1. FCFS       first come first service

This is a relatively simple disk scheduling algorithm. It schedules according to the order in which the process requests access to the disk. The advantage of this algorithm is fair and simple, and the requests of each process can be processed in turn, without the situation that the requests of a process can not be satisfied for a long time. This algorithm does not optimize the seek. In the case of more access requests to the disk, this algorithm will reduce the throughput of the device service, resulting in the average seek time may be longer, but the response time of each process to get the service changes less.

2. Shortest seek time first algorithm (SSTF)

The algorithm chooses such a process, which requires that the track to be accessed is closest to the track where the current head is located, so as to minimize the seek time each time. The algorithm can get better throughput, but it can not guarantee the shortest average seek time. Its disadvantage is that the response opportunity to the user's service request is not equal, so the response time changes greatly. In the case of many service requests, the request to the inner and outer edge tracks will be delayed indefinitely, and the response time of some requests will be unexpected.

3. Scan algorithm (SCAN)       elevator scheduling

The scanning algorithm considers not only the distance between the track to be accessed and the current track, but also the current moving direction of the head. For example, when the head is moving from the inside to the outside, the next access object selected by the scanning algorithm should be the track it wants to access, which is not only outside the current track, but also the closest. In this way, the magnetic arm can only be reversed and moved from the outside to the inside until no more tracks need to be accessed. At this time, the process is also selected to schedule every time, that is, the track it wants to access is within the current track, so as to avoid the occurrence of starvation. Because the moving law of magnetic head in this algorithm is similar to the operation of elevator, it is also called elevator scheduling algorithm. This algorithm basically overcomes the shortcomings of the shortest seek time priority algorithm, which focuses on the middle track and changes greatly in response time, and has the advantages of the shortest seek time priority algorithm, that is, the throughput is large and the average response time is small. However, due to the swing scanning method, the frequency of the two tracks is still lower than the middle track.

4. Cyclic scanning algorithm (CSCAN)

Cyclic scanning algorithm is an improvement of scanning algorithm. If the access request to the track is evenly distributed, there are relatively few access requests falling behind the head when the head reaches one end of the disk and moves in reverse direction. This is because these tracks have just been processed, and the request density at the other end of the disk is quite high, and the waiting time for these access requests is long. In order to solve this situation, the circular scanning algorithm stipulates that the head moves in one direction. For example, when the head moves to the most visited track, the head immediately returns to the most desired track, that is, the minimum track number is followed by the maximum track number to form a cycle for scanning.