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Module 11: File-System Interface File Concept Access :Methods - - PowerPoint PPT Presentation

Module 11: File-System Interface File Concept Access :Methods Directory Structure Protection Consistency Semantics Silberschatz, Galvin, and Gagne 1999 Applied Operating System Concepts 11.1 File Concept Contiguous


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SLIDE 1

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.1

Module 11: File-System Interface

  • File Concept
  • Access :Methods
  • Directory Structure
  • Protection
  • Consistency Semantics
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SLIDE 2

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.2

File Concept

  • Contiguous logical address space
  • Types:

– Data

✴ numeric ✴ character ✴ binary

– Program

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SLIDE 3

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.3

File Structure

  • None - sequence of words, bytes
  • Simple record structure

– Lines – Fixed length – Variable length

  • Complex Structures

– Formatted document – Relocatable load file

  • Can simulate last two with first method by inserting appropriate

control characters.

  • Who decides:

– Operating system – Program

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SLIDE 4

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.4

File Attributes

  • Name – only information kept in human-readable form.
  • 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.

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SLIDE 5

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.5

File Operations

  • create
  • write
  • read
  • reposition within file – file seek
  • 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.

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SLIDE 6

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.6

File Types – name, extension

Executable exe, com, bin or none ready-to-run machine- language program Object

  • bj, o

complied, machine language, not linked Source code c, p, pas, 177, asm, a source code in various languages Batch bat, sh commands to the command interpreter Text txt, doc textual data documents Word processor wp, tex, rrf, etc. various word-processor formats Library lib, a libraries of routines Print or view ps, dvi, gif ASCII or binary file Archive arc, zip, tar related files grouped into one file, sometimes compressed. File Type Usual extension Function

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SLIDE 7

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.7

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

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SLIDE 8

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.8

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.
  • Backups of these two structures are kept on tapes.
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SLIDE 9

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.9

Information in a Device Directory

  • Name
  • Type
  • Address
  • Current length
  • Maximum length
  • Date last accessed (for archival)
  • Date last updated (for dump)
  • Owner ID (who pays)
  • Protection information (discuss later)
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SLIDE 10

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.10

Operations Performed on Directory

  • Search for a file
  • Create a file
  • Delete a file
  • List a directory
  • Rename a file
  • Traverse the file system
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SLIDE 11

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.11

Organize the Directory (Logically) to Obtain

  • Efficiency – locating a file quickly.
  • Naming – convenient to users.

– Two users can have same name for different files. – The same file can have several different names.

  • Grouping – logical grouping of files by properties, (e.g., all Pascal

programs, all games, …)

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SLIDE 12

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.12

Single-Level Directory

  • A single directory for all users.
  • Naming problem
  • Grouping problem
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SLIDE 13

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.13

Two-Level Directory

  • Separate directory for each user.
  • Path name
  • Can have the saem file name for different user
  • Efficient searching
  • No grouping capability
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SLIDE 14

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.14

Tree-Structured Directories

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SLIDE 15

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.15

Tree-Structured Directories (Cont.)

  • Efficient searching
  • Grouping Capability
  • Current directory (working directory)

– cd /spell/mail/prog – type list

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SLIDE 16

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.16

Tree-Structured Directories (Cont.)

  • Absolute or relative path name
  • Creating a new file is done in current directory.
  • Delete a file

rm <file-name>

  • Creating a new subdirectory is done in current directory.

mkdir <dir-name> Example: if in current directory /spell/mail mkdir count mail prog copy prt exp count

  • Deleting “mail” ⇒ deleting the entire subtree rooted by “mail”.
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SLIDE 17

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.17

Acyclic-Graph Directories

  • Have shared subdirectories and files.
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SLIDE 18

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.18

Acyclic-Graph Directories (Cont.)

  • Two different names (aliasing)
  • If dict deletes list ⇒ dangling pointer.

Solutions: – Backpointers, so we can delete all pointers. Variable size records a problem. – Backpointers using a daisy chain organization. – Entry-hold-count solution.

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SLIDE 19

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.19

General Graph Directory

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SLIDE 20

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.20

General Graph Directory (Cont.)

  • How do we guarantee no cycles?

– Allow only links to file not subdirectories. – Garbage collection. – Every time a new link is added use a cycle detection algorithm to determine whether it is OK.

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SLIDE 21

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.21

Protection

  • File owner/creator should be able to control:

– what can be done – by whom

  • Types of access

– Read – Write – Execute – Append – Delete – List

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SLIDE 22

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.22

Access Lists and Groups

  • Mode of access: read, write, execute
  • Three classes of users

RWX a) owner access 7 ⇒ 1 1 1 RWX b) groups 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.

  • wner

group public chmod 761 game

  • Attach a group to a file

chgrp G game

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SLIDE 23

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.23

  • File-System Structure
  • Allocation Methods
  • Free-Space Management
  • Directory Implementation
  • Efficiency and Performance
  • Recovery
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SLIDE 24

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.24

File-System Structure

  • File structure

– Logical storage unit – Collection of related information

  • File system resides on secondary storage (disks).
  • File system organized into layers.
  • File control block – storage structure consisting of information

about a file.

