Diagnosis and Performance Monitoring - Deploying and Managing a Cloud Infrastructure - page 141

mechanical parts, such as the speed of the rotating spindle and the speed of the read/write
head's actuator arm. All these individual measurable variables are added together to come
up with a single value that evaluates the performance of the drive in terms of access time.
This measure varies widely from manufacturer to manufacturer and per model; even dives
of the same class and model may not perform at exactly the same level. Solid-state drives
are not dependent on mechanical parts, so they do not have the same limitations and the
access times are rather short and consistent. The two measurements that define access time
are seek time and rotational latency.
Seek Time
The data on disk media is stored in segments called sectors, which are arranged one after
the other linearly in circular tracks. The disk is in turn made up of multiple tracks arranged
in concentric circles. Depending on the type of disk, some tracks are arranged in a spiral
pattern, but the typical standard is a concentric pattern.
The seek time is the time it takes for the drive's disk head assembly to travel and position
itself over the track where the data needs to be read or written. This is done through the actu-
ator arm, which controls where the read/write head goes. The drive uses the actuator arm to
move the head to the correct track where the sector containing the data is located. If the head
is already on the correct track at the time a read or write command is issued, then the seek
time would be zero. But if the head is currently on the outermost track and it needs data on
the innermost track, the seek time is at the maximum.
A rotating disk drive's seek time is the average of all possible seek times, which is the
sum of all possible seek times divided by the number of those seeks. But in application, the
seek time is actually is actually determined by a statistical algorithm that is approximated
as the seek time over a third of the total number of tracks.
The first hard drives had an immense seek time of 600 milliseconds (ms), but by the
mid 1970s, this was cut down to 25 ms. In the 1980s, voice coil type actuation reduced
seek times to around 20 ms. But that is the extent of the breakthrough because since then
the improvements are at a snail's pace and average seek times today are still only a little
bit below 20 ms, averaging in range from 10 ms to 18ms, depending on the quality of the
HDD model.
Rotational Latency
Rotational latency is the delay caused by waiting for the rotating disk to bring the required
sector under the read/write head. This is entirely dependent on the rotational speed of
a disk through its spindle motor, which is measured in revolutions per minute (rpm).
For most drives, the average rotational latency in milliseconds is half of the rotational
period. On the other hand, the maximum rotational latency is the time that it takes to do
a full rotation without taking into consideration any spin-up time. It is a simple matter
to decrease the rotational latency by simply increasing the rotational speed of the disks,
which also has the added benefit of increasing throughput, especially if the number of sec-
tors per track is increased. Table 5.2 shows the common disk rotational speeds and the
associated rotational latency of each.