It is sometimes said that tape is a sequential-access medium, whereas a 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 rate for a data transfer that is underway, excluding the effect of access latency. In contrast, the “effective transfer rate” is the ratio of total bytes per total seconds, including overhead time such as access latency.
Suppose we have a computer with the following characteristics: 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 $15$ milliseconds 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 second.
$a$. 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 $(1) 512$ bytes,
$(2) 8$ kilobytes, $(3) 1$ megabyte, and $(4) 16$ megabytes ?
$b$. The utilization of a device is the ratio of effective transfer rate to streaming transfer rate. Calculate the utilization of the disk drive for each of the four transfer sizes given in part $a$.
$c$. Suppose that a utilization of $25$ percent (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 transfers larger than bytes and is a sequential access device for smaller transfers.
$e$. Compute the minimum 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 ?