The MS Excel workbook ClosedQN-chap6 can now be used to solve the model. The initial customer populations for classes PDF and ZIP are set at 4 and 16, respectively (see Section 6.4.1). The total Web server traffic intensity (i.e., the total customer population or the total number of concurrent downloads) can be increased (or decreased) in the model by maintaining the 1:4 ratio between the PDF and ZIP downloads.
Suppose that the manager requests to see performance results of the original configuration against a balanced I/O configuration (i.e., one in which PDF and ZIP files are stored on disks 1 through 4 in such a way that access to each category of files is evenly distributed across all four disks). The new set of service demands for this balanced configuration is easily obtained. The CPU demands are the same as those in Table 6.1. The disk demands are computed by taking, for each class, the total disk demand (i.e., 154.2 msec and 471.6 msec for PDF and ZIP files, respectively) and, instead of dividing them evenly over two disks each, dividing them both over all four disks. Thus, for PDF files instead of 77.1 (= 154.2/2) msec for two disks, 38.6 (= 154.2/4) msec of service is required for each of four disks. Similarly, for ZIP files, the average per disk demand is 117.9 (= 471.6/4) msec. Table 6.2 shows the service demands for the balanced configuration.
The original and the balanced QN models are solved using ClosedQN-Chap6.XLS. A range of traffic intensities, varying from 20 to 180 total customers (i.e., maintaining the 1:4 ratio between PDF and ZIP customers), are considered. Figure 6.5 shows the throughput, measured in file downloads/sec, for PDF and ZIP files for both configurations. The corresponding download time results are shown in Fig. 6.6.
Figure 6.5. Throughput vs. number of concurrent users for the original file layout and for the balanced layout.
Figure 6.6. Average file download time vs. number of concurrent users for the original file layout and for the balanced layout.
Observations from the throughput results (i.e., Fig. 6.5) indicate that:
Suppose that the SLAs on download times for PDF and ZIP files are 7 seconds and 20 seconds, respectively (since ZIP files are roughly 3 times larger than PDF files, it is reasonable for the ZIP/PDF SLA ratio to be proportional). Observations from the download time results (i.e., Fig. 6.6) indicate that:
For 160 customers using the balanced configuration, the utilizations of all devices are nearly 100%. Specifically, the model indicates that UCPU = 0.98, Udisk1 = Udisk2 = 0.92, and Udisk3 = Udisk4 = 0.99.