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Unformatted text preview: the instant this operation is
completed. To access information stored on a disk, the disk address of the desired data
has to be specified. We saw that the disk address is specified in terms of the surface
number, the track/cylinder number, and the sector number. Information is always written
from the beginning of a sector and can be read only from the track beginning. Hence disk
access time depends on the following three parameters: .
Seek Time. As soon as a read/write command is received by the disk unit, the
read/write heads are first positioned on to the specified track (cylinder) number by
moving the access arms assembly in the proper direction. The time required to position
the read/write head over the desired track is called the seek time. The seek time varies
depending on the position of the access arms assembly when a read/write command is
received. If the access arms assembly is positioned on the outer most track and the track
to be reached is the inner most one then the seek time will be maximum, and it will be
zero if the access arms assembly already happens to be on the desired track. The average
seek time is thus specified for most systems which is of the order of 10 to 100
We also saw that some disk systems have multiple read/write heads on each access arm.
This is basically for reducing the seek time. For example, a disk system may have two
sets of read/write heads for each surface, one for reading/writing on the inside tracks and
another for the outside tracks. This will reduce the average seek time by half of that for a
disk system which has a single read/write head per surface, because each read/write head
needs to cover and move across only half of the total number of tracks.
Latency. Once the heads are positioned on the desired track, the head on the
specified surface is activated. Since the disk is continuously rotating, this head should
wait for the desired data (specified sector) to come under it. This rotational waiting time,
i.e., the time required to spin the desired sector under the head is called the latency. The
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- Spring '14