SCAN (Elevator) Disk Scheduling Algorithms

Given an array of disk track numbers and initial head position, our task is to find the total number of seek operations to access all the requested tracks if the SCAN disk scheduling algorithm is used.

In the SCAN Disk Scheduling Algorithm, the head starts from one end of the disk and moves towards the other end, servicing requests in between one by one and reaching the other end. Then the direction of the head is reversed and the process continues as the head continuously scans back and forth to access the disk. So, this algorithm works as an elevator and is hence also known as the elevator algorithm. As a result, the requests at the midrange are serviced more and those arriving behind the disk arm will have to wait.

Advantages of SCAN (Elevator) Algorithm

• This algorithm is simple and easy to understand.
• SCAN algorithm has no starvation.
• This algorithm is better than the FCFS Disk Scheduling algorithm.

Disadvantages of the SCAN (Elevator) Algorithm 

• More complex algorithm to implement.
• This algorithm is not fair because it causes a long waiting time for the cylinders just visited by the head.
• It causes the head to move till the end of the disk in this way the requests arriving ahead of the arm position would get immediate service but some other requests that arrive behind the arm position will have to wait for the request to complete.

Algorithm

Step 1: Let the Request array represents an array storing indexes of tracks that have been requested in ascending order of their time of arrival. ‘head’ is the position of the disk head.

Step 2: Let direction represents whether the head is moving towards left or right.

Step 3: In the direction in which the head is moving, service all tracks one by one.

Step 4: Calculate the absolute distance of the track from the head.

Step 5: Increment the total seek count with this distance.

Step 6: Currently serviced track position now becomes the new head position.

Step 7: Go to step 3 until we reach one of the ends of the disk.

Step 8: If we reach the end of the disk reverse the direction and go to step 2 until all tracks in the request array have not been serviced.

Example:

Input: 
Request sequence = {176, 79, 34, 60, 92, 11, 41, 114}
Initial head position = 50
Direction = left (We are moving from right to left)
Output:
Total number of seek operations = 226
Seek Sequence is
41
34
11
0
60
79
92
114
176

The following chart shows the sequence in which requested tracks are serviced using SCAN. 

SCAN Disk Scheduling Algorithm

Therefore, the total seek count is calculated as: 

= (50-41) + (41-34) + (34-11) + (11-0) + (60-0) + (79-60) + (92-79) + (114-92) + (176-114)
= 226

Implementation

The implementation of SCAN is given below. Note that distance is used to store the absolute distance between the head and the current track position. disk_size is the size of the disk. Vectors left and right store all the request tracks on the left-hand side and the right-hand side of the initial head position respectively. 

C++

// C++ program to demonstrate
// SCAN Disk Scheduling algorithm

#include <bits/stdc++.h>
using namespace std;

int size = 8;
int disk_size = 200;

void SCAN(int arr[], int head, string direction)
{
    int seek_count = 0;
    int distance, cur_track;
    vector<int> left, right;
    vector<int> seek_sequence;

    // appending end values
    // which has to be visited
    // before reversing the direction
    if (direction == "left")
        left.push_back(0);
    else if (direction == "right")
        right.push_back(disk_size - 1);

    for (int i = 0; i < size; i++) {
        if (arr[i] < head)
            left.push_back(arr[i]);
        if (arr[i] > head)
            right.push_back(arr[i]);
    }

    // sorting left and right vectors
    std::sort(left.begin(), left.end());
    std::sort(right.begin(), right.end());

    // run the while loop two times.
    // one by one scanning right
    // and left of the head
    int run = 2;
    while (run--) {
        if (direction == "left") {
            for (int i = left.size() - 1; i >= 0; i--) {
                cur_track = left[i];

                // appending current track to seek sequence
                seek_sequence.push_back(cur_track);

                // calculate absolute distance
                distance = abs(cur_track - head);

                // increase the total count
                seek_count += distance;

                // accessed track is now the new head
                head = cur_track;
            }
            direction = "right";
        }
        else if (direction == "right") {
            for (int i = 0; i < right.size(); i++) {
                cur_track = right[i];
                // appending current track to seek sequence
                seek_sequence.push_back(cur_track);

                // calculate absolute distance
                distance = abs(cur_track - head);

                // increase the total count
                seek_count += distance;

                // accessed track is now new head
                head = cur_track;
            }
            direction = "left";
        }
    }

    cout << "Total number of seek operations = "
         << seek_count << endl;

    cout << "Seek Sequence is" << endl;

    for (int i = 0; i < seek_sequence.size(); i++) {
        cout << seek_sequence[i] << endl;
    }
}

// Driver code
int main()
{

    // request array
    int arr[size] = { 176, 79, 34, 60,
                      92, 11, 41, 114 };
    int head = 50;
    string direction = "left";

