- In Chapter 3, we discussed the role of RAM, the computer's main memory.
RAM temporarily holds program instructions, data, and output
until they are no longer needed by the computer. As soon as the
computer finishes with any given program and its data, those items are erased
from RAM. Consequently, if programs, data, and processing results
are to be preserved for future use, a computer system needs more permanent
storage. Storage systems fill this role.
We begin this chapter with a discussion of characteristics common among
storage systems. Then we cover one of the most important kinds
of storage systems in use today-those based on magnetic disks.
While this part of the chapter is primarily about floppy disk drives and
hard disk drives, we also look at other common magnetic storage devices,
such as Zip drives. From there, we study optical discs, namely
CDs and DVDs, and then turn to other types of storage systems, such as
magnetic tape, flash memory, online storage, and smart cards.
The chapter concludes with a summary and comparison of the storage devices
covered in the chapter 9.
PROPERTIES OF STORAGE SYSTEMS
- Several important properties characterize storage systems.
In this section, we consider some of the most significant, including the
two physical parts of a storage system, the non-volatility property of
storage media, the ability to remove storage media from many storage devices,
and the methods used to access and represent data.
- Storage Devices and Media
- There are two parts to any storage system: a storage device and a
storage medium. The storage medium is where the data is
actually stored (such as a floppy disk or CD); a storage medium needs
to be inside the appropriate storage device (such as a floppy drive
or CD drive) to be read from or written to. Often the storage
device and medium are two separate pieces of hardware, though with some systems --
such as a hard drive -- the two parts are permanently sealed together to
form one single piece of hardware.
Storage devices can be internal (located inside the system unit), external
(plugged into an external port on the system unit), or remote (located on
another computer, such as a network server). Internal devices
have the advantage of requiring no additional desk space and are often faster
than their external counterparts. External devices can be more
easily used with multiple computers, or added to a PC that has no room left
inside its system unit. Remote devices are accessed over a network,
such as a home network, a company network, or the Internet. Regardless
of how they are connected to the computer, letters of the alphabet and/or names
are assigned to each storage device, so the devices can be identified when they
need to be used (see Figure 4-1).
- Storage media are non-volatile. This means that when you shut off
power to a storage device, the data stored on that device's storage medium
will still be there when you turn the device back on. This feature
contrasts with RAM, which is volatile. As discussed previously,
data held in RAM is erased once it is no longer needed or the power to the
computer is turned off.
- Removable vs. Fixed Media
- In many storage systems, although the storage device is always connected to
the computer, the storage medium used with that device can be inserted and
removed. These are called removable-media storage systems.
Floppy disks, CDs, and DVDs are examples of removable media.
On the other hand, fixed-media storage systems, such as most
hard drive systems, seal the storage medium (such as the hard disk) inside
the storage device (such as the hard drive) and users cannot remove it.
Fixed-media devices generally provide higher speed and better reliability
at a lower cost than removable-media alternatives. Removable-media
devices have other advantages, however, including the following.
Unlimited storage capacity -- You can insert a new medium into the storage
device to replace one that has become full.
Transportability -- You can easily share media between computers and people.
Backup -- You can make a duplicate copy of valuable data on a
removable medium and store the copy away from the computer, for use if
the original copy is destroyed.
Security -- Sensitive programs or data can be saved on removable
media and stored in a secured area.
Virtually all desktop, notebook, and tablet PCs include both removable-media
and fixed-media storage systems.
- Random vs. Sequential Access
- When the computer system receives an instruction that requires data or programs
located in storage, it must go to the designated location on the appropriate
medium and retrieve the requested data or programs. This procedure
is referred to as access. Two basic access methods are available:
random and sequential.
Random access, also called direct access, means that data can be retrieved
directly from any location on the medium, in any order. With
sequential access, however, the data can only be retrieved in the order
in which it is physically stored on the medium. Most of a PC's
storage devices -- including hard disk drives, floppy disk drives, and CD/DVD
drives -- are random access devices. They work like audio CDs
or movie DVDs—the user can jump directly to a particular selection or location,
as needed. One type of PC storage device that uses sequential access
is a tape drive. Computer tapes work like audio cassette tapes or
videotapes -- to get to a specific location on the tape, you must play or
fast-forward through all of the tape before it. Media that allow
random access are sometimes referred to as addressable media.
This means that the storage system can locate each piece of stored data
or each program at a unique address, which is determined by the computer system.
- Logical vs. Physical Representation
- Anything (such as a program, letter, digital photograph, or song) stored
on a storage medium is referred to as a file. Data files
are also sometimes called documents. When a document that
was just created (such as a memo or letter in a word processing program)
is saved, it is stored in a new file on the storage medium that the user
designates. During the storage process, the user is required to
give the file a name, called a filename; that name is used when the user
requests to see the document at a later time.
To keep files organized, related documents are often stored inside folders
located on the storage medium. For example, one folder might
contain memos to business associates while another might hold a set of
budgets for a specific project (see
To further organize files, you can create subfolders within a folder.
For instance, you might create a "Letters" folder that contains one
subfolder for letters sent to friends and a second sub-folder for
letters sent to potential employers. In
), both Budgets and Memos are subfolders inside the My Documents folder.
Although both the user and the computer use drive letters, folder names,
and filenames to save and retrieve documents, the way a user perceives
this process differs from the way a computer implements this process.
