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Hard disk Guide
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The hard disk can have a huge impact on the performance
of your PC: The fact is that the rotating magnetic media
of the hard disk is one of the severest performance bottlenecks,
causing second-long delays while fat programs spin off
the disk and into RAM. Whereas disk access times are measured
in milliseconds, system RAM performance is counted in
nanoseconds. Understanding hard disk operation - and optimizing
- can eliminate teeth-grinding delays.
The factors that affect the speed of a Hard disk:
Rotation speed
Number of sectors per track
Seek time / head switch time / cylinder switch time
Rotational latency
Data access time
Cache on the HD
How data is organized on the disks
Transfer rates
Interface (EIDE / SCSI)
What are sectors,
tracks, heads and cylinders?
On a Hard disk, data is stored in the magnetic coating
of the disk. The so called head, held by an actor arm,
is used to write and read data. This disk rotates with
a constant turn time, measured in revolutions per minute
(rpm). Data is organized on a disk in cylinders, tracks
and sectors. Cylinders are concentric tracks on the surface
of the disk. A track is divided into sectors. A Hard disk
has a head on each side of a disk. Nowadays, the actuator
arm is moved by a servo-motor (not a step-motor which
needs more time while swinging in after moving over the
desired track). All hard disks have reserved sectors,
which are used automatically by the drive logic if there
is a defect in the media.
Rotation speed
Typical hard disks have a rotation speed from 4,500 to
7,200 rpm, a 10,000 rpm drive just hit the market. The
faster the rotation, the higher the transfer rate, but
also the louder and hotter the HD. You may need to cool
a 7200 rpm disk with an extra fan, or its life would be
much shorter. Modern HD’s read all sectors of a track
in one turn (Interleave 1:1). The rotation speed is constant.
Number of sectors per track
Modern harddisks use different track sizes. The outer
parts of a disk have more space for sectors than the inner
parts. Usually, HD’s begin to write from the outside
to the inside of a disk. Hence, data written or read at
the beginning of a HD is accessed and transferred faster
rate.
Seek time / head switch time
/ cylinder switch time
The fastest seek time occurs when moving from one track
directly to the next. The slowest seek time is the so
called full-stroke between the outer and inner tracks.
Some harddisks (especially SCSI drives) don't execute
the seek command correctly. These drives position the
head somewhere close to the desired track or leave the
head where it was. The seek time everyone is interested
in is the average seek time, defined as the time it takes
to position the drive's heads for a randomly located request.
Yes, you are correct: seek time should be smaller if the
disk is smaller (5 1/4", 3 1/2" etc.).
All heads of a Hard disk are carried on one actuator arm,
so all heads are on the same cylinder. Head switch time
measures the average time the drive takes to switch between
two of the heads when reading or writing data.
Cylinder switch time is the average time it takes to move
the heads to the next track when reading or writing data.
All these times are measured in milliseconds (ms).
Rotational latency
After the head is positioned over the desired track, it
has to wait for the right sector. This time is called
rotational latency and is measured in ms. The faster the
drives spins, the shorter the rotational latency time.
The average time is the time the disk needs to turn half
way around, usually about 4ms (7200rpm) to 6ms (5400rpm).
Data access time
Data access time is the combination of seek time, head
switch time and rotational latency and is measured in
ms.
As you now know, the seek time only tells you about how
fast the head is positioned over a wanted cylinder. Until
data is read or written you will have to add the head
switch time for finding the track and also the rotational
latency time for finding the wanted sector.
Cache
I guess you already know about cache. All modern HD’s
have their own cache varying in size and organization.
The cache is normally used for writing and reading. On
SCSI HD’s you may have to enable write caching, because
often it is disabled by default. This varies from drive
to drive. You will have to check the cache status with
a program like ASPIID from Seagate.
You may be surprised that it is not the cache size that
is important, but the organization of the cache itself
(write / read cache or look ahead cache).
With most EIDE drives, the PC’s system memory is
also used for storing the HD’s firmware (e.g. software
or "BIOS"). When the drive powers up, it reads
the firmware from special sectors. By doing this, manufacturers
save money by eliminating the need for ROM chips, but
also give you the ability to easily update your drives
"BIOS" if it is necessary (Like for the WD drives
which had problems with some motherboard BIOS' resulting
in head crashes!).
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