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Disk Scheduling Algorithm in Operating System
The document discusses disk scheduling, a process used by operating systems to manage I/O requests for disk access. It explains various disk scheduling algorithms such as FCFS, SSTF, Scan, C-Scan, Look, and C-Look, detailing their advantages and disadvantages. Additionally, it defines important terms related to disk access like seek time, rotational latency, transfer time, and response time.
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Introduction of the presenter and their details.
Disk scheduling is a function of operating systems for managing I/O requests for disks.
Definitions of key terms: Seek Time, Rotational Latency, Transfer Time, Disk Access Time, and Disk Response Time.
The main purpose of disk scheduling algorithms is to efficiently schedule I/O requests.
FCFS (First Come First Serve), where requests are handled in the order they arrive. Advantages and disadvantages are discussed.
SSTF (Shortest Seek Time First) reduces average response time and increases throughput, but can cause starvation.
SCAN algorithm services requests in one direction like an elevator but may cause long waiting times for distant requests.
C-SCAN (Circular SCAN) moves in a circular fashion, providing more uniform wait times compared to SCAN.
LOOK and CLOOK algorithms optimize servicing by not traveling to the very end of the disk, thus reducing delays.
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Disk Scheduling Algorithm in Operating System
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- 3.• Disk schedulingis done by operating systems toschedule I/O requests arriving for disk. Diskscheduling is also known as I/O scheduling.• The technique that operating system uses todetermine the request which is to be satisfied next iscalled disk scheduling.
- 4.• some importantterms related to disk scheduling –• Seek time - Seek time is the time taken in locatingthe disk arm to a specified track where theread/write request will be satisfied.• Rotational Latency - It is the time taken by thedesired sector to rotate itself to the position fromwhere it can access the R/W heads.
- 5.• Transfer Time- It is the time taken to transfer thedata.• Disk Access Time - Disk access time is given as,Disk Access Time = Rotational Latency + Seek Time+ Transfer Time• Disk Response Time - It is the average of timespent by each request waiting for the IOoperation.
- 6.• Purpose ofDisk Scheduling - The mainpurpose of disk scheduling algorithm is to select adisk request from the queue of IO requests anddecide the schedule when this request will beprocessed.
- 7.The Disk schedulingalgorithms are as follows –1. FCFS :- FCFS (First come first serve) is thesimplest of all the Disk SchedulingAlgorithms. In FCFS, the requests areaddressed in the order they arrive in the diskqueue.
- 9.ADVANTAGES :-1. Everyrequest gets a fair chance.2. No indefinite postponementDISADVANTAGES :-1. Does not try to optimize seek time.2. May not provide the best possible service
- 10.2. SSTF :-In SSTF (Shortest Seek Time First),requests having shortest seek time areexecuted first. So, the seek time of everyrequest is calculated in advance in queue andthen they are scheduled according to theircalculated seek time. As a result, the requestnear the disk arm will get executed first. SSTFis certainly an improvement over FCFS as itdecreases the average response time andincreases the throughput of system.
- 11.ADVANTAGES :-1. AverageResponse Time decreases.2. Throughput increases.DISADVANTAGES :-1. Can cause Starvation for a request if it hashigher seek time as compared to incomingrequests
- 12.3. SCAN :-In SCAN algorithm the disk armmoves into a particular direction and servicesthe requests coming in its path and afterreaching the end of disk, it reverses itsdirection and again services the requestarriving in its path. So, this algorithm workslike an elevator and hence also knownas elevator algorithm. As a result, the requestsat the midrange are serviced more and thosearriving behind the disk arm will have to wait.
- 13.ADVANTAGES :-1. Highthroughput.2. Average response time.DISADVANTAGES :-1. Long waiting time for requests for locationsjust visited by disk arm
- 14.C-SCAN :- InC-SCAN algorithm in which thedisk arm instead of reversing its direction goesto the other end of the disk and starts servicingthe requests from there. So, the disk arm movesin a circular fashion and this algorithm is alsosimilar to SCAN algorithm and hence it isknown as C-SCAN (Circular SCAN).ADVANTAGES :-1. Provides more uniform wait time compared toSCAN
- 15.5. LOOK: Itis similar to the SCAN diskscheduling algorithm except the differencethat the disk arm in spite of going to the end ofthe disk goes only to the last request to beserviced in front of the head and then reversesits direction from there only. Thus it preventsthe extra delay which occurred due tounnecessary traversal to the end of the disk.
- 16.6. CLOOK: AsLOOK is similar to SCANalgorithm, in similar way, CLOOK is similar toCSCAN disk scheduling algorithm. InCLOOK, the disk arm inspite of going to theend goes only to the last request to be servicedin front of the head and then from there goesto the other end’s last request. Thus, it alsoprevents the extra delay which occurred dueto unnecessary traversal to the end of the disk.
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