File System
AMBEDKAR INSTITUTE OF TECHNOLOGY, DSEU
PRESENTED BY ANIKET PANDEY
File system
A file is a collection of related information that is recorded on secondary storage. Or
file is a collection of logically related entities. From user’s perspective a file is the
smallest allotment of logical secondary storage.
The name of the file is divided into two parts as shown below:
name
extension, separated by a period.
Files attributes and its operations:
Attributes Types Operations
Name Doc Create
Type Exe Open
Size Jpg Read
Creation Data Xis Write
Author C Append
Last Modified Java Truncate
protection class Delete
Close
File type Usual extension Function
Executable exe, com, bin Read to run machine language program
Object obj, o Compiled, machine language not linked
Source Code C, java, pas, 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, doc Various word processor formats
Archive arc, zip, tar Related files grouped into one compressed file
Multimedia mpeg, mov, rm For containing audio/video information
Markup xml, html, tex It is the textual data and documents
Library lib, a ,so, dll It contains libraries of routines for programmers
It is a format for printing or viewing a ASCII or
Print or View gif, pdf, jpg
binary file.
FILE DIRECTORIES:
Collection of files is a file directory. The directory contains information about the files, including
attributes, location and ownership. Much of this information, especially that is concerned with storage,
is managed by the operating system. The directory is itself a file, accessible by various file management
routines.
Information contained in a device directory Operation performed on directory are:
are: • Search for a file
• Name • Create a file
• Type • Delete a file
• Address • List a directory
• Current length • Rename a file
• Maximum length • Traverse the file system
• Date last accessed
• Date last updated
• Owner id
• Protection information
Advantages of maintaining directories are:
Efficiency: A file can be located more quickly.
Naming: It becomes convenient for users as two users can have same name for different files or
may have different name for same file.
Grouping: Logical grouping of files can be done by properties eg. all java programs, all games etc.
SINGLE-LEVEL DIRECTORY
In this a single directory is maintained for all the
users.
Naming problem: Users cannot have same
name for two files.
Grouping problem: Users cannot group files
according to their need.
TWO-LEVEL DIRECTORY
In this separate directories for each user is maintained.
Path name : Due to two levels there is a path name
for every file to locate that file.
Now, we can have same file name for different user.
Searching is efficient in this method.
TREE-STRUCTURED DIRECTORY
Directory is maintained in the form of a tree.
Searching is efficient and also there is grouping
capability. We have absolute or relative path name for
a file.
FILE ALLOCATION METHODS :
1. Continuous Allocation –
A single continuous set of blocks is allocated to a file at the time of file creation.
Thus, this is a pre-allocation strategy, using variable size portions. The file
allocation table needs just a single entry for each file, showing the starting block
and the length of the file. This method is best from the point of view of the
individual sequential file. Multiple blocks can be read in at a time to improve I/O
performance for sequential processing. It is also easy to retrieve a single block.
For example, if a file starts at block b, and the ith block of the file is wanted, its
location on secondary storage is simply b+i-1
Disadvantage –
• External fragmentation will occur, making it difficult to find contiguous
blocks of space of sufficient length. Compaction algorithm will be necessary
to free up additional space on disk.
• Also, with pre-allocation, it is necessary to declare the size of the file at the
time of creation.
2. Linked Allocation(Non-contiguous allocation) –
Allocation is on an individual block basis. Each block contains a pointer to the next block in
the chain. Again the file table needs just a single entry for each file, showing the starting
block and the length of the file. Although pre-allocation is possible, it is more common
simply to allocate blocks as needed. Any free block can be added to the chain. The blocks
need not be continuous. Increase in file size is always possible if free disk block is available.
There is no external fragmentation because only one block at a time is needed but there can
be internal fragmentation but it exists only in the last disk block of file.
Disadvantage –
• Internal fragmentation exists in last disk block of file.
• There is an overhead of maintaining the pointer in every disk block.
• If the pointer of any disk block is lost, the file will be truncated.
• It supports only the sequential access of files.
3. Indexed Allocation –
It addresses many of the problems of contiguous and chained allocation. In this case, the file
allocation table contains a separate one-level index for each file: The index has one entry for each
block allocated to the file. Allocation may be on the basis of fixed-size blocks or variable-sized
blocks. Allocation by blocks eliminates external fragmentation, whereas allocation by variable-size
blocks improves locality. This allocation technique supports both sequential and direct access to the
file and thus is the most popular form of file allocation.
Disk Free Space Management :
Just as the space that is allocated to files must be managed ,so the space that
is not currently allocated to any file must be managed. To perform any of the
file allocation techniques, it is necessary to know what blocks on the disk are
available. Thus we need a disk allocation table in addition to a file allocation
table. The following are the approaches used for free space management.
1.Bit Tables : This method uses a vector containing one bit for each block on
the disk. Each entry for a 0 corresponds to a free block and each 1
corresponds to a block in use.
For example: 00011010111100110001
2.Free Block List : In this method, each block is assigned a number
sequentially and the list of the numbers of all free blocks is maintained in a
reserved block of the disk.