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SLIDE 25

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.25

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.
  • Wasteful of space (dynamic storage-allocation problem).
  • Files cannot grow.
  • Mapping from logical to physical.

LA/512 Q R – Block to be accessed = ! + starting address – Displacement into block = R

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SLIDE 26

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.26

Linked Allocation

  • Each file is a linked list of disk blocks: blocks may be scattered

anywhere on the disk. pointer block =

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SLIDE 27

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.27

  • Allocate as needed, link together; e.g., file starts at block 9
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SLIDE 28

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.28

Linked Allocation (Cont.)

  • Simple – need only starting address
  • Free-space management system – no waste of space
  • No random access
  • Mapping

– Block to be accessed is the Qth block in the linked chain of blocks representing the file. – Displacement into block = R + 1

  • File-allocation table (FAT) – disk-space allocation used by MS-

DOS and OS/2. LA/511 Q R

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SLIDE 29

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.29

Indexed Allocation

  • Brings all pointers together into the index block.
  • Logical view.

index table

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SLIDE 30

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.30

Example of Indexed Allocation

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SLIDE 31

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.31

Indexed Allocation (Cont.)

  • Need index table
  • Random access
  • Dynamic access without external fragmentation, but have
  • verhead of index block.
  • Mapping from logical to physical in a file of maximum size of

256K words and block size of 512 words. We need only 1 block for index table. LA/512 Q R – Q = displacement into index table – R = displacement into block

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SLIDE 32

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.32

Indexed Allocation – Mapping (Cont.)

  • Mapping from logical to physical in a file of unbounded length

(block size of 512 words).

  • Linked scheme – Link blocks of index table (no limit on size).

LA / (512 x 511) Q1 R1

– Q1 = block of index table – R1 is used as follows:

R1 / 512 Q2 R2

– Q2 = displacement into block of index table – R2 displacement into block of file:

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SLIDE 33

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.33

Indexed Allocation – Mapping (Cont.)

  • Two-level index (maximum file size is 5123)

LA / (512 x 512) Q1 R1

– Q1 = displacement into outer-index – R1 is used as follows:

R1 / 512 Q2 R2

– Q2 = displacement into block of index table – R2 displacement into block of file:

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SLIDE 34

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.34

Indexed Allocation – Mapping (Cont.)

  • uter-index

index table file

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SLIDE 35

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.35

Combined Scheme: UNIX (4K bytes per block)

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SLIDE 36

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.36

Free-Space Management

  • Bit vector (n blocks)

0 1 2 n-1 bit[i] =

  • 0 ⇒ block[i] free

1 ⇒ block[i] occupied

  • Block number calculation

(number of bits per word) * (number of 0-value words) +

  • ffset of first 1 bit
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SLIDE 37

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.37

Free-Space Management (Cont.)

  • Bit map requires extra space. Example:

block size = 212 bytes disk size = 230 bytes (1 gigabyte) n = 230/212 = 218 bits (or 32K bytes)

  • Easy to get contiguous files
  • Linked list (free list)

– Cannot get contiguous space easily – No waste of space

  • Grouping
  • Counting
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SLIDE 38

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.38

Free-Space Management (Cont.)

  • Need to protect:

– Pointer to free list – Bit map

✴ Must be kept on disk ✴ Copy in memory and disk may differ. ✴ Cannot allow for block[i] to have a situation where bit[i] =

1 in memory and bit[i] = 0 on disk. – Solution:

✴ Set bit[i] = 1 in disk. ✴ Allocate block[i] ✴ Set bit[i] = 1 in memory

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SLIDE 39

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.39

Directory Implementation

  • Linear list of file names with pointer to the data blocks.

– simple to program – time-consuming to execute

  • Hash Table – linear list with hash data structure.

– decreases directory search time – collisions – situations where two file names hash to the same location – fixed size

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SLIDE 40

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.40

Efficiency and Performance

  • Efficiency dependent on:

– disk allocation and directory algorithms – types of data kept in file’s directory entry

  • Performance

– disk cache – separate section of main memory for frequently sued blocks – free-behind and read-ahead – techniques to optimize sequential access – improve PC performance by dedicating section of memroy as virtual disk, or RAM disk.

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SLIDE 41

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.41

Various Disk-Caching Locations

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SLIDE 42

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.42

Recovery

  • Consistency checker – compares data in directory structure with

data blocks on disk, and tries to fix inconsistencies.

  • Use system programs to back up data from disk to another

storage device (floppy disk, magnetic tape).

  • Recover lost file or disk by restoring data from backup.
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SLIDE 43

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.43

Memory-mapped Files

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SLIDE 44

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.44

Sequential-access File

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SLIDE 45

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.45

Example of Index and Relative Files

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SLIDE 46

Applied Operating System Concepts Silberschatz, Galvin, and Gagne 1999 11.46

Typical File-System Organization

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SLIDE 47

Layered File System

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SLIDE 48

Contiguous Allocation of Disk Space

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SLIDE 49

File-Allocation Table

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SLIDE 50

Linked Free-Space List on Disk