International Journal of Application or Innovation in Engineering & Management (IJAIEM)
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Volume 1, Issue 4, December 2012
ISSN 2319 - 4847
File Allocation Methods Performance over
Disk scheduling algorithms
Miss.Archana Wamanrao Bhade1, Miss.Seema R.Wankhade2 (Corresponding author)
1, 2
Assistant Professor Department of Information Technology,
Government College of Engineering, Amravati, India
ABSTRACT
Files are the most obvious objects in the operating systems manipulate. Everything is typically stored in files: programs, data,
output, and so on. One problem in the file management is how to allocate space for files so that disk space is utilized effectively
and files can be accessed quickly. Three major methods of allocating disk space are contiguous, linked and indexed. Each
method has its advantages and disadvantages.
Disk subsystem performance can be dramatically im-proved by dynamically ordering, or scheduling, pending re-quests. Via
strongly validated simulation, we examine the impact of complex logical-to-physical mappings and large prefetching caches on
scheduling electiveness. Hard disks are being used to store huge information/data in all modem computers. Disk drives must
provide faster access time in order to optimize speed of I/O operations. This paper describes development of a simulator which
uses four disk scheduling algorithms (FCFS, SSTF, LOOK for both upward and downward direction, and C-LOOK) to measure
their performance in terms of total head movement.
As well as a new algorithm SPFF. SPFF keeps the advantage of SCAN and, at the same time, absorbs the strength of SSTF.
The algorithm SPFF not only shows the more superiority than other scheduling polices, but also has higher adjustability to
meet the computer system’s different demands. [3]
In multitasking system with many processes, disk performance can be improved by incorporating a scheduling algorithm.
There are two objectives for any disk scheduling algorithm:
1. Minimize the throughput - the average number of requests satisfied per time unit.
2. Maximize the response time - the average time that a request must wait before it is satisfied.
Keywords: SPFF, Disk scheduling, File Allocation Methods
1. INTRODUCTION
A hard drive is a collection of plates called platters. Both sides of each platter are covered with some kind of a
magnetization medium that allows ones and zeros to be stored. Each surface is divided into circles called tracks.
Furthermore, each track is divided into smaller pieces called sectors. Disk I/O is done sector by sector. A group of
tracks that are positioned on top of each other is called a cylinder. There is a head connected to an arm for each surface,
which handles all I/O operations. Usually, all arms are attached to each other so the heads are always in the same
cylinder. For each I/O request, first, a head must be selected. This is done electronically, and the time it takes is not
significant. Then the head is moved over the destination track. After that, the disk is rotated to position the desired
sector under the head. Finally, the I/O operation is performed. Arm movements and disk rotations are where the delay
occurs.
There
are
two
objectives
for
any
disk
scheduling
algorithm:
1. Minimize the throughput - the average number of requests satisfied per time unit.
2. Maximize the response time - the average time that a request must wait before it is satisfied.
Whenever a process needs I/O to or from the disk, it issues a system call to the operating system. This request specifies
several pieces of information:
(1) Type of I/O operation,
(2) Address of disk (drive, cylinder, surface, block),
(3) Address of memory, and
(4) Amount of information is to be transferred
The operating system is responsible for using hardware efficiently — for the disk drives, this means having a fast
access time and disk bandwidth.
1.Access time has two major components
Seek time is the time for the disk are to move the heads to the cylinder containing the desired sector.
2.Rotational latency is the additional time waitin for the disk to rotate the desired sector to the disk head.
Minimize seek time Seek time  seek distance
Disk bandwidth is the total number of bytes transferred, divided by the total time between the first request for service
and the completion of the last transfer
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Different algorithms such as FCFS, SSTF, LOOK and CLOOK are used for selecting request for servicing from the
queue of requests. We illustrate them with a request queue (0-199).
98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53.
FCFS
First-Come, First-Served (FCFS) is the simplest form of disk scheduling. This algorithm is easy to implement using
FIFO queue.
Figure 1:-FCFS
Illustration shows total head movement of 640 cylinders.
SSTF
Selects the request with the minimum seek time from the current head position. SSTF scheduling is a form of SJF
scheduling; may cause starvation of some requests.
Figure 2:- SSTF
Illustration shows total head movement of 236 cylinders.
SCAN
The disk arm starts at one end of the disk, and moves toward the other end, servicing requests until it gets to the other
end of the disk, where the head movement is reversed and servicing continues. Sometimes called the elevator
algorithm. .
Figure 3:-SCAN
Illustration shows total head movement of 208 cylinders
C-SCAN
Provides a more uniform wait time than SCAN.
