Inch ∗ (TPI).
• Bit Density - Number of bits recorded per unit length of a track, de fi ned
in units of Bits per Inch (BPI).
• Areal Density - Number of bits recorded per unit area of the disk
surface. It is equivalent to the product of track density and bit density,
and is de fi ned in units of bits per square inch.
• Access Time - This is the time required to retrieve a block of data
from the disk and is equal to sum of seek time and average latency, both
de fi ned below.
• Seek Time - Time taken by the head positioning servomechanism to
move the read/write head from one track to another.
• Single Track Seek Time - Seek Time for moving the head from one
track to the adjacent track.
• Average Seek Time -ExactSeek Time depends on the seek length, i.e.,
the difference between the initial track and destination track. Average
Seek Time is an average of seek times for all possible seek lengths.
• One-third Stroke Seek Time - Seek Time formovingheadovera
distance equal to one third of the maximum stroke.
• Latency - The process of reading or writing can not be initiated imme-
diately after positioning the head over the destination track as the exact
location of the track may not be under the head at that moment. The
read-write process must wait for some time before the desired sector of
data is available. This waiting time is the latency, and it contributes to
the access time.
• Average Latency - Each data retrieval process has a different latency.
Average latency is the time equal to half the time required for one revo-
lution of the disk.
During the process of seek from one track to another, the error between
the position of the head and the destination track gradually becomes smaller.
However, it is practically impossible to bring the error to zero and maintain
it there from that time onward. Even though the head positioning servomech-
anism tries to make the head follow the center of a track while reading or
writing data, it is practically not possible to make the error zero. So the end
of a seek process does not imply zero position error. In fact, the seek is assumed
to come to an end if the position error remains less than some pre-speci fi ed
limit for few consecutive samples. The limit is typically 15% of the track pitch
before a reading operation is all owed, and 10% of track pitch for a seek prior to
Imperial units are widely used in the HDD industry.