Unformatted text preview: y, also known as the rotational delay time, is also a variable and depends on the
distance of the desired sector from the initial position of the head on the specified track. It
also depends on the rotational speed of the disk which may be anywhere from 300 rpm
(rotations per minute) to 7200 rpm. An average latency time is thus normally specified which is of the order of 5 to 80 milliseconds. Note that the average latency of a disk
system is equal to half the time taken by the disk to rotate once. Hence the average
latency of a disk system whose rotational speed is 3600 rpm will be 0.5/3600 minutes =
3. Transfer Rate. Transfer rate refers to the rate at which data are read from or written
to the disk. Once the read/write head is positioned over the desired sector, the data are
read/written at a speed determined by the rotational speed of the disk. If the rotational
speed of a disk is 3600 rpm, the disk has 125 sectors/track, and 512 bytes/sector, then in
one full revolution of the disk, the amount of data transferred will be 125 x 512=64,000
bytes = 64 k bytes (approximately). Hence the transfer rate of the disk system will be
64,000 * 3600/60 bytes/second = 38,40,000 bytes/second = 3.8 Megabytes/second
(approximately). Notice that the transfer rate of a disk system depends on the density of
the stored data and the rotational speed of the disk.
Since the data transfer time is negligible (due to high transfer rate) as compared to seek
time and latency, the average access time for a disk system is the sum of its average seek
time and average latency. The average access time for different types of disk systems
varies over a wide range and may be anywhere from 10 to 600 milliseconds.
Since the access time for a piece of data stored on a disk depends on the physical location
of the data, it is more correct to refer to a disk system as direct access storage device
instead of random access storage device. Random access refers to a storage device in
which the access time is independent of the physical loca...
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This document was uploaded on 04/07/2014.
- Spring '14