Disk Scheduling Revisited

Margo Seltzer, Peter Chen, John Ousterhout


Since the invention of the movable head disk, people have improved I/O performance by intelligent scheduling of disk accesses. We have applied these techniques to systems with large memories and potentially long disk queues. By viewing the entire buffer cache as a write buffer, we can improve disk bandwidth utilization by applying some traditional disk scheduling techniques. We have analyzed these techniques, which attempt to optimize head movement and guarantee fairness in response time, in the presence of long disk queues. We then propose two algorithms which take rotational latency into account, achieving disk bandwidth utilizations of nearly four times a simple first come first serve algorithm. One of these two algorithms, a weighted shortest total time first, is particularly applicable to a file server environment because it guarantees that all requests get to disk within a specified time window.
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