The head moves from one end of the disk to the other. servicing requests as it goes. When it reaches the other end,
however, it immediately returns to the beginning of the disk, without servicing any requests on the return trip.
Treats the cylinders as a circular list that wraps around from the last cylinder to the first one.
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Figure 4:- C-SCAN
C-LOOK
Version of C-SCAN-Arm only goes as far as the last request in each direction, then reverses direction immediately,
without first going all the way to the end of the disk.
Figure 5:- C-LOOK
SPFF (Shortest path first -fit first)
This algorithm is based on the shortest path of disk head motion constructed by all the pendent requests. From view of
the head-moving distance, it has the stronger globosity than SSTF. From view of the head-moving direction, it has the
better flexibility than SCAN. Therefore, SPFF keeps the advantage of SCAN and, at the same time, absorbs the
strength of SSTF. The algorithm SPFF not only shows the more superiority than other scheduling polices, but also has
higher adjustability to meet the computer system’s different demands. [3]
2.
SOFTWARE DEVELOPMENT
A simulator has been developed in JAVA language for disk scheduling algorithms which contains six major modules
(i.e. FCFS, SSTF, SCAN,C-SCAN,LOOK and C-LOOK) and one new algorithm SPFF. This simulator runs each and
presents results based upon service requests and number of tracks involved in the given test sample. Tracks requests are
read by the program from the relevant file. Algorithm automatically offers reordered list of the read requests and a
queue is displayed to service the requests. When all requests are serviced, then the result in the form of total head
movement is displayed. After this the graph is shown on screen to check performance of each disk scheduling
algorithm. Each module can be run to get new data of requests from the keyboard. The problem of disk scheduling on a
single disk is studied from the viewpoint of the characteristics peculiar to the program functions that need guaranteed
service. It is shown that a conventional disk scheduler possesses an upper bound to disk utilization which may be as low
as 70 percent for large task sets. It is also shown that full disk utilization can be achieved by dynamically assigning
processes on the basis of seek time.
3. IMPLEMENTATION AND TESTING
The lists of scheduling algorithms are implemented using java, a simulation program is created and the algorithms are
selected during the assignment phase of the request to the scheduler. Once the set of track requests are selected then we
need to select the particular file allocation method and scheduling policies. Then need to run the simulation to observe
the output. In the testing phase we select the example file which contains the list of track request .Then select the
particular File Allocation Method and then the scheduling algorithm, then select run simulation execute the project.
Following are the input parameters consider for execution according the scheduling policies. The output is calculated
according the scheduling policies.
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Volume 1, Issue 4, December 2012
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Figure 2.1 shows the flowchart of the system. It will help to how to use the system. Initially start the system then select
the process.
3.1 Input Parameters:
No. of request 5
No. of request 5
Tracks:-12, 85, 40, 100, 75
File Allocation Method used:-Linked / Indexed
Table 3.1 Sample request and the input parameters.
Service
d
request
1
2
3
4
5
Pending
request
1,2,3,4,
52,3,4,5
3,4,5
4,5
5
Selecte
d
request
1
2
3
4
5
See
k
tim
e 5
3 7
3 4
5 6
0
5
2
Head
positio
n
65
12
85
40
100
3.2 Output parameters
3.2.1Total tracks traversed (Non contiguous-static request)
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Algorit
hm/
Track
reques
t
P1
FCFS
SSTF
SCAN
CSCAN
LOOK
CLOOK
53
10
50
50
10
53
P2
73
10
15
15
10
28
P3
P4
45
60
15
60
25
35
25
35
15
60
35
10
P5
Total
25
256
28
123
28
165
28
165
28
123
15
151
3.2.2 Total seek time (Non contiguous-static request)
S
N
Tota
l
track
s
Algorith
m
Initial
Head
Positio
n
Head
Movem
ent
Directio
n
Total
tracks
traver
sed
Ave.
Seek
length
Seek
Time
(Linked
)
Seek Time
Indexed
1
5
FCFS
65
NA
256
51
1290
1290
2
5
SSTF
65
NA
123
24
625
625
3
5
SCAN
65
165
33
835
835
4
5
C-SCAN
65
Toward
s0
Upward
165
33
835
835
5
5
LOOK
65
NA
123
24
625
625
6
5
C-LOOK
65
NA
151
30
765
765
3.3 Input Parameter
No.of request 8
Tracks:- 8,9,10,11,12,13,14,15
File Allocation Method used:-Contiguous
TABLE 3.2 Sample request and the input parameters.
S.
N.
