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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>SCAN - Disk Scheduling Algorithms</title>
<!-- Bootstrap -->
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.2.1/css/bootstrap.min.css"
integrity="sha384-GJzZqFGwb1QTTN6wy59ffF1BuGJpLSa9DkKMp0DgiMDm4iYMj70gZWKYbI706tWS" crossorigin="anonymous">
<!-- Font Awesome 5 -->
<link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.6.3/css/all.css"
integrity="sha384-UHRtZLI+pbxtHCWp1t77Bi1L4ZtiqrqD80Kn4Z8NTSRyMA2Fd33n5dQ8lWUE00s/" crossorigin="anonymous">
<!-- Custom style -->
<link rel="stylesheet" type="text/css" href="css/style.css">
<!-- Favicon -->
<link rel="icon" href="images/icon.png">
</head>
<body>
<!-- ============================================
===================== HEADER =================== -->
<header class="shadow-md">
<!-- Navbar -->
<nav class="navbar navbar-expand-lg navbar-dark">
<div class="container">
<!-- Logo -->
<a class="navbar-brand mr-5" href="index.html"><img src="images/heading.png" href="index.html" alt=""
width="400px"></a>
<!-- menu -->
<button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent"
aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
<span class="navbar-toggler-icon"></span>
</button>
<div class="collapse navbar-collapse" id="navbarSupportedContent">
<!-- ID = DATA TARGET FOR COLLAPSE BUTTON-->
<ul class="navbar-nav ml-auto">
<!-- margin left auto: ml-auto: does auto margin to left -->
<li class="nav-item">
<a class="nav-link" href="index.html">Home</a>
</li>
<!-- dropdown menu -->
<li class="nav-item dropdown active">
<a class="nav-link dropdown-toggle" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false"
style="cursor: pointer;">Simulate</a>
<div class="dropdown-menu" id="custom-dropdown-menu">
<a class="dropdown-item" href="FCFS/fcfs.html">FCFS</a>
<a class="dropdown-item" href="SSTF/sstf.html">SSTF</a>
<a class="dropdown-item" href="SCAN/scan.html">SCAN</a>
<a class="dropdown-item" href="CSCAN/cscan.html">C-SCAN</a>
<a class="dropdown-item" href="LOOK/look.html">LOOK</a>
<a class="dropdown-item" href="CLOOK/clook.html">C-LOOK</a>
</div>
</li>
<li class="nav-item">
<a class="nav-link" href="about.html">About Us</a>
</li>
</ul>
</div>
<!-- /menu -->
</div>
</nav>
<!-- /Navbar -->
</header>
<!-- ============================================
===================== MAIN =================== -->
<div class="py-3 mb-3">
<div class="container">
<h1>SCAN (Elevator Algorithm)</h1>
</br>
<p>Scan is often called Elevator Scheduling Algorithm, due to the way it works.<br>
In SCAN, disk arm starts from one end of the disk and executes all the requests in its way till it reaches the
other end. <br> As soon as it reaches the other end, it reverses its direction, hence it moves back and forth
continuously (like an elevator) to access the disk.
</p>
</br>
<h3> Advantages:</h3>
<ul>
<li>
There is no starvation.
</li>
<li>
Average Response time, and its variance is low.
</li>
<li>
High throughput.
</li>
<li>
Simple to understand and better in performance compared to FCFS
</li>
</ul>
</br>
<h3> Disadvantages:</h3>
<ul>
<li>
Its Implementation can be difficult and complex.
</li>
<li>
The requests at the midrange are serviced more and those arriving behind the disk arm will have to wait. Thus,
fair chance is not given to each request.
</li>
<li>
Arm will move to end even when there is no request to execute.
</li>
</ul>
<h3> Example: </h3>
<ul>
<li>
Consider a disk containing 200 tracks (0-199) and the request queue includes the track number 176, 79, 34, 60,
92, 11, 41, 114 respectively. <br>The current position of read//write head is 50, and direction is towards the
larger value.
</li>
<li>
Calculate the total number of cylinders moved by the head using SCAN disk scheduling.
</li>
<li>
As mentioned in the following example, the disk contains 200 tracks. So, we take a track line between 0 to
199.
</li>
<li>
The current position of the read/write head is 50. So, we start at 50, then move the read/write head. <br>
When all
the requests are addressed, then we calculate a total number of cylinders moved by the head.
</li>
<br>
<img src="images/scan-eg-graph.jpg" alt="graph of example">
<br> <br>
<li>
Total Number of cylinders moved by head <br>
= (50-41)+(41-34)+(34-11)
+(11-0)+(60-0)+(79-60)
+(92-79)+(114-92)+(176-114) <br>
= 226
</li>
</ul>
<h3>Steps to Implement Algorithm:</h3>
<ol>
<li>
Let 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 disk head.
</li>
<li>
Let direction represents whether the head is moving towards left or right.
</li>
<li>
In the direction in which head is moving service all tracks one by one.
</li>
<li>
Calculate the absolute distance of the track from the head.
</li>
<li>
Increment the total seek count with this distance.
</li>
<li>
Currently serviced track position now becomes the new head position.
</li>
<li>
Go to step 3 until we reach at one of the ends of the disk.
</li>
<li>
If we reach at the end of the disk reverse the direction and go to step 2 until all tracks in request array
have not been serviced.
</li>
</ol>
<p>
<b> Time Complexity: O ( N * logN )   Auxiliary Space: O ( N ) </b>
</p>
<a class="simulate" href="SCAN/scan.html">Simulate</a>
</div>
<!-- /container -->
</div>
<!-- ============================================
===================== FOOTER =================== -->
<div class="footer">
<div class="container">
<!-- Logo - About - Contact -->
<div class="row py-3 text-light">
<!-- Logo -->
<div class="col-lg-3">
<a href="index.html"><img src="images/footer.png" class="footer-logo img-fluid" width="200" draggable="false"
alt="Logo">
</a>
</div>
<!-- About -->
<div class="col-lg">
<h3>About</h3>
<p>
This project was done by Team 2 Group 5 as a part of Operating
Systems Lab.
</p>
</div>
<div class="col-lg-2">
<h3>Quick Links</h3>
<div class="footer-links">
<a href="fcfs-info.html">FCFS</a>
<a href="sstf-info.html">SSTF</a>
<a href="scan-info.html">SCAN</a>
<a href="cscan-info.html">C-SCAN</a>
<a href="look-info.html">LOOK</a>
<a href="clook-info.html">C-LOOK</a>
</div>
</div>
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</div>
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</div>
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integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo"
crossorigin="anonymous"></script>
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</body>
</html>