Information Technology Reference
In-Depth Information
must first seek to the right track, then wait for the first desired sector to rotate
to the head, and then transfer the blocks. So, the time for a disk access is:
disk access time = seek time + rotation time + transfer time
Seek. The disk must first
seek |move its arm over the desired track.
Definition: seek
To seek, the disk first activates a motor that moves the arm assembly to
approximately the right place on disk. Then, as arm stops vibrating from
the motion of the seek, the disk begins reading positioning information
embedded in the sectors to determine exactly where it is and to make
fine-grained positioning corrections to settle on the desired track. Once
Definition: settle
the head has settled on the right track, the disk uses signal strength and
positioning information to make minute corrections in the arm position to
keep the head over the desired track.
A request's seek time depends on how far the disk arm has to move.
A disk's minimum seek time is the time it takes for the head to move
Definition: minimum seek
time
from one track to an adjacent one. For short seeks, disks typically just
\resettle" the head on the new track by updating the target track number
in the track-following circuitry.
Minimum seek times of 0.3{1.5 ms are
typical.
If a disk is reading the tth track on one surface, its head switch time is the
Definition: head switch
time
time it would take to begin reading the tth track on a different surface.
Tracks can be less than a micron wide and tracks on different surfaces
are not perfectly aligned. So, a head switch between the same tracks on
different surfaces has a cost similar to a minimum seek: the disk begins
using the sensor on a different head and then resettles the disk on the
desired track for that surface.
A disk's maximum seek time is the time it takes the head to move from
Definition: maximum seek
time
the innermost track to the outermost one or vice versa. Maximum seek
times are typically over 10 ms and can be over 20 ms.
A disk's average seek time is the average across seeks between each possible
Definition: average seek
time
pair of tracks on a disk. This value is often approximated as the time to
seek one third of the way across the disk.
Rotate. Once the disk head has settled on the right track, it must wait
for the target sector to rotate under it. This waiting time is called the
rotational latency . Today, most disks rotate at 4200 RPM to 15,000 RPM
Definition: rotational
latency
(15 ms to 4 ms per rotation), and for many workloads a reasonable esti-
mate of rotational latency is one-half the time for a full rotation|7.5 ms{
2 ms.
Once a disk head has settled on a new track, most disks immediately begin
reading sectors into their buffer memory, regardless of which sectors have
been requested. This way, if there is a request for one of the sectors that
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