Pending
request
1
Selected
request
Seek time
Head
position
1,2,3,4,5,6,7
1
45
53
2,3,4,5,6,7,8
2
1
61
3
3,4,5,6,7,8
3
1
62
4
4,5,6,7,8
4
1
63
5
5,6,7,8
5
1
64
6
6,7,8
6
1
65
7
7,8
7
1
66
8
8
8
1
67
2
,8
3.3.1 Total tracks traversed (Contiguous Allocation)
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Algo/
Track
reques
tP1
FCF
S
SST
F
SCA
N
CSCAN
LOOK
CLOO
K
45
38
38
53
38
45
P2
1
1
1
8
1
1
P3
1
1
1
1
1
1
P4
1
1
1
1
1
1
P5
1
1
1
1
1
1
P6
P7
1
1
1
1
1
1
1
1
1
1
1
1
P8
Total
1
52
1
45
8
53
1
68
1
45
1
52
Seek
Time
Contig
uous
3.3.2 Total seek time (Contiguous Allocation)
S To
. tal
N tra
ck
s
1 8
2
8
3
8
4
8
5
8
6
8
Al
gor
ith
m
Initial
Head
Positio
n
Total
tracks
traverse
d
53
Head
Movem
ent
Directio
n
NA
52
Av
e.
Se
ek
len
6
FC
FS
SS
TF
53
NA
45
5
225
SC
A
LO
O
CLO
CSC
53
53
6
265
45
5
225
53
Toward
s0
U/WAR
D
NA
52
6
260
53
NA
68
8
340
53
260
3.4SPFF Scheduling
Sample 1:-Input Parameter-Dynamic request
No. of request:-7
Tracks:- 183,37,122,14,124,65,67
File Allocation Method used:- Linked/Indexed
TABLE 3.3 Sample request and the input parameters
Alg
orit
hm
De
tai
ls
Serviced Request
1st
2nd
3rd
requ requ requ
est
est
est
FCF
S
PR
HP
1,2,
3
98
2,3,
4
183
SR
ST
PR
1
85
1,2
HP
SR
SST
F
4th
reque
st
5th
reques
t
6th
reque
st
7th
requ
est
3,4,
5
37
4,5,6
5,6,7
6,7,
7
122
14
124
65
2
146
1,3
3
85
1,4
4
108
4,5
5
110
4,6
6
59
6,7
7
2
7
98
37
183
124
14
65
67
2
3
1
5
4
6
7
Volume 1, Issue 4, December 2012
Tot
al
See
k
Tim
e
595
429
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SC
AN
CSC
AN
LO
OK
CLO
OK
SPF
F
ST
61
61
85
110
59
51
2
PR
1,2
1,3
1,4
4,5
4,6
6,7,
7
HP
98
37
183
124
14
65
67
SR
2
3
1
5
4
6
7
ST
61
159
61
59
59
51
81
PR
1,2
2,3
3,4
4,5
4,6
6,7
7
HP
98
183
37
122
124
14
65
SR
1
2
3
5
4
6
7
ST
85
220
85
2
138
51
2
PR
1,2
2,3,
2,4
4,5
5,6,
6,7
7
HP
98
183
122
37
14
65
67
SR
1
3
2
4
6
7
5
ST
85
61
85
23
51
2
57
PR
1,2
2,3
3,4
4,5
4,6
6,7
7
HP
98
183
37
122
124
14
65
SR
1
2
3
5
4
6
7
ST
85
146
85
2
110
51
2
PR
1,2
2,3
4,6
4
98
183
2,4,6
,7
124
2,4,6,
HP
2,4,
5
122
67
65
37
SR
1
3
5
7
6
2
4
ST
85
61
2
57
2
28
23
531
583
364
532
258
PR:-Pending request
HP:-Head position
SR:-Selected Request
ST:-Seek Time
3.4.1Total tracks traversed
Algorit
hm /
Track
P1
FCFS
SSTF
SC
AN
CSCAN
LO
OK
SPF
F
85
CLOO
K
85
85
61
61
85
P2
146
61
61
220
85
146
28
P3
85
85
159
85
61
85
61
P4
108
110
59
138
23
110
23
P5
110
59
59
2
57
2
2
P6
59
51
51
51
51
51
2
P7
2
2
81
2
2
2
57
595
429
454
583
364
532
258
Total
85
3.4.2 Total seek time
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To
tal
tra
ck
s
Algor
ithm
Initia
l
Head
Posit
ion
Head
Move
ment
Direct
ion
Total
track
s
trave
rsed
Av
Se
ek
len
gth
1
8
98
NA
595
85
2
8
FCF
S
SSTF
98
NA
429
61
3
8
98
75
8
583
83
5
8
98
Towar
ds 0
U/WA
RD
NA
454
4
364
52
6
8
98
NA
532
76
7
8
SCA
N
LOO
K
CLOO
CSCA
SPFF
98
NA
258
36
98
Seek
Time
Link
ed
Seek
Time
Index
ed
2989
2985
2159
2155
2284
2280
2929
2925
1834
1830
2674
2670
1304
1300
3.5. Observations:
3.5.1Seek Time Contiguous:The fig.3.1 shows the seek time required by each scheduling algorithm on contiguous file allocation method
Seek Tim e Contiguous
400
350
300
250
200
150
100
50
0
340
260
265
225
225
260
Seek Time
Contiguous
FCFS SSTF SCAN LOOK
CCLOOK SCAN
Figure .3.1 Comparison of seek time
3.5.2 Seek Time-Non Contiguous(static request)
Linked
Indexed
1500 1290
1500 1290
835 835
1000
625
625
1000
