Minimizing Service Time for a Fixed Capacity and Request Size

For a given bit density, the capacity of a MEMS device is determined by the product of active tips and movement ranges in $X$ and $Y$. We looked for a configuration that minimized mean response time for a probe-based storage device with a fixed capacity of 2GB.

Given this constraint, our model identified an optimal configuration for each workload as shown in Figure 7b. We can see that the optimum number of active tips is at the top of the range for the values of that parameter. This is not surprising, because additional active tips lower service time. The movement ranges in $X$ and $Y$ are small, which also reduces service time. If we compare the values of the optimal configurations to the behavior of the experimentally obtained values for the service time in Figure 7a we can see that our predictions point out very well where the minimum of the service time will occur.

Note that the error is greatest for the tpcd workload, where the model generally underestimates the service time. Specifically, the model underestimates the seek time for tpcd because the distribution of seeks in the real workload does not fit our assumption of a uniformly random distribution. The tpcd workload has a significant number of large seeks that increase the overall average. As we change the movement range in $X$ and $Y$, the model prediction changes more quickly than the real workload. Thus, at higher values of $\delta_y$, the simulated and predicted service times converge.