The document discusses various aspects of file systems and file organization in operating systems. It describes files as named collections of related information stored on secondary storage devices like disks. It covers different file access methods, directory structures like single-level, two-level and tree-structured directories. It also discusses file allocation methods like contiguous, linked and indexed allocation and free space management techniques.

1. Chapter 9: File-System Interface
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Mr. S. V. Viraktamath
Dept of E&CE,
SDMCT, Dharwad
svvitc2011@gmail..com

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3. Chapter 10: File-System Interface
• File Concept
• Access Methods
• Directory Structure
• File-System Mounting
• File Sharing
• Protection

4. Introduction
• So far - Discussed
– CPU scheduling
– Process issues- Critical section problem
– Deadlock
– Main memory management
– Virtual memory
• File system is a most visible aspect of OS.
• It provides the mechanism for on-line storage
of and access to both data and programs
(OS+User)
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5. • File?
• Directories?
• Partitions?
• In this topic  various aspects of files and the
major directory structures.
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6. • Storage Medias
• Magnetic disks, magnetic tapes, and optical
disks.
• Files are mapped, by the operating system,
onto physical devices.
• These storage devices are usually nonvolatile.
• A file is a named collection of related
information that is recorded on secondary
storage.
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7. Summary
• File system is a most visible aspect of OS.
• A file is an abstract data type defined and
implemented by the operating system.
• Collection of files  Directories Partitions.
• Storage medias magnetic disks, magnetic
tapes, and optical disks
• Files are mapped by OS onto physical devices.
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8. File
• A file is the smallest allotment of logical
secondary storage.
• Data cannot be written to secondary storage
unless they are within a file.
• Data files may be numeric, alphabetic,
alphanumeric, or binary.
• In general, a file is a sequence of bits, bytes,
lines, or records.
• The information in a file is defined by its
creator
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9. • A file has a certain defined structure
according to its type.
• A text file is a sequence of characters
organized into lines
• Source file, Object file, executable file ?
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10. File Attributes
• A File name is usually a string of characters,
such as example1.c
• When a file is named, it becomes independent
of the process, the user, and even the system
that created it.
• A file has certain attributes
– Name -Type - Size
– Location - Protection
– Time, Date and user identification..
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11. Snapshot
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12. Snapshot of directory
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13. File operations
• The operating system can provide system calls
to create, write, read, reposition, delete, and
truncate files.
• Truncating a file: The user may want to erase
the contents of a file but keep its attributes.
• Other common operations - appending new
information to the end of an existing file and
renaming an existing file.
• Searching the directory for the file..
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14. Open files
• The OS keeps a small table containing
information about all open files (the open-file
table).
• File operation requested – no searching is
required
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15. Types of files
• A common technique for implementing file
types is to include the type as part of the file
name.
• The name is split into two parts-a name and an
extension, usually separated by a period
character. (Name and family name/address, USN)
• User and OS can understand which type of file
it is.
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16. • MS-DOS, a name can consist of up to eight
characters followed by a period and terminated
by an extension of up to three characters.
• A file with a .com, .exe, or .bat extension can
be executed.
• Assemblers expect source files to have an .asm
extension,
• The word processor expects its file to end with
a .doc extension..
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17. Common file types
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18. • File names
– Operatingsystem1
– ↓↨♥♣•◘¶☻▬↨
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19. Access Methods
• Files store information.
• This information must be accessed and read
into computer memory.
• Methods
– Sequential Access
– Direct Access
– Other Access Methods ..
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20. Access Methods
• Sequential Access
– Simplest access method
• Used : editors and compilers usually access
files in this fashion
• Bulk operations –reads and writes.
• read next; write next;
• no read after last write
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21. Sequential-access file.
• Sequential access is based on a tape model of a
file.
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22. Direct Access
• This method is based on a disk model of a file
• Disks allow random access to any file block
• A direct-access file allows arbitrary blocks to be
read or written.
• may read block 14, then read block 53, back to 7
• read n, where n is the block number, rather than
read next, and write n rather than write next.
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23. Other access method
• Can be built on top of a direct-access method.
• An index in the back of a book- like index -
contains pointers to the various blocks.
• first search the index, and then use the pointer
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24. Directory Structure
• The file systems of computers can be
extensive.
• How many files may be there in u r system?
• Some systems store millions of files on
terabytes of disk.
• First, disks are split into one or more
partitions-- files and directories reside.
• Partitions can also store multiple operating
systems, allowing a system to boot and run
more than one.
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25. A typical file-system organization
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26. The operations - on a directory
• Search for a file- search directory for a file
• Create a file- new file added to directory
• Delete a file
• List a directory- able to list the files in a directory
• Rename a file- content changes
• Traverse the file system- the file can be copied
to tape.
• ………………………..
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27. Single-Level Directory
• The simplest directory structure
• All files are contained in the same directory
• As the number of files and users increases-
limitations
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All Users-common folder