    SCAN(arr, head, direction);

    return 0;
}

Java

// Java program to demonstrate
// SCAN Disk Scheduling algorithm
import java.util.*;

class GFG
{

static int size = 8;
static int disk_size = 200;

static void SCAN(int arr[], int head, String direction)
{
    int seek_count = 0;
    int distance, cur_track;
    Vector<Integer> left = new Vector<Integer>(),
                    right = new Vector<Integer>();
    Vector<Integer> seek_sequence = new Vector<Integer>();

    // appending end values
    // which has to be visited
    // before reversing the direction
    if (direction == "left")
        left.add(0);
    else if (direction == "right")
        right.add(disk_size - 1);

    for (int i = 0; i < size; i++)
    {
        if (arr[i] < head)
            left.add(arr[i]);
        if (arr[i] > head)
            right.add(arr[i]);
    }

    // sorting left and right vectors
    Collections.sort(left);
    Collections.sort(right);

    // run the while loop two times.
    // one by one scanning right
    // and left of the head
    int run = 2;
    while (run-- >0)
    {
        if (direction == "left")
        {
            for (int i = left.size() - 1; i >= 0; i--)
            {
                cur_track = left.get(i);

                // appending current track to seek sequence
                seek_sequence.add(cur_track);

                // calculate absolute distance
                distance = Math.abs(cur_track - head);

                // increase the total count
                seek_count += distance;

                // accessed track is now the new head
                head = cur_track;
            }
            direction = "right";
        }
        else if (direction == "right")
        {
            for (int i = 0; i < right.size(); i++)
            {
                cur_track = right.get(i);

                // appending current track to seek sequence
                seek_sequence.add(cur_track);

                // calculate absolute distance
                distance = Math.abs(cur_track - head);

                // increase the total count
                seek_count += distance;

                // accessed track is now new head
                head = cur_track;
            }
            direction = "left";
        }
    }

    System.out.print("Total number of seek operations = "
                        + seek_count + "\n");

    System.out.print("Seek Sequence is" + "\n");

    for (int i = 0; i < seek_sequence.size(); i++)
    {
        System.out.print(seek_sequence.get(i) + "\n");
    }
}

// Driver code
public static void main(String[] args)
{

    // request array
    int arr[] = { 176, 79, 34, 60,
                    92, 11, 41, 114 };
    int head = 50;
    String direction = "left";

    SCAN(arr, head, direction);
}
}

// This code is contributed by 29AjayKumar

Python3

# Python3 program to demonstrate
# SCAN Disk Scheduling algorithm
size = 8
disk_size = 200

def SCAN(arr, head, direction):

    seek_count = 0
    distance, cur_track = 0, 0
    left = []
    right = []
    seek_sequence = []

    # Appending end values
    # which has to be visited
    # before reversing the direction
    if (direction == "left"):
        left.append(0)
    elif (direction == "right"):
        right.append(disk_size - 1)

    for i in range(size):
        if (arr[i] < head):
            left.append(arr[i])
        if (arr[i] > head):
            right.append(arr[i])

    # Sorting left and right vectors
    left.sort()
    right.sort()

    # Run the while loop two times.
    # one by one scanning right
    # and left of the head
    run = 2
    while (run != 0):
        if (direction == "left"):
            for i in range(len(left) - 1, -1, -1):
                cur_track = left[i]

                # Appending current track to
                # seek sequence
                seek_sequence.append(cur_track)

                # Calculate absolute distance
                distance = abs(cur_track - head)

                # Increase the total count
                seek_count += distance

                # Accessed track is now the new head
                head = cur_track

            direction = "right"

        elif (direction == "right"):
            for i in range(len(right)):
                cur_track = right[i]

                # Appending current track to seek
                # sequence
                seek_sequence.append(cur_track)

                # Calculate absolute distance
                distance = abs(cur_track - head)

                # Increase the total count
                seek_count += distance

                # Accessed track is now new head
                head = cur_track

            direction = "left"

        run -= 1

    print("Total number of seek operations =",
          seek_count)

    print("Seek Sequence is")

    for i in range(len(seek_sequence)):
        print(seek_sequence[i])

# Driver code

# request array
arr = [ 176, 79, 34, 60,
         92, 11, 41, 114 ]
head = 50
direction = "left"

SCAN(arr, head, direction)

# This code is contributed by divyesh072019

C#

// C# program to demonstrate
// SCAN Disk Scheduling algorithm
using System;
using System.Collections.Generic;

class GFG
{

static int size = 8;
static int disk_size = 200;

static void SCAN(int []arr, int head, String direction)
{
    int seek_count = 0;
    int distance, cur_track;
    List<int> left = new List<int>(),
                    right = new List<int>();
    List<int> seek_sequence = new List<int>();

