Computer Maintenance
Hard Drives
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Copyright © Texas Education Agency, 2011. All rights reserved.
What is a Hard Drive?
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The hard disk drive (HDD) is a high-volume,
disk-storage device with fixed, high-density,
rigid media composed of relatively inflexible
aluminum, glass platters, or disks. This
inflexibility led to the name hard disk drive. In
the past, the hard drive was typically not
removable, which is why IBM has referred to
hard drives as fixed disk drives.
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Hard Disk Basics
Hard disks were invented in the 1950s. They started as large disks,
up to 20 inches in diameter, holding just a few megabytes. They
were originally called "fixed disks" or "Winchesters" (a code name
used for a popular IBM product). They later became known as "hard
disks" to distinguish them from "floppy disks." Hard disks have a
hard platter that holds the magnetic medium, as opposed to the
flexible plastic film found in tapes and floppies.
At the simplest level, a hard disk is not that different from a cassette
tape. Both hard disks and cassette tapes use the same magnetic
recording techniques. Hard disks and cassette tapes also share the
major benefits of magnetic storage – the magnetic medium can be
easily erased and rewritten, and it will "remember" the magnetic flux
patterns stored onto the medium for many years.
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Hard Disk Basics
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The hard-disk platter is then polished to mirror-like
smoothness.
On a hard disk, you can move to any point on the
surface of the disk almost instantly.
In a hard disk, the read/write head "flies" over the
disk, never actually touching it.
A hard-disk platter can spin underneath its head at
speeds up to 3,000 inches per second; that’s about
170 mph or 272 kph!
The information on a hard disk is stored in extremely
small magnetic domains. The size of these domains
is made possible by the precision of the platter and
the speed of the medium.
IT: Computer Maintenance - Hard Drives
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Hard Disk Basics

A modern desktop machine will have a hard disk with a
capacity between 20 and 80 gigabytes. Data is stored on the
disk in the form of files.
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A file is simply a named collection of bytes. The bytes might be
the ASCII codes for the characters of a text file, the instructions
from a software application for the computer to execute, the
records of a data base, or the pixel colors for a GIF image.
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No matter what it contains, however, a file is simply a string of
bytes. When a program running on the computer requests a
file, the hard disk retrieves its bytes and sends them to the CPU
one at a time.
IT: Computer Maintenance - Hard Drives
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Hard Disk Basics
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There are two ways to measure the
performance of a hard disk:
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Data rate – The data rate is the number of
bytes per second that the drive can deliver to
the CPU. Rates between 5 and 40 megabytes
per second are common.
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Seek time – The seek time is the amount of
time between when the CPU requests a file
and when the first byte of the file is sent to the
CPU. Times between 10 and 20 milliseconds
are common.
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Inside a Hard Disk
It is a sealed aluminum box with
controller electronics attached
to one side.
The electronics
 control
the read/write mechanism
and the motor that spins the
platters
 assemble the magnetic domains
on the drive into bytes (reading)
and turn bytes into magnetic
domains (writing)
 are all contained on a small board
that detaches from the rest of the
drive
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Inside a Hard Disk
Underneath the board are
the connections for the
motor that spins the
platters, as well as a
highly-filtered vent hole
that allows internal and
external air pressures to
equalize.
IT: Computer Maintenance - Hard Drives
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Inside a Hard Disk
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The platters, which typically spin at
5,400 or 7,200 rpm when the drive
is operating, are manufactured to
amazing tolerances, and are mirrorsmooth.
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The arm that holds the read/write
heads is controlled by the
mechanism in the upper-left corner,
and is able to move the heads from
the hub to the edge of the drive.
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its movement mechanism is extremely
light and fast
on a typical hard-disk drive, it can move
from hub to edge and back up to 50
times per second
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Inside a Hard Disk
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In order to increase the amount of information
the drive can store, most hard disks have
multiple platters. This drive has three platters
and six read/write heads.
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Inside a Hard Disk
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The mechanism that moves the arms on a hard disk
has to be incredibly fast and precise. It can be
constructed using a high-speed linear motor.
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Inside a Hard Disk
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Many drives use a "voice coil" approach -- the
same technique used to move the cone of a speaker
on your stereo is used to move the arm.
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Storing the Data
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Data is stored on the
surface of a platter in
sectors and tracks.
Tracks are concentric
circles, and sectors are
pie-shaped wedges on a
track.
