OS - Disk Scheduling / Management

Disk Access Time= Seek time + Rotational Delay + Controller Overhead + Queuing delay

Disk Access Time Formula

FCFS SSTF SCAN CSCAN LOOK CLOOK

Disk Scheduling Algorithms

reduce total seek time

The purpose of disk scheduling algorithms is to

SSTF

This algorithm is founded on the idea that the read/write should proceed to the track that is closest to its current position. This scheme enables the R/W to service a particular location immediately in just a short distance travel. This may sound very efficient but it actually has a drawback.

SCAN

This algorithm is performed by moving the access arm back-and-forth the innermost and outermost track. As it scans the tracks from end to end, it sweeps all the requests found in the direction it is headed. This scheme ensures that all tracks, whether in the outermost, middle or innermost location, will be traversed by the access arm thereby finding all the requests. This algorithm is also known as the Elevator Algorithm.

SSTF

This algorithm selects the disk I/O request which requires the least disk arm movement from its current position regardless of the direction. It allows the head to move to the closest track in the service queue.

Disk Scheduling

is done by operating systems to schedule I/O requests arriving for the disk. Disk scheduling is also known as I/O scheduling.

Variance Response Time

is measure of how individual request are serviced with respect to average response time.

LOOK

is similar to the SCAN algorithm except for the end-to-end reach of each sweep. Instead, the read/write head is only tasked to go to the farthest location in need of servicing. Since, it is also a directional algorithm, as soon as it is done with the last request in one direction it then sweeps in the other direction.

Response Time

is the average of time spent by a request waiting to perform its I/O operation.

Average Response Time

is the response time of the all requests.

FCFS

is the simplest disk scheduling algorithm which services disk locations based on the arrival order of the requests. Since it follows the order of arrival, it causes wild swings from the innermost to the outermost tracks of the disk or vice versa. The farther the location to be serviced by the read/write head from its current location the higher the seek time will be.

Rotational Latency

is the time taken by the desired sector of disk to rotate into a position so that it can access the read/write heads. So the disk scheduling algorithm that gives minimum rotational latency is better.

Seek Time

is the time taken to locate the disk arm to a specified track where the data is to be read or write. So, the disk scheduling algorithm that gives minimum average seek time is better.

Transfer Time

is the time to transfer the data. It depends on the rotating speed of the disk and number of bytes to be transferred.

- Multiple I/O requests may arrive by different processes and only one I/O request can be served at a time by the disk controller. Thus, other I/O requests need to wait in the waiting queue and need to be scheduled. - Two or more request may be far from each other so can result in greater disk arm movement. - Hard drives are one of the slowest parts of the computer system and thus need to be accessed in an efficient manner.

Disk scheduling is important because:

CSCAN

sweeps the disk from end-to-end, but as soon as it reaches one of the end tracks it then moves to the other end track without servicing any requesting location. As soon as it reaches the other end track it when starts to look and grant service requests. This algorithm improves the unfair situation of the end tracks against the middle tracks. Notice that in this algorithm an alpha symbol was used to represent the dash line. This return sweep is sometimes given a numerical value which is included in the computation of the THM. The alpha symbolizes a reset of the access arm to the starting end of the disk track.

CLOOK

which uses a return sweep before processing a set of disk requests. Like the Look Algorithm, C-Look does not reach the end of the tracks unless there is a request, either a read or a write, on such disk location.