Ppt on file management

1. File System
Management
Prepared by: Janki Kansagra

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

3. Objectives
 To explain the function of file systems
 To describe the interfaces to file systems
 To discuss file-system design tradeoffs, including access
methods, file sharing, file locking, and directory structures
 To explore file-system protection

4. 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

5. File Operations
 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

6. 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

7. File Types – Name, Extension

8. Access Methods
 Sequential Access
read next
write next
reset
no read after last write
(rewrite)
 Direct Access
read n
write n
position to n
read next
write next
rewrite n
n = relative block number

9. Sequential-access File

10. Directory Structure
 A collection of nodes containing information about all files.
F 1 F 2
F 3
F 4
F n
Directory
Files
Both the directory structure and the files reside on disk

11. Operations Performed on
Directory
 List directory contents
 Search for a file
 Create a file
 Delete a file
 Rename a file
 Traverse the file system

12. Single-Level Directory
 A single directory for all user called the root
directory
 Pros: Simple, easy to quickly locate files
 Cons: inconvenient naming (uniqueness), no
grouping

13. Two-Level Directory
 Separate directory for each user
 Introduces the notion of a path name
 Can have the same file name for different user
 Efficient searching
 No grouping capability

14. Tree-Structured
Directories
 Directories can now contain files and
subdirectories
 Efficient searching, allows grouping

15. Path Names
 To access a file, the user should either:
 Go to the directory where file resides, or Specify the
path where the file is
 Path names are either absolute or relative
 Absolute: path of file from the root directory
 Relative: path from the current working directory
 Most OSes have two special entries in each directory:
 “.” for current directory and “..” for parent

16. Acyclic-Graph Directories
 Allow sharing of subdirectories and files

17. Protection
 File owner/creator should be able to control:
 what can be done
 by whom
Types of access
 Read
 Write
 Execute
 Append
 Delete
 List

18. Categories of Users
Individual user
 Log in establishes a user-id
 Might be just local on the computer or could be
through interaction with a network service
Groups to which the user belongs
 For example, “nahum” is in “w4118”
 Again could just be automatic or could involve
talking to a service that might assign, say, a
temporary cryptographic key

19. UNIX Access Rights
 Mode of access: read, write, execute
 Three classes of users
RWX
a) owner access 7  1 1 1
RWX
b) group access 6  1 1 0
RWX
c) public access 1  0 0 1
 Ask manager to create a group (unique name), say
G, and add some users to the group.
 For a particular file (say game) or subdirectory,
define an appropriate access.
owner group public
chmod 761 game

20. Issues with Access Rights
 Just a single owner, a single group and the public
 Pro: Compact enough to fit in just a few bytes
 Con: Not very expressive
 Access Control List: This is a per-file list that tells
who can access that file
 Pro: Highly expressive
 Con: Harder to represent in a compact way

21. Disk Scheduling
 The operating system is responsible for using
hardware efficiently — for the disk drives, this means
having a fast access time and disk bandwidth.
 Access time has two major components
 Seek time is the time for the disk are to move the
heads to the cylinder containing the desired sector.
 Rotational latency is the additional time waiting
for the disk to rotate the desired sector to the disk
head.

22. Disk Scheduling
 Minimize seek time
 Seek time  seek distance
 Disk bandwidth is the total number of bytes
transferred, divided by the total time between the
first request for service and the completion of the last
transfer.

23. Disk Scheduling
 Several algorithms exist to schedule the servicing of
disk I/O requests.
 We illustrate them with a request queue (0-199).

98, 183, 37, 122, 14, 124, 65, 67
 Head pointer 53

24. FCFS Disk Scheduling

25. SSTF Disk Scheduling
 Selects the request with the minimum seek time from
the current head position.
 SSTF scheduling is a form of SJF scheduling; may
cause starvation of some requests.
 Illustration shows total head movement of 236
cylinders.

26. SSTF Disk Scheduling

27. SCAN Disk Scheduling
 The disk arm starts at one end of the disk, and moves
toward the other end, servicing requests until it gets
to the other end of the disk, where the head
movement is reversed and servicing continues.
 Sometimes called the elevator algorithm.
 Illustration shows total head movement of 208
cylinders.

28. SCAN Disk Scheduling

29. C-SCAN Disk Scheduling
 Provides a more uniform wait time than SCAN.
 The head moves from one end of the disk to the
other. servicing requests as it goes. When it reaches
the other end, however, it immediately returns to the
beginning of the disk, without servicing any requests
on the return trip.
 Treats the cylinders as a circular list that wraps
around from the last cylinder to the first one.

30. C-SCAN Disk Scheduling

31. LOOK and C-LOOK Disk
Scheduling
 In LOOK and C-LOOK the arm goes as far as the final
request in each direction then reverses direction
immediately without first going all the way to the end
of the disk.
 Versions of SCAN and C-SCAN that follow this
pattern are called LOOK and C-LOOK.

32. LOOK Disk Scheduling

33. C-LOOK Disk Scheduling

34. Disk Allocation Methods
Three allocation methods:
 Contiguous allocation
 Linked allocation
 Indexed allocation

35. Contiguous Allocation
 Each file occupies a set of contiguous blocks on the
disk
 Pros:
Simple – only starting location (block #) and
length (number of blocks) are required
Random access
 Cons:
Wasteful of space (dynamic storage-allocation
problem)
External fragmentation: may need to compact
space
Files cannot grow

36. Contiguous Allocation

37. Contiguous Allocation
 Some newer file systems (i.e. high-performance
Veritas File System) use a modified contiguous
allocation scheme
 Extent-based file systems allocate disk blocks in
extents
 An extent is a contiguous chunk of blocks (similar to
clusters)
 Extents are allocated when the file grows
 Extents are linked
 A file consists of one or more extents
 Extent size can be set by owner of file

38. Linked Allocation
 Each file is a linked list of disk blocks
 Blocks may be scattered anywhere on the disk
 Pros:
Simple – need only starting address
Free-space management system – no waste of
space
 Cons:
No efficient random access

39. Linked Allocation

40. File Allocation Table
 Variation on linked list allocation.
 FAT is located in contiguous space on disk
 Each entry corresponds to disk block number
 Each entry contains a pointer to the next block or 0
 Used by MS-DOS and OS/2

41. Linked Allocation

42. Indexed Allocation
 Brings all pointers together into the index block
 Pros:
 Efficient random access
 Dynamic access without external fragmentation
 Cons:
 Index table storage overhead

43. Indexed Allocation

44. Indexed Allocation
 Linked Scheme: If the size of index block is too
large link several index blocks together.
 Last pointer in the block points next index block.
 Nil indicates the end of index file.

45. Indexed Allocation
 Multi level Index: First level index block points to
second level index blocks which point to file blocks.

outer-index
index table file

46. Indexed Allocation
 Combined Scheme: first n-3 pointers point to data
blocks, last three are multilevel indexes.
 First block points to single level index, second block
points to second level. Third block points to third
level.

47. Indexed Allocation