Page 63 - UNI 101 Computer Science Handout.

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Once the sector is reached, the transfer of data can begin. Since the disk is rotating at a fixed speed,

the time required to transfer the block, known as transfer time, is defined by the number of sectors on

a track, since this establishes the percentage of the track that is used by a single data block. The transfer
time is defined by

Transfer time= 1/ (number of sectors × rotational speed)

If the hard drive in the example contains 30 sectors per track, the transfer time for a single block would
be

Transfer time= 1/ (30×60) =0.55milliseconds

Figure below shows a table of typical disks of different types, comparing various characteristics of the

disks. Since the total time required to access a disk block is approximately the sum of these three

numbers, a typical disk access might require 20 to 25msec. To put these speeds in perspective, consider
that the typical modern computer can execute an instruction in less than 1 nanosecond. Thus, the CPU

is capable of executing millions of instructions in the time required for a single disk access. This should

make it very clear to you that disk I/O is a major bottleneck in processing and also that it is desirable to
find other work that the CPU can be doing while a program is waiting for disk I/O to take place.

An expansion of part of a track to show a single data block is shown in Figure below. The block consists

of a header,512 bytes of data, and a footer. An inter block gap separates the block from neighboring

blocks.

Figure 6 A Single Data Block.

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