You can use simple estimates t

First let's consider the pure disk system. One terabyte is 1024 GB. To be correct, we need 103 disks at 10 GB each. But since this question is about approximations, we will simplify the arithmetic by rounding off the numbers. The pure disk system will have 100 drives. The cost of the disk drives would be $100,000, plus about 20% for cables, power supplies, and enclosures, i.e., around $120,000. The aggregate data rate would be 100 x 5 MB/ s, or 500 MB/ s. The average waiting time depends on the workload. Suppose that the requests are for transfers of size 8 KB, and suppose that the requests are randomly distributed over the disk drives. If the system is lightly loaded, a typical request will arrive at an idle disk, so the response time will be 15 ms access time plus about 2 ms transfer time. If the system is heavily loaded, the delay will increase, roughly in proportion to the queue length.

Now let's consider the hierarchical storage system. The total disk space required is 5% of 1 T B, which is 50 GB. Consequently, we need 5 disks, so the cost of the disk storage is $5,000 (plus 20%, i.e., $6,000). The cost of the 950 GB tape library is $9500. Thus the total storage cost is $15,500. The maximum total data rate depends on the number of drives in the tape library. We suppose there is only 1 drive. Then the aggregate data rate is 6 x 10 MB/ s, i.e., 60 MB/s. For a lightly loaded system, 95% of the requests will be satisfied by the disks with a delay of about 17 ms. The other 5% of the requests will be satisfied by the tape library, with a delay of slightly more than 20 seconds. Thus the average delay will be (95 >< 0.017+5 >< 20)/ 100, or about 1 second. Even with an empty request queue at the tape library, the latency of the tape drive is responsible for almost all of the system's response latency, because 1/20th of the workload is sent to a device that has a 20 second latency. If the system is more heavily loaded, the average delay will increase in proportion to the length of the queue of requests waiting for service from the tape drive.

The hierarchical system is much cheaper. For the 95% of the requests that are served by the disks, the performance is as good as a pure-disk system. But the maximum data rate of the hierarchical system is much worse than for the pure-disk system, as is the average response time.