1. Understanding Operating Systems Seventh Edition
Chapter 8 File Management

2. Learning Objectives
After completing this chapter, you should be able to describe:
The fundamentals of file management
File-naming conventions, including the role of extensions
The difference between fixed-length and variable-length record format
The advantages and disadvantages of several file storage techniques
Comparisons of sequential and direct file access
Access control techniques and how they compare
The role of data compression in file storage
Understanding Operating Systems,7e

3. Introduction File management system
Software responsible for creating, deleting, modifying, controlling access to files
File Manager’s efficiency directly affected by:
How the system’s files are organized
How files are stored
How each file’s records are structured
How user access to all files is protected
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4. Responsibilities of the File Manager
Four tasks
File storage
Keeping track of where each file is stored
Policy implementation:
Determine where and how files are stored
Efficiently use available storage space
Determine who (users) will have access
Provide efficient file access for users using device-independent commands
Understanding Operating Systems,7e

5. File Manager Responsibilities (con’td)
File allocation (once user access cleared)
Activate secondary storage device, load file into memory, and update file
File deallocation
Update file tables, rewrite file (if revised), and communicate file availability
Understanding Operating Systems,7e

6. Responsibilities of the File Manager (cont'd.)
User access factors:
flexibility of access to information
Share files
Provide distributed access
Allow users to browse public directories
subsequent protection
Prevent system malfunctions
Security checks
Accounts \ passwords
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7. Definitions Program files Data (document) files Directories (folders)
Contain executable instructions
Data (document) files
Contain data – created \ used by programs
Directories (folders)
Listings of filenames and their attributes (used for organization)
Understanding Operating Systems,7e

8. Data Definitions / Data Hierarchy
Bit  Byte  Field  Record  File  Database
Bit – smallest data element
Byte – 8 or 16 bits / representing a character
Field – group of related bytes, identified by user (name, type, size)
Record – group of related fields
File – group of related records, data used/generated by specific application program
Database – group of related files, interconnected at various levels, giving users flexibility of access to stored data
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9. Interacting with the File Manager
Common user commands
OPEN, DELETE, RENAME, COPY
(figure 8.2)
Typical menu of file options. © Cengage Learning 2014
Understanding Operating Systems,7e

10. Interacting with the File Manager
Device-independent
Physical location: knowledge not needed
Cylinder, surface, sector
Device medium: knowledge not needed
Tape, magnetic disk, optical disc, flash storage
Network knowledge: not needed
Device-independence facilitated through device driver software
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11. Interacting with the File Manager (cont'd.)
Logical commands
Broken into lower-level steps
Example: READ
Move read/write heads to record cylinder
Wait for rotational delay (sector containing record passes under read/write head)
Activate appropriate read/write head and read record
Transfer record to main memory
Send flag indicating free device for another request
System monitors for error conditions
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12. Typical Volume Configuration
Secondary storage unit (removable, nonremovable)
Multi-file volume
Contains many files
Multi-volume files
Extremely large files spread across several volumes
Volume name (label)
File manager manages
Easily accessible
Innermost part of CD, beginning of tape, first sector of outermost track
Understanding Operating Systems,7e

13. Typical Volume Configuration (cont'd.)
(figure 8.3)
The volume descriptor, which is stored at the beginning of each volume, includes this vital information about the storage unit. © Cengage Learning 2014
Understanding Operating Systems,7e

14. Typical Volume Configuration (cont'd.)
Master file directory (MFD)
Stored immediately after volume descriptor
Lists
Names and characteristics of every file in volume
File names (program files, data files, system files)
Subdirectories
If supported by file manager
Remainder of volume
Used for file storage
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15. Typical Volume Configuration (cont'd.)
Single directory per volume
Supported by early operating systems
Disadvantages
Long search time for individual file
Directory space filled before disk storage space filled
Users cannot create subdirectories
Users cannot safeguard their files
Each program needs unique name
Even those serving many users
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16. Introducing Subdirectories
File managers
Create MFD for each volume
Contains file and subdirectory entries
Subdirectory
Created upon account opening
Treated as file
Flagged in MFD as subdirectory
Unique properties
Improvement over single directory scheme
Problems remain: unable to logically group files
Understanding Operating Systems,7e

