chap 4

• Disk scheduling is done by operating systems to schedule I/O requests arriving for the disk.
• Seek Time is the time taken to locate the disk arm to a specified track where the data is to be read or written.
• The disk scheduling algorithm that gives a minimum average seek time is better.
• Rotational Latency is the time taken by the desired sector of the disk to rotate into a position so that it can access the read/write heads.
• The disk scheduling algorithm that gives minimum rotational latency is better.
• Transfer Time is the time to transfer the data, which depends on the rotating speed of the disk and the number of bytes to be transferred.
• Response Time is the average time spent by a request waiting to perform its I/O operation.
• The average Response time is the response time of all requests.
• Variance Response Time is the measure of how individual requests are serviced with respect to average response time.
• The disk scheduling algorithm that gives minimum variance response time is better.
• FCFS is the simplest of all Disk Scheduling Algorithms.
• In FCFS, the requests are addressed in the order they arrive in the disk queue.
• Advantages of FCFS include providing a fair chance to every request and avoiding indefinite postponement.
• Disadvantages of FCFS include not optimizing seek time and potentially providing the worst possible service.
• In the SCAN algorithm, the disk arm moves in a particular direction and services the requests coming in its path, then reverses its direction and again services the request arriving in its path.
• The SCAN algorithm, also known as an elevator algorithm, is characterized by high throughput, low variance of response time, and average response time.
• Disadvantages of SCAN include long waiting time for requests for locations just visited by the disk arm and potentially providing the worst possible service.
• CSCAN algorithm is similar to SCAN, but instead of reversing its direction, the disk arm goes to the other end of the disk and starts servicing the requests from there.
• Advantages of C-SCAN include providing more uniform wait time compared to SCAN.
• SSTF (Shortest Seek Time First), requests having the shortest seek time are executed first.
• The seek time of every request is calculated in advance in the queue and then they are scheduled according to their calculated seek time.
• As a result, the request near the disk arm will get executed first.
• SSTF is an improvement over FCFS as it decreases the average response time and increases the throughput of the system.
• Advantages of Shortest Seek Time First include a decrease in the average response time and an increase in throughput.
• Disadvantages of Shortest Seek Time First include an overhead to calculate seek time in advance, the potential for starvation if a request has a higher seek time than incoming requests, and the high variance of response time as SSTF favors only some requests.
• LOOK Algorithm is similar to the SCAN disk scheduling algorithm except for the difference that the disk arm in spite of going to the end of the disk goes only to the last request to be serviced in front of the head and then reverses its direction from there only.
• Total Seek Time = Total head Movement * Seek Time
• Disk Access Time = Seek Time + Rotational Latency + Transf
• File management is a basic but important feature provided by the operating system.
• The file system in the operating system is capable of managing individual as well as groups of files present in the computer system.
• The file system in the operating system tells us about the location, owner, time of creation and modification, type, and state of a file present on the computer system.
• Files can be read-only or read-write.
• Files are simply used as a medium for providing input(s) and getting output(s).
• An Operating System is a software program that acts as an interface between the hardware, the application software, and the users.
• The main aim of an operating system is to manage all the computer resources.