Hard Disk
A hard disk drive (often shortened as
hard disk, hard drive, or HDD) is a non-volatile storage device that stores
digitally encoded data on rapidly rotating rigid (i.e. hard) platters with
magnetic surfaces.
Introduction:
A hard disk is part of a unit, often
called a "disk drive," "hard drive," or "hard disk
drive," that store and provides relatively quick access to large amounts
of data on an electromagnetically charged surface or set of surfaces. Today's
computers typically come with a hard disk that contains several billion bytes
(gigabytes) of storage.
A hard disk is really a set of
stacked "disks," each of which, like phonograph records, has data
recorded electromagnetically in concentric circles or "tracks" on the
disk. A "head" (something like a phonograph arm but in a relatively
fixed position) records (writes) or reads the information on the tracks. Two
heads, one on each side of a disk, read or write the data as the disk spins.
Each read or write operation requires that data be located, which is an
operation called a "seek." (Data already in a disk cache, however,
will be located more quickly.)
A hard disk/drive unit comes with a
set rotation speed varying from 4500 to 7200 rpm. Disk access time is measured
in milliseconds. Although the physical location can be identified with
cylinder, track, and sector locations, these are actually mapped to a logical
block address (LBA) that works with the larger address range on today's hard
disks.
Capacity and Performance:
A typical desktop machine will have a
hard disk with a capacity of between 10 and 40 gigabytes. Data is stored onto
the disk in the form of files. A file is simply a named collection of bytes.
The bytes might be the ASCII codes for the characters of a text file, or they
could be the instructions of a software application for the computer to
execute, or they could be the records of a data base, or they could be the
pixel colors for a GIF image. 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.
There are two ways to
measure the performance of a hard disk:
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.
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.
The other important parameter is the capacity of the drive,
which is the number of bytes it can hold.
Operation of Hard Disk:
A hard disk uses round, flat disks
called platters, coated on both sides with a special media material designed to
store information in the form of magnetic patterns. The platters are mounted by
cutting a hole in the center and stacking them onto a spindle. The platters
rotate at high speed, driven by a special spindle motor connected to the
spindle. Special electromagnetic read/write devices called heads are mounted
onto sliders and used to either record information onto the disk or read
information from it. The sliders are mounted onto arms, all of which are
mechanically connected into a single assembly and positioned over the surface
of the disk by a device called an actuator. A logic board controls the activity
of the other components and communicates with the rest of the PC.
Each surface of each platter on the
disk can hold tens of billions of individual bits of data. These are organized
into larger "chunks" for convenience, and to allow for easier and
faster access to information. Each platter has two heads, one on the top of the
platter and one on the bottom, so a hard disk with three platters (normally)
has six surfaces and six total heads. Each platter has its information recorded
in concentric circles called tracks. Each track is further broken down into
smaller pieces called sectors, each of which holds 512 bytes of information.
The entire hard disk must be
manufactured to a high degree of precision due to the extreme miniaturization
of the components, and the importance of the hard disk's role in the PC. The
main part of the disk is isolated from outside air to ensure that no
contaminants get onto the platters, which could cause damage to the read/write
heads.
Exploded line drawing of a modern hard disk, showing
the major components.
Though the specifics vary greatly between different
designs, the basic
Components you see above are typical of almost all PC
hard disks.
Here's an example case showing in
brief what happens in the disk each time a piece of information needs to be
read from it. This is a highly simplified example because it ignores factors
such as disk caching, error correction, and many of the other special
techniques that systems use today to increase performance and reliability. For
example, sectors are not read individually on most PCs; they are grouped
together into continuous chunks called clusters. A typical job, such as loading
a file into a spreadsheet program, can involve thousands or even millions of
individual disk accesses, and loading a 20 MB file 512 bytes at a time would be
rather inefficient:
The first step in accessing the disk is to figure
out where on the disk to look for the needed information. Between them, the
application, operating system, system BIOS and possibly any special driver
software for the disk, do the job of determining what part of the disk to read.
The location on the disk undergoes
one or more translation steps until a final request can be made to the drive
with an address expressed in terms of its geometry. The geometry of the drive
is normally expressed in terms of the cylinder, head and sector that the system
wants the drive to read. (A cylinder is equivalent to a track for addressing
purposes). A request is sent to the drive over the disk drive interface giving
it this address and asking for the sector to be read.
The hard disk's control program first
checks to see if the information requested is already in the hard disk's own
internal buffer (or cache). It if is then the controller supplies the
information immediately, without needing to look on the surface of the disk
itself.
In most cases the disk drive is
already spinning. If it isn't (because power management has instructed the disk
to "spin down" to save energy) then the drive's controller board will
activate the spindle motor to "spin up" the drive to operating speed.
The controller board interprets the
address it received for the read, and performs any necessary additional
translation steps that take into account the particular characteristics of the
drive. The hard disk's logic program then looks at the final number of the
cylinder requested. The cylinder number tells the disk which track to look at
on the surface of the disk. The board instructs the actuator to move the
read/write heads to the appropriate track.
When the heads are in the correct
position, the controller activates the head specified in the correct read
location. The head begins reading the track looking for the sector that was
asked for. It waits for the disk to rotate the correct sector number under
itself, and then reads the contents of the sector.
The controller board coordinates the
flow of information from the hard disk into a temporary storage area (buffer).
It then sends the information over the hard disk interface, usually to the
system memory, satisfying the system's request for data.
Types of Hard Disk
v Zip /Bernoulli Disks:
A 3.5" removable disk drive from
Iomega. Zip disks come in 100MB, 250MB and 750MB varieties, with the latter
introduced in 2002 using USB and FireWire interfaces. The 250MB drives,
introduced in 1998, also read and write 100MB disks. The 750MB drives read all,
but only write 250MB and 750MB disks.
Zip disks are a floppy-like
technology that uses design concepts from hard disks and Iomega's earlier
Bernoulli disks. The drive is bundled with software that can catalog the disks
and lock the files for security. Zip drives were introduced in 1995 and became
very popular as a transportable, rewritable storage medium.
Disk Packs:
An early removable hard disk module
used in minicomputers and mainframes that contained two or more platters housed
in a dust-free container. For mounting, the protective bottom of the unit was
removed, and the disk pack was placed into the drive. Then the top part of the
disk pack was unscrewed and removed.
Winchester disk
An early removable disk drive from
IBM that put the heads and platters (disks) in a sealed unit for greater speed.
Introduced in 1973 as the model 3340, the drive had one permanent and one
removable spindle, each holding 30MB. The term Winchester comes from an early
type of disk drive developed by IBM that had 30MB of fixed storage and 30MB of
removable storage and offered a 30 millisecond (ms) access time. So its
inventors called it a Winchester in honor of its 30/30 rifle.
What made this drive significant is
the technique where the read/write heads would rise or lift off from the disk
while the disk increased and lowered when the disk decelerated. This enabled
the heads to not have to move off the disk each time the speed was increased or
decreased. Future hard drives after the 3340 began to use this as a standard
for hard disk drive development. Although modern disk drives are faster and
hold more data, the basic technology is the same, so Winchester has become
synonymous with hard.
