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
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)
12.
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
16.
17.
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