17. Introducing Subdirectories (cont'd.)
File managers today
Users create own subdirectories (folders)
Related files grouped together
Implemented as inverted tree structure
Efficient system searching of individual directories
May require several directories to reach file
Understanding Operating Systems,7e

18. Understanding Operating Systems,7e
(figure 8.4)
File directory inverted tree structure. The “root” is the MFD shown at the top, each node is a directory file, and each branch is a directory entry pointing to either another directory or to a real file. All program and data files subsequently added to the tree are the leaves, represented by circles. © Cengage Learning 2014
Understanding Operating Systems,7e

19. Introducing Subdirectories (cont'd.)
File descriptor
Filename: each must be unique
File type: organization and usage
System dependent
File size: for convenience
File location
Identifies first physical block (or all blocks)
Date and time of creation
Owner
Protection information: access restrictions
Record size: fixed size, maximum size
Understanding Operating Systems,7e

20. File-Naming Conventions
Filename components
Relative filename and extension
Complete filename (absolute filename)
Includes all path information
Relative filename
Name without path information
Appears in directory listings, folders
Provides filename differentiation within directory
Varies in length
One to many characters
Operating system specific
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21. File-Naming Conventions (cont'd.)
Extensions
Appended to relative filename
Two to four characters
Separated from relative filename by period
Identifies file type or contents and associated program
Example
BASIA_TUNE.AVI
Unknown extension
Requires user intervention
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22. File Extension / Type – Associated Program (Windows)

23. Understanding Operating Systems,7e
(table 8.1)
File name parameters for several operating systems.
© Cengage Learning 2014
Understanding Operating Systems,7e

24. File-Naming Conventions (cont'd.)
Operating system specifics
Windows
Drive label and directory name, relative name, and extension
C:\users\bob\documents\assignment2.docx
UNIX/Linux
Forward slash (root), first subdirectory, sub-subdirectory, file’s relative name
/usr/mary/photos/portrait.jpg
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25. File Organization / Record Format
File organization - arrangement of records within files
Record formats
Fixed-length records
Direct access: easy
Record size critical
Variable-length records
Direct access: difficult
No empty storage space and no character truncation
File descriptor stores record format
Used with files accessed sequentially
Text files, program files
Index used to access records
Understanding Operating Systems,7e

26. Record Format (cont'd.) Understanding Operating Systems,7e
(figure 8.5)
Data stored in fixed length fields (top) that extends beyond the field limit is truncated. Data stored in variable length fields (bottom) is not truncated.
© Cengage Learning 2014
Understanding Operating Systems,7e

27. Physical File Organization
Record arrangement and medium characteristics
Magnetic disks file organization
Sequential, direct, indexed sequential
File organization scheme selection considerations
Data volatility
File activity
File size
Response time
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28. Physical File Organization (cont'd.)
Sequential record organization
Records stored and retrieved serially
One after the other
Easiest to implement
File search: beginning until record found
Optimization features may be built into system
Select key field from record and sort before storage
Complicates maintenance algorithms
Preserve original order when records added, deleted
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29. Physical File Organization (cont'd.)
Direct record organization
Direct access files
Requires direct access storage device implementation
Random organization
Random access files
Relative address record identification
Known as logical addresses
Computed when records stored and retrieved
Hashing algorithms
Transform each key into a number
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30. Physical File Organization (cont'd.)
Direct record organization (cont'd.)
Advantages
Fast record access
Sequential access if starting at first relative address and incrementing to next record
Updated more quickly than sequential files
No preservation of records order
Adding, deleting records is quick
Disadvantages
Hashing algorithm collision: records with unique keys may generate the same logical address
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31. Understanding Operating Systems,7e
(figure 8.6)
The hashing algorithm causes a collision. Using a combination of street address and postal code, it generates the same logical address ( ) for three different records. © Cengage Learning 2014
Understanding Operating Systems,7e

32. Physical File Organization (cont'd.)
Indexed sequential record organization
Best of sequential and direct access
Indexed Sequential Access Method (ISAM) software
Advantage: no collisions (no hashing algorithm)
Generates index file for record retrieval
Divides ordered sequential file into equal sized blocks
Each entry in index file contains the highest record key and physical data block location
Search index file
Overflow areas
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33. Contiguous Storage Records stored one after another Advantages
Any record found once starting address, size known
Easy direct access
Disadvantages
Difficult file expansion; fragmentation
(figure 8.8)
With contiguous file storage, File A can’t be expanded without being rewritten to a larger storage area. File B can be expanded, by only one record replacing the free space preceding File C. © Cengage Learning 2014
Understanding Operating Systems,7e