A typical track is shown in
yellow; a typical sector is
shown in blue. A sector
contains a fixed number
of bytes -- for example,
256 or 512.
Either at the drive or the
operating system level,
sectors are often grouped
together into clusters.
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Storing the Data
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The process of low-level formatting a
drive establishes the tracks and sectors
on the platter. The starting and ending
points of each sector are written onto the
platter. This process prepares the drive to
hold blocks of bytes.
High-level formatting then writes the filestorage structures, like the file-allocation
table, into the sectors. This process
prepares the drive to hold files.
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Tracks, Cylinders, and Sectors
The tracks are numbered, starting from zero, starting at
the outside of the platter and increasing as you go in. A
modern hard disk has tens of thousands of tracks on
each platter.
Data is accessed by moving the heads from the inner to
the outer part of the disk, driven by the head actuator.
This organization of data allows for easy access to any
part of the disk, which is why disks are called random
access storage devices.
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Tracks, Cylinders, and Sectors
Each track can hold many thousands of bytes of data. It
would be wasteful to make a track the smallest unit of
storage on the disk, since this would mean that small
files would waste a large amount of space. Therefore,
each track is broken into smaller units called sectors.
Each sector holds 512 bytes of user data, plus as many
as a few dozen additional bytes used for internal drive
control, and for error detection and correction. The first
PC hard disks typically held 17 sectors per track.
Today's hard disks can have thousands of sectors in a
single track, and make use of zoned recording to allow
more sectors on the larger outer tracks of the disk.
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The Difference Between Tracks and
Cylinders
A hard disk is usually made up of multiple platters,
each of which uses two heads to record and read
data: one for the top of the platter and one for the
bottom.
The heads that access the platters are locked together
on an assembly of head arms. This means that all the
heads move in and out together, so each head is
always physically located at the same track number. It
is not possible to have one head at track 0 and
another at track 1,000.
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The Difference Between Tracks and
Cylinders
Because of this arrangement, the track location of the
heads is not often referred to as a track number but as
a cylinder number. A cylinder is basically the set of
the tracks at which all the heads are currently located.
So if a disk had four platters, it would (normally) have
eight heads, and cylinder number 720 (for example)
would be made up of the set of eight tracks (one per
platter surface) at track number 720.
The name comes from the fact that if you mentally
visualize these tracks, they form a skeletal cylinder
because they are equal-sized circles stacked one on
top of the other in space.
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The Difference Between Tracks and
Cylinders
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Interleaving
A common operation when working with a hard disk is
reading or writing a number of sectors of information in
sequence. After all, a sector only contains 512 bytes of
user data, and most files are much larger than that.
Let's assume that the sectors on each track are numbered
consecutively, and say that we want to read the first 10
sectors of a given track on the hard disk. Under ideal
conditions, the controller would read the first sector, then
immediately read the second, and so on, until all 10
sectors had been read. Just like reading the first 10 words
in this sentence in a row.
IT: Computer Maintenance - Hard Drives
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Interleaving
However, the physical sectors on a track are adjacent to
each other and not separated by very much space. Reading
sectors consecutively requires a certain amount of speed
from the hard disk controller. The platters never stop
spinning, and as soon as the controller is done reading all of
sector #1, it has little time before the start of sector #2 is
under the head.
Many older controllers used with early hard disks did not
have sufficient processing capacity to be able to do this.
They would not be ready to read the second sector of the
track until after the start of the second physical sector had
already spun past the head, at which point it would be too
late.
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Interleaving
If the controller is slow in this manner, and no
compensation is made for the controller, it must wait for
almost an entire revolution of the platters before the start
of sector #2 comes around and it can read it. Then, of
course, when it tries to read sector #3, the same thing
would happen, and another complete rotation would be
required.
All of this waiting around would kill performance: if a disk
had 17 sectors per track, it would take 17 times as long to
read those 10 sectors as it should have in the ideal case!
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Interleaving
To address this problem, older controllers employed a
function called interleaving, allowing the setting of a disk
parameter called the interleave factor. When interleaving
is used, the sectors on a track are logically re-numbered
so that they do not correspond to the physical sequence
on the disk.
The goal of this technique is to arrange the sectors so that
their position on the track matches the speed of the
controller, to avoid the need for extra "rotations.”
Interleave is expressed as a ratio, "N:1", where "N"
represents how far away the second logical sector is from
the first, how far the third is from the second, and so on.
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Interleaving
An example is the easiest way to demonstrate this
method. The standard for older hard disks was 17 sectors
per track.
