1. Chapter 3: File Management
Part 1

2. Learning outcome:
By the end of this lecture, students will be able to:
1) Identify basic function of file system
2)Describe file organization techniques
3) Describe types of file structure
4)Describe various methods of file allocation

3. File Management : Introduction
File management system can be define as a system that an
operating system uses to keep track of different files.
Unlike the CPU and memory management aspects of the
operating system which aim mainly towards an optimum
use of the CPU, file management aims to provide a
convenient programming environment for the users of the
system.
There are several functions that must be performed by an
efficient file system:
 include storing of files in an orderly fashion
 accessing the stored files
 appending the stored files and protecting the files from
loss of data

4. File Structure

5. File organization techniques
 Concerned with how records are arranged &
characteristics of medium used to store it.
 On magnetic disks, files can be organized as:
1) Sequential
2) Direct
3) Indexed sequential

6. Characteristics Considered When
Selecting File Organization
Volatility of data—frequency with which additions &
deletions made.
Activity of file—% records processed during a given
run.
Size of file.
Response time—amount of time user is willing to
wait before requested operation is completed.

7. 1) Sequential
Easiest to implement because records are stored &
retrieved serially, one after other.
To speed process some optimization features may be
built into system.
E.g., select a key field from record & then sort records
by that field before storing them.
Aids search process.
Complicates maintenance algorithms because original
order must be preserved every time records added or
deleted.

8. 2) Direct
Uses direct access files which can be implemented
only on direct access storage devices.
Give users flexibility of accessing any record in any
order without having to begin search from beginning
of file.
Records are identified by their relative addresses
(their addresses relative to beginning of file).
Logical addresses computed when records are stored
& again when records are retrieved.
Use hashing algorithms

9. Advantages:
1) Fast access to records.
2)Can be updated more quickly than sequential files
because records quickly rewritten to original addresses
after modifications
Disadvantage:
1) Several records with unique keys may generate same
logical address (collision)

10. 3) Indexed sequential
Combines best of sequential & direct access.
Created & maintained through Indexed Sequential
Access Method (ISAM) software package.
Doesn’t create collisions because it doesn’t use result of
hashing algorithm to generate a record’s address.
 Uses info to generate index file through which records retrieved.
Divides ordered sequential file into blocks of equal size.
 Size determined by File Manager to take advantage of physical storage
devices & to optimize retrieval strategies.
Each entry in index file contains highest record key &
physical location of data block where this record, &
records with smaller keys, are stored.

11. Methods of file allocation
File manager works with files
As whole units
As logical units or records
Within file
Records must have same format
Record length may vary
Records subdivided into fields
Application programs manage record structure
Three methods:
1) Contiguous file allocation
2) Linked list non-contiguous (using blocks)
3) Linked list non-contiguous (using index)

13. 1) Contiguous file allocation
Records stored one after another
Advantages
 Any record found once starting address, size known
 Easy direct access
Disadvantages
 Difficult file expansion, fragmentation

14. 2) Linked list non-contiguous
(using blocks)
Files use any available disk storage space
File records stored in contiguous manner
If enough empty space
Remaining file records and additions
Stored in other disk sections (extents)
Extents
 Linked together with pointers
 Physical size determined by operating system
 Usually 256 bytes

15. File extents linked in two ways
Storage level
 Each extent points to next one in sequence
 Directory entry
 Filename, storage location of first extent, location of last
extent, total number of extents (not counting first)
Directory level
 Each extent listed with physical address, size, pointer to next
extent
 Null pointer indicates last one
Advantage
Eliminates external storage fragmentation
Eliminates need for compaction
Disadvantage
No direct access support
 Cannot determine specific record’s exact location

18. 3) Linked list non-contiguous
allocation (using index)
Allows direct record access
Brings pointers together
 Links every extent file into index block
Every file has own index block
Disk sector addresses for file
Lists entry in order sectors linked
Supports sequential and direct access
Does not necessarily improve storage space use
Larger files experience several index levels