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Databases for Soft Real Time System

In disk based scheduling, the disks are located and traced by traversing the sectors and tracks. Tracks are concentric circles and sectors are just originating from the center of the disks.
So, disk scheduling algorithms are slower when compared with the memory based scheduling. Under disk based scheduling
Ta=Tw+Tp+Tt
Ta is the access time
Tw is the time spent in queue
Tp is the time to position the arm to locate the sectors and track
Tt is the time taken to transfer the block of data.
Since Tp is in the order of few milliseconds, but CPU time is hardly around 50nano seconds, so disk based scheduling algorithms are not suitable for hard real time systems. However these algorithms, can be made useful for soft real time systems. Disk based scheduling will be useful for Soft Real time system based on the following algorithms
First Come First Serve (FCFS)

The task or transaction which comes first will be scheduled and then the next, here the main problem is if the requests are huge then this algorithm is not suitable.
Elevator or Scan policy

Scans in one direction and serves the requests, if not then comes to starting position and runs again from the beginning. In the following diagram, the requests are dark lines and servings are given in dotted arrow lines. In this first case, the order of servings will be 3,4,5,9,1,1,2 (as 3,4,5,9 are over, it comes back to its position and starts from 1 and 1 and then 2) the other method will be scans in one direction and serves the requests, if not reverse the direction and serves the request and comes to starting position and then starts for the next cycle . The following example depicts both. In this the order of serving will be 3,4,5,9,2,1,1 (after 3,4,5 and9, the servings are reversing, so that 2,1,1) or the third method will be allocation of priority to requests. But who will give and how to set priority to the new requests.




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