765
500
Linked
835 835
625
625
765
Indexed
500
0
0
FCFS SSTF SCAN C- LOOK CSCAN
LOOK
FCFS SSTF SCAN C- LOOK CSCAN
LOOK
3.5.3 Seek Time-FCFS Scheduling
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FAM-Comparision
3000
2500
2000
Algorithm
1500
1000
FCFS
500
0
Seek Time
Indexed
Seek Time
(Linked)
Seek Time
Contiguous
3.5.4 Seek Time-SSTF Scheduling
FAM-COMPARISION
1500
1170
1176
1000
Algorithm
500
225
SSTF
0
Seek Time
Indexed
Seek Time
(Linked)
Seek Time
Contiguous
3.5.5 Seek Time SCAN Scheduling
1270
1400
1276
1200
1000
800
Algorithm
600
SCAN
265
400
200
0
Seek Time
Indexed
Seek Time
(Linked)
Seek Time
Contiguous
3.5.6 Seek Time C-SCAN Scheduling
2500
1945
2000
1951
1500
Algorithm
1000
C-SCAN
500
225
0
Seek Time
Indexed
Seek Time
(Linked)
Seek Time
Contiguous
3.5.7 Seek Time LOOK Scheduling
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1685
1800
1600
1400
1200
1000
800
600
400
200
0
1691
Algorithm
LOOK
260
Seek Time
Indexed
Seek Time
(Linked)
1835
1841
Seek Time
Contiguous
3.5.8. Seek Time C-LOOK Scheduling
2000
1800
1600
1400
1200
1000
800
600
400
200
0
Algorithm
C-LOOK
340
Seek Time
Indexed
Seek Time
(Linked)
Seek Time
Contiguous
3.5.9 Seek Time-Non Contiguous (Dynamic request)
3500
3500
2985
2925
3000
2500
2000
2670
2155 2280
3000
SSTF
2500
SCAN
1830
1300
1500
FCFS
LOOK
C-LOOK
1000
C-SCAN
500
SPFF
2000
2989
2929
FCFS
2674
2159
SSTF
2284
SCAN
1834
1304
1500
LOOK
C-LOOK
1000
C-SCAN
500
SPFF
0
0
Seek Time Indexed
Seek Time Linked
Input parameters and output parameters of the tracks request are found first, then select some example tracks request
for execution on the different scheduling algorithms are selected. Select some scheduling policies and execute the
selected request on them then observe the output parameters such as total number of tracks traversed, total seek time
and file allocation method used on the particular input pattern. The following outputs and performance has been
observed using scheduling policies and file allocation methods on the basis of the input parameters supplied as in
example file.
For Static request as per First Come First Serve scheduling policy it is found that the Seek Time is 260, 2585, 2591 for
File allocation methods Contiguous, Indexed and Linked respectively.
As per SSTF Scheduling policy it is found that the seek time is 225,1170,1176 for File allocation methods Contiguous,
Indexed and Linked respectively.
As per SCAN Scheduling policy it is found that the seek time is 265, 1270, 1276 for File allocation methods
Contiguous, Indexed and Linked respectively.
As per C-SCAN Scheduling policy it is found that the seek time is 225,1945,1951 for File allocation methods
Contiguous, Indexed and Linked respectively.
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As per LOOK Scheduling policy it is found that the seek time is 260, 1685, 1691 for File allocation methods
Contiguous, Indexed and Linked respectively.
As per C-LOOK Scheduling policy it is found that the seek time is 340, 1835, 1841 for File allocation methods
Contiguous, Indexed and Linked respectively.
As per SPFF Scheduling policy, it is found that the seek time is 1300, 1304 using File Allocation methods Indexed and
Linked
Hence it has been observed that the Contiguous File Allocation method provides better result for each Scheduling
algorithm and minimized the Seek Time.