34. Access Control Verification
File sharing
Data files, user-owned program files, system files
Advantages
Save space, synchronized updates, resource efficiency
Disadvantage
Need to protect file integrity
File actions
READ only, WRITE only, EXECUTE only, DELETE only, or a combination
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35. Access Control Matrix Advantages Easy to implement
Works well in system with few files, users
(table 8.3)
The access control matrix showing access rights for each user for each file.
© Cengage Learning 2014
Understanding Operating Systems,7e

36. Access Control Matrix (cont'd.)
Disadvantages
As files and/or users increase, matrix increases
Possibly beyond main memory capacity
Wasted space: due to null entries
(table 8.4)
The five access codes for User 2 from Table 8.3. The resulting code for each file is created by assigning a 1 for each checkmark, and a 0 for each blank space.
© Cengage Learning 2014
Understanding Operating Systems,7e

37. Access Control Lists Modification of access control matrix technique
(table 8.5)
An access control list showing which users are allowed to access each of the five files. This method uses storage space more efficiently than an access control matrix.
© Cengage Learning 2014
Understanding Operating Systems,7e

38. Access Control Lists (cont'd.)
Contains user names granted file access
User denied access grouped under “WORLD”
Shorten list by categorizing users
SYSTEM (ADMIN)
Personnel with unlimited access to all files
OWNER (USER)
Absolute control over all files created in own account
GROUP
All users belonging to appropriate group have access
WORLD
All other users in system
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39. Understanding Operating Systems,7e

40. Capability Lists Lists every user and respective file access
Can control access to devices as well as to files
Most common
(table 8.6)
A capability list shows files for each user and requires less storage space than an access control matrix. © Cengage Learning 2014
Understanding Operating Systems,7e

41. Data Compression Compression - technique used to reduce file size
Saves storage space required for files
Improves speed / efficiency of transferring files
Two algorithm types
Lossless: retains all data in the file
Typically used for text or arithmetic files applications
Requires decompression
Lossy: removes some data - often with minimal loss quality
Typically used for image and sound file applications
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42. Text Compression Records with repeated characters Repeated terms
Repeated characters are replaced with a code
Repeated terms
Compressed using symbols to represent most commonly used words
University student database common words
Student, course, grade, department each are represented with single character
Front-end compression
Entry takes given number of characters from previous entry that they have in common
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43. Text Compression Compressing repeating characters or terms
For example: Data in fixed length field may not use all characters in field.
LastName – fix length field of 25 characters
The name Adams would be followed 20 spaces\blanks
Adamsbbbbbbbbbbbbbbbbbbbb (25 characters)
Repeating spaces\blanks could be compressed to Adamsb (8 characters)
Reducing size by 17 characters
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44. Image and Sound Compression
Lossy compression
Irreversible: original file cannot be reconstructed
Compression algorithm highly dependent on file type
JPEG: still images
MPEG: video images
International Organization for Standardization (ISO)
World’s leading developer of international standards
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45. Image and Sound Compression
Typical uncompressed CD quality audio file such as .wav or .aiff requires about 10MB for 1 minute music
3.5 minute song ~ 35 MB file
MP3: common lossy compression technique that reduces file size (90%) to about 10% of original with some (minimal) loss of sound quality
3.5 minute song ~ 3.5 MB file
Common loseless compression formats such as WavPack and ALAC (Apple loseless) result in a 50% reduction in file size with no loss of quality
3.5 minute song ~ 17.5 MB file
Trade off between size reduction and processing\ decompression time
PCM audio format typically stored in .wav or .aiff
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46. Conclusion File manager
Controls every file and processes user commands
Manages access control procedures
Maintain file integrity and security
File organizations
Sequential, direct, indexed sequential
Physical storage allocation schemes
Contiguous, noncontiguous, indexed
Record types
Fixed-length versus variable-length records
Access control methods
Data compression techniques
Understanding Operating Systems,7e