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6.1 / MAGNETIC DISK 193

time (time after positioning the head over the target track until track identification
is confirmed).
Much improvement comes from smaller and lighter disk components. Some
years ago, a typical disk was 14 inches (36 cm) in diameter, whereas the most com-
mon size today is 3.5 inches (8.9 cm), reducing the distance that the arm has to
travel.A typical average seek time on contemporary hard disks is under 10 ms.
ROTATIONAL DELAY Disks, other than floppy disks, rotate at speeds ranging from
3600 rpm (for handheld devices such as digital cameras) up to, as of this writing,
20,000 rpm; at this latter speed, there is one revolution per 3 ms. Thus, on the aver-
age, the rotational delay will be 1.5 ms.

TRANSFER TIME The transfer time to or from the disk depends on the rotation
speed of the disk in the following fashion:
b
T =
rN
where
T = transfer time
b = number of bytes to be transferred
N = number of bytes on a track
r = rotation speed, in revolutions per second

Thus the total average access time can be expressed as
1 b
= T + +
T a s
2r rN
where T is the average seek time. Note that on a zoned drive, the number of bytes
s
per track is variable, complicating the calculation. 2
A TIMING COMPARISON With the foregoing parameters defined, let us look at two
different I/O operations that illustrate the danger of relying on average values. Con-
sider a disk with an advertised average seek time of 4 ms, rotation speed of 15,000 rpm,
and 512-byte sectors with 500 sectors per track. Suppose that we wish to read a file
consisting of 2500 sectors for a total of 1.28 Mbytes. We would like to estimate the
total time for the transfer.
First, let us assume that the file is stored as compactly as possible on the disk.
That is, the file occupies all of the sectors on 5 adjacent tracks (5 tracks × 500 sectors/
track = 2500 sectors). This is known as sequential organization. Now, the time to
read the first track is as follows:
Average seek 4 ms
Average rotational delay 2 ms
Read 500 sectors 4 ms
10 ms

2 Compare the two preceding equations to Equation (4.1).