After comparing two non contiguous file allocation method, it has been observed that indexed file allocation method
provides the better result than the linked file allocation method for static as well as dynamic request.
4. RESULTS & CONCLUSION
Various test samples of size 4, 8, 16, 20, up to 50 tracks request (static requests) were used to check the performance of
each and every disk scheduling policy on different file allocation methods. Initial head position for each case was
different. Head movements of each algorithm have been illustrated using the test samples and figures. After comparing
the workload and the File Allocation Methods used, tabulated results indicated that FCFS provides the worst
performance due to presence of wild swings. FCFS goes for a lengthy seek to service a distant waiting request even
though another request may have just arrived near the track where the read/write head is currently positioned. Its total
head movement is 256, 515,52 tracks for samples 1, 2 and 3 respectively. In SSTF algorithm, a substantial
improvement has been observed due to elimination of large swings. It reduces the head movements and provides results
of 123,232,45 tracks for three test samples. LOOK algorithm works like SSTF except that it uses either upward or
downward direction to choose the requests from the rearranged queue. Performance of this algorithm was 123,335,45
tracks for three test sample. Results obtained for SCAN are 165, 252,53 tracks for three test sample. Results obtained
for C-SCAN are 165, 387, 68 tracks and C-LOOK are 151,335,52 tracks for three samples. The problem with the SSTF
is that it provides some process to wait for long time until its request(s) are satisfied if new requests with shorter seek
time keep arriving. SCAN provides better results than the SSTF and more fair than the SSTF. C-LOOK and C-SCAN
are always better than the FCFS and the SSTF for heavy loads only.These results provide the best combination of disk
scheduling algorithm and File system which improve the performance of disk I/O. after testing all the scheduling
algorithms on each file allocation methods it is found that Contiguous File Allocation always provides better seek time
for any number of request, but this allocation method is very much hypothetical for large file. Comparing two noncontiguous file allocation methods for all scheduling algorithms, it has been observed that up to load of 4 Linked File
Allocation Method is better using SSTF scheduling.For load 8, 16,20 Indexed File Allocation is better using
SSTF/SCAN scheduling. For load 24-50 Indexed File Allocation is better using SCAN scheduling. It has been observed
that if the requests are arriving dynamically then comparing with all scheduling algorithm on Linked and Indexed File
Allocation methods, LOOK and C-LOOK provides better results than other conventional scheduling algorithm. The
SPFF(shortest path first-fit first)scheduling policy provides best result on both Linked and Indexed File allocation
method. After comparing the results of SPFF on Linked and Indexed File Allocation Methods, it is found that SPFF
provides best result on Indexed File Allocation. These results can be used to analyze the underlying file allocation
method used and depending upon the File Allocation and work load the scheduling algorithm may be choose which
provides the optimal result. These results are extremely useful for designing more efficient and effective disk
scheduling algorithms to reduce seek time in multiprogramming environment.
REFERENCES
[1] B. L. Worthington, G. R. Ganger, Y. N. Patt, “Scheduling Algorithms for Modern Disk Drives,” In ACM
Sigmetrics Conference, pp. 241-25, May, 1994.
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Algorithm Selection,” WSEAS Transactions On Information Science And Applications, Issue 7, Vol 3, pp. 13611368, July 2006.
[3] H. Ming, “A disk scheduling algorithm: SPFF,”Wuhan University Journal of Natural Sciences, Vol 10 Number 6
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International Journal of Application or Innovation in Engineering & Management (IJAIEM)
Web Site: www.ijaiem.org Email: editor@ijaiem.org, editorijaiem@gmail.com
Volume 1, Issue 4, December 2012
ISSN 2319 - 4847
[7] M. Y. Javed and Mr. I. U. Khan, “Simulation and Performance Comparison of Four Disk Scheduling
Algorithms”, Vol 2, 888379, IEEE TRANSACTIONS, ISBN: 0-7803-6355-8,TENCON, pp.10-15, 2000.
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AUTHOR
Miss. Archana W. Bhade received the B.E. and M.E. degrees in Computer science & engineering from Government College of
Engineering,Amravati and PRMIT & R, Badnera in 2001 and 2009, respectively. Currently she is working as Assistant Professor in
Government College of Engineering, Amravati.
Miss. Seema R. Wankhade received the B.E. and M.E. degrees in Computer science & engineering from Anuradha College of
Engineering,Amravati and PRMIT & R, Badnera in 2001 and 2009, respectively. Currently she is working as Assistant Professor in
Government College of Engineering, Amravati.
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