28. Two-Level Directory
• Create a separate directory for each user.
• Each user has her own user file directory
(UFD)
• It lists only the files of a single user. It isolates
one user from another user.
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29. • When a user job starts or a user logs in, the
system's master file directory (MFD) is
searched.
• The MFD is indexed by user name or account
number, points to the UFD for that user. (C pgms)
 To name a particular file uniquely we must give
both user name and file name.
 System files stored in directory user 0.
 Can have the same file name for different user
 Efficient searching
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30. Tree-Structured Directories
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31.  MS DOS –structures files as a tree
 Tree is most common directory structure.
 Tree has root directory.
 Path name is path from root, through all
subdirectories to a specified file.
 A directory contains a set of files or
subdirectories.
• Each user has a current directory.
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32. • Absolute path name: It begins at the root and
follows a path down to the specified file, giving
the directory names on the path.
• A relative path name:
Defines a path from the current directory.
Ex. Current directory root/spell/mail then
• Absoluteroot/spell/mail/prt/first
• Relative prt/first
• If directory is empty, it can be deleted. (MSDOS)
• Unix rm command removes /deletes a directory.
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33. Acyclic-Graph Directories
• Consider two programmers who are working
on a joint project.
• There may be common subdirectory between
them.
• An acyclic graph allows directories to have
shared subdirectories and files.
• The same file or subdirectory may be in two
different directories.
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35. • A tree structure prohibits the sharing of files or
directories.
• An acyclic graph allows directories to have
shared subdirectories and files.
• An acyclic graph, that is, a graph with no
cycles, is a natural generalization of the tree
structured directory scheme.
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36. • With a shared file, only one actual file exists,
so any changes made by one person are
immediately visible to the other.
• One approach to implementing shared files is
simply to duplicate all information about them
in both sharing directories.
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37. General Graph Directory
• when we add links to an existing tree-
structured directory, the tree structure is
destroyed resulting in a simple graph
structure.
• If cycles are allowed to exist in the directory,
we likewise want to avoid searching any
component twice.
• A poorly designed algorithm might result in an
infinite loop continually searching through the
cycle and never terminating.
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38. General Graph Directory

39. • How do we guarantee no cycles?
– Allow only links to file not subdirectories
– Every time a new link is added - use a cycle
detection algorithm to determine whether it is OK
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41. Allocation Methods
• An allocation method refers to how disk
blocks are allocated for files:
– Contiguous allocation
– Linked allocation
– Indexed allocation
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42. Contiguous Allocation
• Each file occupies a set of contiguous blocks
on the disk.
• Simple – only starting location (block #) and
length (number of blocks) are required
• Random access – possible.
• Wasteful of space (dynamic storage-allocation
problem)
• Files cannot grow
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43. Contiguous Allocation of Disk Space

44. • Accessing block b + 1 after block b normally
requires no head movement.
• When head movement is needed, it is only one
track.
• The number of disk seeks required for
accessing contiguously allocated files is
minimal.
– The amount of time that a disk drive's head takes
to move to a specific location on a disk.
• Both sequential and direct access can be
supported by contiguous allocation.
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45. Disadv
• Difficulty is finding space for a new file.
• First fit and best fit are the most common
strategies used to select a free hole from the set
of available holes.
• Algorithms suffer from the problem of
external fragmentation. Why?
• Used in 
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46. • Older microcomputer systems used contiguous
allocation on floppy disks.
• Run a repacking routine- copy to another
floppy or tape.
• Another problem with contiguous allocation is
determining how much space is needed for a
file.
• If we allocate too little space to a file – it may
be extended
• If you allocate too much space -inefficient.
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47. Linked Allocation
• Linked allocation solves all problems of
contiguous allocation.
• With linked allocation, each file is a linked list
of disk blocks; the disk blocks may be
scattered anywhere on the disk.
• The directory contains a pointer to the first and
last blocks of the file.
• Each block contains a pointer to the next block
• These pointers are not made available to the
user. (Game)
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Pointers –adr is not correct

