Page 247 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes the density of bits per track is uniform. The farther a track is from the center of the disk, the
greater its length, so the more sectors it can hold. As we move from outer zones to inner
zones, the number of sectors per track decreases. Tracks in the outermost zone typically
hold 40 percent more sectors than do tracks in the innermost zone. The drive increases its
rotation speed as the head moves from the outer to the inner tracks to keep the same rate
of data moving under the head. This method is used in CD-ROM and DVD-ROM drives.
Alternatively, the disk rotation speed can stay constant, and the density of bits decreases
from inner tracks to outer tracks to keep the data rate constant. This method is used in hard
disks and is known as constant angular velocity (CAV). The number of sectors per track
has been increasing as disk technology improves, and the outer zone of a disk usually has
several hundred sectors per track. Similarly, the number of cylinders per disk has been
increasing; large disks have tens of thousands of cylinders.

The number of sectors per track has been increasing as disk technology
improves, and the outer zone of a disk usually has several hundred sectors
per track. Similarly, the number of cylinders per disk has been increasing;
large disks have tens of thousands of cylinders.

7.3 Disk Scheduling

In multiprogramming systems, many processes may be generating requests for reading and
writing disk records. Because these processes often make requests faster than they can be
serviced by the moving head disks, waiting queues are build up for each devices. In order to
stop unbounded increase in the queue length these pending requests must be examined and
serviced in an efficient manner. Disk scheduling involves a careful examination of pending
requests to determine the most efficient ways to service the waiting requests terms.
Latency Time: The time it takes for the data block to rotate from its current to just under the
read-write head is called latency time.
Seek Time: The time it takes to position the read-write head on the top of the track where data
block is stored.

Transfer Time: The time it takes to transfer a block of data from the disk to memory.
These times >> CPU processing time.

Transfer rate of RL81 (VAX Disk) = 2.2 mb/sec
Data block size = 512 bytes.
Total block transfer time (Latency+Seek+Transfer) = about 0.1 sec.
CPU will take about 9600 ns (0.0000096 sec) to read this block.

Give the detail how to schedule the disk in operating system.

Disk Scheduling Algorithms
First Come First Served: Process the first request then the next and so on.

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