Using an interleave factor of 1:1, the sectors would be
numbered 1, 2, 3, .. , 17, and the problem described
above with the controller not being ready in time to read
sector #2 would often occur for sequential reads.
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Interleaving
Instead, an interleave factor of 2:1 could be used. With this
arrangement, the sectors on a 17-sector track would be
numbered as follows: 1, 10, 2, 11, 3, 12, 4, 13, 5, 14, 6, 15, 7,
16, 8, 17, 9. Using this interleave factor means that while sector
1 is being processed, sector 10 is passing under the read head,
and so when the controller is ready, sector 2 is just arriving at
the head.
To read the entire track, two revolutions of the platters are
required. This is twice as long as the ideal case (1:1
interleaving with a controller fast enough to handle it) but it is
almost 90% better than what would result from using 1:1
interleaving with a controller that is too slow (which would mean
17 rotations were required).
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Interleaving
What if the controller was too slow for a 2:1 interleave? It
might only be fast enough to read every third physical
sector in sequence. If so, an interleave of 3:1 could be
used, with the sectors numbered as follows: 1, 7, 13, 2, 8,
14, 3, 9, 15, 4, 10, 16, 5, 11, 17, 6, 12.
Again here, this would reduce performance compared to
2:1, if the controller was fast enough for 2:1, but it would
greatly improve performance if the controller couldn't
handle 2:1.
IT: Computer Maintenance - Hard Drives
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Interleaving
So this begs the question then: how do you know
what interleave factor to use? Well, on older hard disks, the
interleave factor was one parameter that had to be tinkered with to
maximize performance. Setting it too conservatively caused the
drive to not live up to its maximum potential, but setting it too
aggressively could result in severe performance hits due to extra
revolutions being needed.
The perfect interleave setting depended on the speeds of the hard
disk, the controller, and the system. Special utilities were written to
allow the analysis of the hard disk and controller, and would help
determine the optimal interleave setting. The interleave setting
would be used when the drive was low-level formatted, to set up
the sector locations for each track.
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Interleaving
On modern disk drives, the interleave setting is
always 1:1. Controller too slow? Today's controllers are so
fast, much of the time they sit around waiting for the
platters. How did this situation come to change so
drastically in 15 years?
Well, it's pretty simple. The spindle speed of a hard disk has
increased from 3,600 RPM on the first hard disks, to today's
standards of 5,400 to 10,000 RPM. An increase in speed of
50% to 177%. The faster spindle speed means that much
less time is needed for the controller to be ready before the
next physical sector comes under the head.
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Interleaving
However, look at what processing power has done in
the same time frame: CPUs have gone from 4.77 MHz
speeds to the environs of 1 GHz; an increase of over
20,000%! The speed of other chips in the PC and its
peripherals have similarly become faster by many multiples.
As a result of this increase in speed in modern circuits,
controller speed is no longer an issue for current drives.
There is in fact no way to set the interleave for a modern
drive; it is fixed at 1:1 and no other setting would be
necessary. Understanding interleaving is still important
because the concept forms the basis for more advanced
techniques such as head and cylinder skew, which are used
on modern drives.
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Maintaining Your Hard Drive
To maintain your hard drive, you should know
how to
• remove unnecessary files and clutter
• check the integrity of your hard drive
• defrag your hard drive
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Removing Unnecessary Files
Every time you run a program, install, uninstall, or go on the
web, junk files get left behind. It is good to remove these junk
files. In the System Tools of your computer is a utility called
Disk Cleanup.
• Disk Cleanup will search your hard drive and remove the
files you no longer need.
• There are other programs that can also perform a more
thorough cleanup of your drive.
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Check the integrity of your
hard drive
The Windows Operating System includes a utility called
Scan Disk. Scan Disk is located in the System Tools folder
with the Disk Cleanup utility.
Scan Disk will check the hard drive for errors in the file
system and attempt to fix anything it finds. It can also check
for defects on the platters themselves.
Scan Disk should be run before defragging.
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Defragging A Hard Drive
As you use your computer, some files can become
fragmented, meaning that part of a file may be stored in
one location, and the rest of it in another. In order for your
computer to read the file, it will need to go to both
locations, resulting in slowed performance.
It makes sense that if the computer only had to look in one
location to get an entire file, it would perform faster.
Defragging a hard drive will accomplish this.
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Fragmented vs. Defragmented
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Reference
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www.howstuffworks.com
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