SSTF Scheduling

It seems reasonable to service all the requests close to the current head position before moving the head far away to service other requests. This assumption is the basis for the shortest-seek-time-first (SSTF) algorithm.
The SSTF algorithm selects the request with the minimum seek time from the current head position.
Since seek time increases with the number of cylinders traversed by the head, SSTF chooses the pending request closest to the current head position.
For our example request queue (see Fig. 12.9);
```
53, 65, 67, 37, 14, 98, 122, 124, 183
```
Figure 12.9: SSTF disk scheduling.

$\includegraphics[scale=0.7]{figures/12-06}$
This scheduling method results in a total head movement of only 236 cylinders-little more than one-third of the distance needed for FCFS scheduling of this request queue. This algorithm gives a substantial improvement in performance.
SSTF scheduling is essentially a form of shortest-job-first (SJF) scheduling; and like SJF scheduling, it may cause starvation of some requests (steady supply of shorter seek time requests).
Although the SSTF algorithm is a substantial improvement over the FCFS algorithm, it is not optimal. Consider;
```
53, 37, 14, 65, 67, 98, 122, 124, 183
```
This strategy reduces the total head movement to 208 cylinders.

**Figure 12.9:** SSTF disk scheduling.
$\includegraphics[scale=0.7]{figures/12-06}$

Cem Ozdogan 2010-05-25