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50. • To create a new file, we simply create a new
entry in the directory.
• This pointer is initialized to nil (the end-of-list
pointer value) to signify an empty file.
• There is no external fragmentation with linked
allocation
• The size of a file does not need to be declared
when that file is created.
• A file can continue to grow as long as free
blocks are available.
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51. Dis adv
• It can be used effectively only for sequential-
access files.
• To find the ith block of a file, we must start at
the beginning of that file.
• Another disadv-the space required for the
pointers.
• If a pointer requires 4 bytes out of a 512-byte
block, then 0.78 percent of the disk used.
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52. • Solution - to collect blocks into multiples,
called clusters and to allocate the clusters
rather than blocks.
• Yet another problem of linked allocation is
reliability.
• If a pointer were lost or damaged.
• Partial solutions are to use doubly linked lists.
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53. Indexed Allocation
• Linked Allocation –Adv?
– No external fragmentation & Size declaration pblm
• Linked allocation Disadv?
– No direct Access
• IndexedNo External fragmentation & Direct
access
• All pointers brought into one location called
index block.
• Each file has its own index block.
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54. • How to access 3rd block of a file?
• The directory contains the address of the index
block.
•
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55. • Indexed allocation does suffer from wasted
space Pointer overhead is more.
• Ex. a file of only one or two blocks.
• If the index block is too small - it will not be
able to hold enough pointers for a large file.
• Solution:
– Linked Scheme :link together several index blocks
– Multilevel index: first level index block to point to
a set of second-level index blocks, which in turn
point to the file blocks.
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56. Free-Space Management
• Disc space limited-reused.
• The system maintains a free-space list.
• To create a file -search the free-space list-
there- removed from the free-space list.
• File deleted -its disk space is added to the free-
space list.
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57. Bit Vector
• The free-space list is implemented as a bit map
or bit vector.
• Each block is represented by 1 bit. If the block
is free, the bit is 1.
• Blocks 2, 3,4,5, 8, 9, 10, 11, 12, 13, 17, 18, 25,
26, and 27 are free, and the rest of the blocks
are allocated.
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58. • The main advantage-relatively simplicity.
• Disadvantage
– Bit vectors are inefficient unless the entire vector
is kept in main memory.
– A 1.3-GB disk with 512-byte blocks would need a
bit map of over 332 KB to track its free blocks
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59. Linked List
• Linked Allocation?
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60. Linked List
• Another approach is to link together all the
free disk blocks.
• Keeping a pointer to the first free block.
• First block contains a pointer to the next free
disk block, and so on.
• Ex. keep a pointer to block 2, it contain a
pointer to block 3, which would point to block
4, which would point to block 5.
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61. Linked free space list on disk
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62. • Disadvantages
– we must read each block
• Grouping:
– A modification of the free-list approach is to store
the addresses of n-1 free blocks in the first free
block.
– The last block contains the addresses of another n
free blocks, and so on.
– Adv to compared to linked as most of the free
blocks addresses are available in one block.
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63. • Counting:
– WKT several contiguous blocks may be allocated
or freed simultaneously (in contiguous-allocation)
– Rather than keeping a list of n free disk addresses
keep the address of the first free block and the
number n.
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64. 11/15/2014 SDM SVV OS 65
End of Topic

65. ಆನಂದಾಶ್ರಮ ಮತ್ತು ಮಹಿಷಿ ಟ್ರಸ್ಟದಲ್ಲಿ ಎಸ್ ಡಿ ಎಮ್ ಕನನಡ ವೆೇದಿಕೆಯ ಸ್ಹಯೇಗದೆ ಂದಿಗೆ
ಹಮ್ಮಿಕೆ ಂಡಿರತವ ಕಿರತ ಕಾಣಿಕೆ ಹಾಗ ಸ್ಂಗೇತ್ ಕಾಯಯಕರಮದಲ್ಲಿ ಧ್ವನಿ ತ್ಂಡ ನಡೆಸಿದ
ಸ್ಂಗೇತ್ ಕಾಯಯಕರಮ ಅಲ್ಲಿಯ ಜನರ ಮನಗೆದತು ಅವರನತನ ಖತಷಿಯಾಗಸಿತ್ತ.
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66. • Daughter of Pandit Mallikarjun Mansoor
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67. 11/15/2014 SDM SVV OS 68

68. 11/15/2014 SDM SVV OS 69

69. End of Topic
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71. 11/15/2014 SDM SVV OS 72
Chandrakant Kanavi

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73. • Prof. Shakuntala
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74. End of Topic
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