LOOK algorithm

One variant of LOOK is circular LOOK (C-LOOK). It is an effort to remove the bias in LOOK for track clusters at the edges of the platter. C-LOOK basically only scans in one direction. Either you sweep from the inside out, or the outside in. When you reach the end, you just swing the head all the way back to the beginning. This actually takes advantage of the fact that many drives can move the read/write head at high speeds if it's moving across a large number of tracks (e.g. the seek time from the last track to track 0 is smaller than one would expect and usually considerably less than the time it would take to seek there one track at a time). The huge jump from one end request to the other is not considered as a head movement as the cylinders are treated as a circular list.

N and F LOOK were designed to offset LOOK’s bias towards recent jobs. Both algorithms partition the request queue into smaller sub queues and process the sub queues in order (oldest first). N-LOOK is so-called because the request queue is divided into N sub queues. F-LOOK is a simplification where there are only 2 queues, but they are used in a double-buffered fashion. While F-LOOK is processing one queue, all new requests go into the other one. To explain these algorithms we’re going to use the example of a disk with 200 tracks, and the read/write head starts at track 100. The request queue, in order, contains requests for tracks: 55, 58, 18, 90, 160, 38, we assume that the request queue is split into two, with the oldest one containing the requests for tracks: 55, 58, 18, 90. In this instance, N-LOOK and F-LOOK behave the same. Also notice, that in this configuration, it doesn’t matter which direction the head was moving in, all requested tracks are less than 100 so it will only move in the direction of decreasing tracks. Even through the average number of tracks traversed is the same as LOOK in the worst case, N and F LOOK are in some sense, more fair than plain old LOOK. The sub queue system caps the maximum latency a process can expect between a request and it being serviced (unlike SSTF that can starve processes for arbitrary lengths of time).

The shortest LOOK (S-LOOK) algorithm is an extension of the LOOK algorithm to handle the cases where the disk head is located between the far-end requests. The algorithm is designed to make a decision of which direction should be served first instead of only continuing to seek in the same direction before the new requests have arrived. Since the seek time is directly proportional to the seek distance, our goal is to minimize the seek distance, and hence, reduce the seek time.