    // appending end values
    // which has to be visited
    // before reversing the direction
    if (direction == "left")
        left.Add(0);
    else if (direction == "right")
        right.Add(disk_size - 1);

    for (int i = 0; i < size; i++)
    {
        if (arr[i] < head)
            left.Add(arr[i]);
        if (arr[i] > head)
            right.Add(arr[i]);
    }

    // sorting left and right vectors
    left.Sort();
    right.Sort();

    // run the while loop two times.
    // one by one scanning right
    // and left of the head
    int run = 2;
    while (run-- >0)
    {
        if (direction == "left")
        {
            for (int i = left.Count - 1; i >= 0; i--)
            {
                cur_track = left[i];

                // appending current track to seek sequence
                seek_sequence.Add(cur_track);

                // calculate absolute distance
                distance = Math.Abs(cur_track - head);

                // increase the total count
                seek_count += distance;

                // accessed track is now the new head
                head = cur_track;
            }
            direction = "right";
        }
        else if (direction == "right")
        {
            for (int i = 0; i < right.Count; i++)
            {
                cur_track = right[i];

                // appending current track to seek sequence
                seek_sequence.Add(cur_track);

                // calculate absolute distance
                distance = Math.Abs(cur_track - head);

                // increase the total count
                seek_count += distance;

                // accessed track is now new head
                head = cur_track;
            }
            direction = "left";
        }
    }

    Console.Write("Total number of seek operations = "
                        + seek_count + "\n");
    Console.Write("Seek Sequence is" + "\n");
    for (int i = 0; i < seek_sequence.Count; i++)
    {
        Console.Write(seek_sequence[i] + "\n");
    }
}

// Driver code
public static void Main(String[] args)
{

    // request array
    int []arr = { 176, 79, 34, 60,
                    92, 11, 41, 114 };
    int head = 50;
    String direction = "left";

    SCAN(arr, head, direction);
}
}

// This code is contributed by 29AjayKumar

Javascript

<script>

    // Javascript program to demonstrate
    // SCAN Disk Scheduling algorithm

    let size = 8;
    let disk_size = 200;

    function SCAN(arr, head, direction)
    {
        let seek_count = 0;
        let distance, cur_track;
        let left = [], right = [];
        let seek_sequence = [];

        // appending end values
        // which has to be visited
        // before reversing the direction
        if (direction == "left")
            left.push(0);
        else if (direction == "right")
            right.push(disk_size - 1);

        for (let i = 0; i < size; i++)
        {
            if (arr[i] < head)
                left.push(arr[i]);
            if (arr[i] > head)
                right.push(arr[i]);
        }

        // sorting left and right vectors
        left.sort(function(a, b){return a - b});
        right.sort(function(a, b){return a - b});

        // run the while loop two times.
        // one by one scanning right
        // and left of the head
        let run = 2;
        while (run-- >0)
        {
            if (direction == "left")
            {
                for (let i = left.length - 1; i >= 0; i--)
                {
                    cur_track = left[i];

                    // appending current track to seek sequence
                    seek_sequence.push(cur_track);

                    // calculate absolute distance
                    distance = Math.abs(cur_track - head);

                    // increase the total count
                    seek_count += distance;

                    // accessed track is now the new head
                    head = cur_track;
                }
                direction = "right";
            }
            else if (direction == "right")
            {
                for (let i = 0; i < right.length; i++)
                {
                    cur_track = right[i];

                    // appending current track to seek sequence
                    seek_sequence.push(cur_track);

                    // calculate absolute distance
                    distance = Math.abs(cur_track - head);

                    // increase the total count
                    seek_count += distance;

                    // accessed track is now new head
                    head = cur_track;
                }
                direction = "left";
            }
        }

        document.write("Total number of seek operations = "
                            + seek_count + "</br>");
        document.write("Seek Sequence is" + "</br>");
        for (let i = 0; i < seek_sequence.length; i++)
        {
            document.write(seek_sequence[i] + "</br>");
        }
    }

    // request array

    let arr = [ 176, 79, 34, 60, 92, 11, 41, 114 ];
    let head = 50;
    let direction = "left";

    SCAN(arr, head, direction);

</script>

Output

Total number of Seek Operations = 226

Seek Sequence: 41, 34, 11, 0, 60, 79, 92, 114, 176

Time Complexity: O(N * logN)

Auxiliary Space: O(N)

FAQs on SCAN (Elevator) Disk Scheduling Algorithms

1. Will the SCAN Algorithm consider all pending requests?

Answer:

The SCAN Algorithm considers all pending requests in one direction before changing the direction.

2. Is SCAN Preemptive or Non-Preemptive?

Answer:

SCAN is a non-preemptive disk scheduling algorithm, after starting the requests, it completes them without any interruptions.