It is sometimes said that tape is a sequential-access medium, whereas magnetic d

Question

It is sometimes said that tape is a sequential-access medium, whereas magnetic disk is a random-access medium. In fact, the suitability of a storage device for random access depends on the transfer size. The term streaming transfer rate denotes the data rate for a transfer that is underway, excluding the effect of access latency. By contrast, the effective transfer rate is the ratio of total bytes per total seconds, including overhead time such as the access latency.

Suppose that, in a computer, the level 2 cache has an access latency of 8 nanoseconds and a streaming transfer rate of 800 megabytes per second, the main memory has an access latency of 60 nanoseconds and a streaming transfer rate of 80 megabytes per second, the magnetic disk has an access latency of 15 millisecond and a streaming transfer rate of 5 megabytes per second, and a tape drive has an access latency of 60 seconds and a streaming transfer rate of 2 megabytes per seconds.

a. Random access causes the effective transfer rate of a device to decrease, because no data are transferred during the access time. For the disk described, what is the effective transfer rate if an average access is followed by a streaming transfer of 512 bytes, 8 kilobytes, 1 megabyte, and 16 megabytes?

b. The utilization of a device is the the ratio of effective transfer rate to streaming transfer rate. Calculate the utilization of the disk drive for random access that performs transfers in each of the four sizes given in part a.

c. Suppose that a utilization of 25% (or higher) is considered acceptable. Using the performance figures given, compute the smallest transfer size for disk that gives acceptable utilization.

d. Complete the following sentence: A disk is a random-access device for transfer larger than bytes, and is a sequential-access device for smaller transfers.

e. Compute theminimum transfer sizes that give acceptable utilization for cache,memory, and tape.

f. When is a tape a random-access device, and when is it a sequential-access device?

Explanation / Answer

a. For the disk, the transfer time (excluding access time) is calculated as bytes bytespersecond . The effective transfer rate is the number of bytes divided by the number of seconds. For 512 bytes, the transfer time is about 0.1 millisecond, so the total time is about 15.1 milliseconds, and the effective transfer rate is about 34,000 bytes per second. Similarly, for 8 kilobytes, the transfer time is about 1.6 ms, so the effective transfer rate is about 490,000 bytes per second. For 1 MB, the effective transfer rate is about 4,900,000 bytes per second, and for 16 MB, the effective transfer rate is 5,218,419 bytes per second.

b. At 512 bytes, the utilization is about 34,000/5,242,880, i.e., about 0.65%. At 8 KB, the utilization is about 9.3%, at 1 MB the utilization is about 93.5

c. A 25% utilization means that 1/4 of the time is spent transferring data, and 3/4 of the time is spent on access overhead

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