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Enlist different file allocation methods? Explain contiguous
allocation method in detail.
Ans From the user's point of view, a file is an abstract data type. It can be created. Im- listing,
opened, written, read, closed and deleted without any real concern for its 2m for
implementation. The implementation of a file is a problem far the operating diagram, 3m
system. for
The main problem is how to allocate space to these files so that disk space is explanation
effectively utilized and files can be quickly accessed.
Three major methods of allocating disk space are in wide use:
Contiguous
Linked
Indexed
Contiguous Allocation
The contiguous allocation method requires each file to occupy
a set of contiguous addresses on the disk. Disk addresses define
a linear ordering on the disk. Contiguous allocation of a file is
defined by the disk address of the first block and its length. If
the file is 'n' blocks long and starts at location 'b', then it
occupies blocks b, b+l, b+2, - --- - b+n-1. The directory entry
for each file indicates the address of the starting block and the
length of the area allocated for this file.
Contiguous allocation suppots both sequential and direct
25 | 2 6
MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION
(Autonomous)
(ISO/MEC- 27001 -2013 Certified)
accesS.
For direct access to block i of a file, which starts at block 'b',
we can imnediately access block b+i. The difficulty with
contiguous allocation is finding space for a new file.
For direct access to block i' of a file, which starts at block 'b',
we can imnediately access block b+i.
The difficulty with contiguous allocation is finding space for a
new file.
If file to be created ure 'n' blocks long, we must search free
space list for 'n' free contiguous blocks.
Directory
Filo Start Longth
cOunt
1 2 3 cOunt
tr
4 56
8 910 11O
tr
121 13 14 5
17 18 19
na
20 21 23[
24 25 26st 27O
28 29 30 31
Advantages of Contiguous File Allocation Method:
1. Supports both sequential and direct access methods.
2. Contiguous allocation is the best form of allocation for
sequential files. Multiple blocks can be brought in at a time to
improve /O performance for sequential processing.
3. It is also easy to retrieve a single block from a file. For
example, ifa file starts at block 'n' and the ith block of the file
is wanted, its location on secondary storage is simply n + i.
4. Reading all blocks belonging to each file is very fast.
5. Provides good performance.
Disadvantages of Contiguous File Allocation Method:
1. Suffers from external fragmentation.
2. Very difficult to find contiguous blocks of space for new files.
3. Also with pre-allocation, it is necessary to declare the size of
the file at the time of creation which many a times is difficult to
estimate.
4. Compaction may be required and it can be very expensive.
e) Deseribe free space management technique. (Any two) 4M
Ans. 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 Any 2
disk.There are mainly four approaches by using which, the free techniques
blocks in the disk are managed. Correct
Explanation
2M each
Bit Vector
2 Linked List
1)Bit Vector:
The free-space list is implemented as a bit map or bit vector.
Each block is represented by I bit. If the block is free, the bit is 1; if
the block is allocated, the bit is 0.
For example,consider a disk where blocks 2, 3, 4, 5,8, 9, 10, 11, 12,
13 are free and the rest of the blocks are allocated.
The free-space bit map would be : 0011110011111100
0 2 3 5 6 7 9 10|1l| 12 13 14 15
0 0 1 | 1 |1 | 1 0 01 1 00
l=Free block
0= Allocated block
The main advantage of this approach is its relative simplicity and its
efficiency in finding the first free block or n consecutive free blocks
on the disk.
2) Linked List
In this approach, the free disk blocks are linked together ie. a free
block contains a pointer to the next free block. The block number of
the very first disk block is stored at a separate location on disk and is
also cached in memnory. In this approach, link all the disk blocks
together, keeping a pointer to the first free block. This block contains
a pointer to the next free disk block, and so on.
Page 16 / 21
MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION
(Autonomous)
(ISO/IEC - 27001 - 2005 Certified)
WINTER- 2022EXAMINATION
MODEL ANSWER
22516
Subject: Operating System Subject Code:
fre
kathe
3. Indexed allocation
(MSBTE -W-08, W-18)
Q.6.5.12 Explain Indexed allocaton method.
Ref Sec. 6.5(3) W-08, 4 Marks
O. 6.5.13 Descrlbe ndexed alfocation method with
advantage and disadvantage.
Ret. Sec. 6.5(3) W-18, 4Marks
FAT stores the directory entries present with starting
block in it. Both FAT table address and file address
written in the directory can match and file is read.
..A SACHINSHAHVenture
6-1:
Operating System (CO/IF/Sem 5/MSBTE)
The linked allocation cannot perform well if FAT is not
there. This problem is recovered by indexed allocation.
Index allocation brings all pointers together into one
location called as indexed block.
address
If we wish to search ih block of file. we use i"
created,
of index block entry. Initially when the file is
be set to nil.
all the pointers in the indexed block will
entries
When the file is written from its " block, the
willbe made in index block.
directory
file
index block
ABC 19
e 10110
12 18J14
1671D 19 21
25
24J2s28er
28J2930J31
Fig.6.5.5: Indexed allocation
from wasted
Index support direct access. It suffers
than linked
space problem. The pointer head is more
allocation as the block size of indexed block is more.
Advantages
1
This method is faster than contiguous and linked
allocation.
2.
Block creates the set of pointer at one location.
3. It does not suffer from extra fragmentation.
4 Accessing index block is easy.
Disadvantages
1 It suffers because of space wasted.
2 Deciding indexed block size for each file is
complicated process.
3 Storing many addresses i.e. pointers becomes an
overhead.
