresses of another n free block and so on..
Counting
Store address of the first free block and number n of free contiguos blocks that follow in the first block