The document discusses device management and disk scheduling algorithms. Device management involves tracking devices, allocating devices to processes, and deallocating devices. Disk scheduling is necessary when there are multiple disk requests, to determine the order to satisfy the requests. Common disk scheduling algorithms described are first come first serve (FCFS), shortest seek time first (SSTF), scan, circular scan (C-SCAN), and look. Examples are given comparing the total head movements using each algorithm on a sample disk queue.

1. DEVICE
MANAGEMENT
BY ANIL KUMAR
DEPT. OF COMPUTER SCIENCE
KABARAK UNIVERSITY, KENYA

2. THE MAIN FUNCTIONS OF DEVICE MANAGEMENT ARE:-
• Keep track of all device/peripherals.
• Decide which process gets the device, when and for how much time.
• Allocate the device and initiate the I/O operation.
• Deallocate the devices. This is done at two levels:
When I/O command has been executed, the device is temporarily
released.
As the process terminated, the device is permanently released.

3. DISK STRUCTURE

4. THE TIME TO READ OR WRITE A DISK BLOCK CONSISTS
OF THREE FACTORS:-
• Seek time: - It is the time consumed to move the arm to proper cylinder.
• Latency time: - It is the time required for searching the required sector.
• Transfer time: - It is the time required to transfer data from the sector to the
memory.
Thus the total time to access a disk request is equal to the sum of seek time,
latency time and the transfer time.
When a disk drive receives one request, there is no problem. But when there is
more than one request, then which one is to be scheduled first, is the challenge.
The process of scheduling a request among several requests is called the disk
scheduling.

5. DISK SCHEDULING ALGORITHMS
The algorithm used to select which I/O request is going to be satisfied first is
called “disk scheduling algorithm”.
Various disk scheduling algorithms are:-
• First come first serve (FCFS) scheduling.
• Shortest seek time first (SSTF) scheduling.
• Scan scheduling.
• Circular scan (C-SCAN) scheduling.
• Look scheduling.

6. DISK SCHEDULING ALGORITHMS , EXAMPLES
Lets consider a disk queue with request of I/O to block on cylinder . 98, 183, 37, 122,
14, 124, 65, 67
If the disk head is initially at cylinder 53, then calculate total number of head
movements using following algorithms:
Starts from next slide………

7. FCFS
0 14 37 53 65 67 98 122 124 19183 9
USING FCFS SCHEDULING TOTAL HEAD MOVEMENT IS:
= (98-53)+(183-98)+(183-37)+(122-37)+(122-14)+(124-14)+(124-
65)+(67-65)
= 45+ 85+146+85+108+110+59+2
= 640 cylinders.

8. SSTF
0 14 37 5 3 65 67 98 122 124 183 199
USING SSTF SCHEDULING TOTAL HEAD MOVEMENT IS:
= (65-53)+(67-65)+(67-37)+(37-14)+(98-14)+(122-98)+(124-122)+(183-
124)
= 12+2+30+23+84+24+2+59
= 236 cylinders.

9. SCAN
0 14 37 53 65 67 98 122 124 183 199
USING SCAN SCHEDULING TOTAL HEAD MOVEMENT IS:
=(53-37)+(37-14)+(14-0)+(65-0)+(67-65)+(98-67)+(122-98)+(124-
122)+(183-124)
= 16+23+14+65+2+31+24+2+59
= 236 cylinders.

10. C-SCAN
0 14 37 53 65 67 98 122 124 183 199
USING C-SCAN SCHEDULING TOTAL HEAD MOVEMENT IS:
= (65-53)+(67-65)+(98-67)+(122-98)+(124-122)+(183-124)+(199-
183)+(14-0)+(37-14)
=12+2+31+24+2+59+169+14+23
=336 cylinders

11. LOOK
0 14 37 53 65 67 98 122 124 183 199
USING LOOK SCHEDULING TOTAL HEAD MOVEMENT IS:
= (65-53)+(67-65)+(98-67)+(122-98)+(124-122)+(183-124)+(183-
14)+(37-14)
=12+2+31+24+2+59+169+23
=322 cylinders