4. Allocation method provides a way in which the disk will
be utilized and the files will be accessed.
Selection of an appropriate allocation method will
significantly affect the performance and efficiency of
the system.
There are various methods which can be used to
allocate disk space to the files.
Introduction:
7. Contiguous Allocation
If the blocks are allocated to the
file in such a way that all the
logical blocks of the file get the
contiguous physical block in the
hard disk then such allocation
scheme is known as contiguous
allocation.
8. Linked file
allocation
In linked list allocation, each file is considered as the linked list of
disk blocks.
However, the disks blocks allocated to a particular file need not to
be contiguous on the disk.
Each disk block allocated to a file contains a pointer which points
to the next disk block allocated to the same file.
9. File Allocation
Table
File Allocation Table overcomes this
drawback of linked list allocation.
File allocation table needs to be
cached in order to reduce the number
of head seeks. Now the head doesn't
need to traverse all the disk blocks in
order to access one successive block.
It simply accesses the file allocation
table, read the desired block entry
from there and access that block.
10. Indexed
Allocation
Instead of maintaining a file allocation
table of all disk pointers, indexed
allocation scheme stores all the disk
pointers in one of the blocks called as
Indexed block.
Indexed block doesn't hold the file
data, but it holds the pointers to all
the disk blocks allocated to that
particular file.
Directory entry will only contain the
index block address.
11. Multilevel Indexed
Allocation
In Multilevel index allocation, we have
various levels of indices.
There are outer level index blocks which
contain the pointers to the inner level
index blocks and the inner level index
blocks contain the pointers to the file
data.
The outer level index is used to find the inner
level index.
The inner level index is used to find the
desired data block.
12. Inode
In UNIX based operating systems, each file is indexed by an Inode.
Inode are the special disk block which is created with the creation of the
file system.
The number of files or directories in a file system depends on the number
of Inodes in the file system.
14. Free Space
Management
A file system is responsible to allocate the free blocks to the file
therefore it has to keep track of all the free blocks present in the
disk.
There are mainly two approaches by using which, the free blocks in
the disk are managed.
Bit vector.
Linked list.
15. Bit
vector
In this approach, the free space list is implemented as a bit map
vector. It contains the number of bits where each bit represents each
block.
If the block is empty then the bit is 1 otherwise it is 0. Initially all the
blocks are empty therefore each bit in the bit map vector contains 1.
LAs the space allocation proceeds, the file system starts allocating
blocks to the files and setting the respective bit to 0.
16. Linked
List
It is another approach for free space
management.
This approach suggests linking together all the
free blocks and keeping a pointer in the cache
which points to the first free block.
Therefore, all the free blocks on the disks will be
linked together with a pointer.
Whenever a block gets allocated, its previous
free block will be linked to its next free block.
18. In this, we concluded
that
File allocation methods are an important aspect
of computer operating systems, as they
determine how files are stored and accessed on
disk .
Overall, the choice of free space management
technique depends on the specific requirements
of the operating system and the storage devices
being used.