Typically, a user views how data is stored (what we have
discussed so far in this section and what appears in the Windows Explorer
) using logical file representation. That
is, we view a document stored as one complete unit in a particular folder
on a particular drive. In contrast, the physical way data is
stored and organized on the storage media (as viewed by the computer) is
called physical file representation. For example, the ABC
Industries Proposal Memo file shown in
is logically located
within the Memos folders inside the My Documents folder on the hard drive C,
but the data in these folders could be physically stored in many different
pieces scattered across that hard drive. When this occurs, the
computer keeps track of the various locations used and the logical
representation (filename, folder names, and drive letter) that is being
used to identify that file. Fortunately, we don't have to be
concerned with how files are physically stored on a disk, because the
computer keeps track of that and retrieves files for us whenever we request them.
MAGNETIC DISK SYSTEMS
- Speedy access to data, relatively low cost, and the ability to erase and
rewrite data make magnetic disks the most widely used storage media on today's
computers. With magnetic storage systems, data is written by read/write
heads magnetizing particles a certain way on a medium's surface.
The particles retain their magnetic orientation until the orientation is changed
again, so files can be stored, rewritten to the disk, and erased, as needed.
Storing data on a magnetic disk is illustrated in Figure 4-3.
The most common type of magnetic disk is the hard disk; another
common type of magnetic disk is the floppy disk.
- Floppy Disks and Drives
- Over the years, most PCs have been set up to use a floppy disk -- sometimes
called a diskette or disk -- to accommodate removable storage needs.
Floppy disks are a removable medium and very inexpensive, so
they are handy for such tasks as backing up small amounts of data, sending
small files to others, and sharing data between two computers— such as a
computer at home and one at school. Floppy disks are written to
and read by floppy disk drives (commonly called just floppy drives).
Because floppy drives are relatively slow and their capacity
is relatively small compared to newer removable storage options, some
manufacturers refer to the floppy drive as a legacy drive and are no
longer automatically including one as part of their computer systems.
Instead of having an internal floppy drive, some PC buyers
are opting for an external portable floppy drive that they can move from
PC to PC, as needed.
Floppy Disk Characteristics
- A floppy disk consists of a round piece of flexible plastic coated with
a magnetizable substance. The disk is protected by a square,
rugged plastic cover lined with a soft material that wipes the disk
clean as it spins (see Figure 4-4).
The surface of a floppy
disk is organized into circular rings called tracks, and pie-shaped sectors.
On most PC systems, the smallest storage area on a disk is a
cluster -- the part of a track that crosses a specific number
(always two or more) of adjacent sectors (see
is stored along the tracks of the disk; tracks, sectors, and clusters
are numbered so that the computer can record where data is stored and
can retrieve it at a later time. To accomplish this, the PC
keeps a directory -- called the file directory or file allocation table (FAT)
-- of where each file is physically stored, its size, and what filename
the user has assigned to it. When the user requests a
document (always by filename), the computer uses the FAT to retrieve it.
A cluster is the smallest addressable area on a disk;
consequently, everything stored on a disk always takes up at least
one cluster of space on the disk. When a file takes up
more than one cluster of space, each cluster contains directions
pointing to the next cluster used, so the computer can retrieve
all pieces of the file in the proper order when it is needed.
Most floppy disks in use today measure 3 1/2 inches in diameter
(small enough to fit into a shirt pocket) and can store 1.44 MB of
data, which is sufficient to store about 500 or so pages of double-spaced
text created using a common word processing program. Digital
photographs, music files, or documents containing a lot of images usually
require a higher capacity removable storage media—such as a CD, DVD,
or high-capacity magnetic disk— which are discussed shortly.
Using Floppy Disks
- To use a floppy disk, it must first be inserted into a floppy drive
(with the label area facing up and closest to the user, as illustrated
in Figure 4-6).
When it is completely inserted, the disk clicks
into place, the metal shutter is moved aside to expose the surface of the
disk, and the eject button on the front of the drive pops out.
Because the drive openings for some other types of removable disks
(such as Zip disks, discussed shortly) are similar in size and
appearance to a floppy drive opening, be careful when inserting a
floppy disk to ensure that you are using the proper drive.
If the disk does not fit into or doesn't "click" into place inside the
drive opening, you are likely inserting it into the wrong drive.
Before a floppy disk can be used, it must ^formatted to prepare it for
use. Most floppy disks sold today are already formatted for
either IBM or Macintosh computers and, therefore, are ready to use.
Formatting a disk that already contains data erases
everything on the disk. Although in the past users would
reformat floppy disks if they became unreliable, today users typically
discard floppy disks when they become unreliable because of their very
low cost. The formatting process is sometimes used, however,
to quickly erase a floppy disk for reuse.
When the floppy disk needs to be accessed, the drive begins to rotate
the disk within its plastic cover. The drive's read/write
head can read (retrieve) data from or write (store) data onto the actual
surface of the disk while the disk is spinning. The read/write
heads move in and out, allowing the read/write head access to all tracks
on the disk. While the disk is spinning, the drive's indicator
light goes on—don't remove the floppy disk while this light is on.
To remove the disk, wait until the light goes off, and then you
can press the eject button to remove the disk.
High-Capacity Removable Magnetic Disks and Drives
A number of higher-capacity removable magnetic storage media -- sometimes called
superdiskettes -- have emerged in recent years, either as replacements for
standard floppy disks or as supplemental storage solutions.
Although some of these systems have a large installed base and are still
widely used at the present time, that may not be the case in the future as
recordable optical disc technology improves. High-capacity
removable disks include Zip disks and SuperDisks.
Zip disks, introduced by Iomega Corporation in 1995, are high-capacity magnetic
disks that can be read from and written to only with Zip drives.
Zip disks are similar in size and appearance to floppy disks (see Figure 4-7)
but have a capacity of 100, 250, or 750 MB.&
Zip drives are downward
compatible, meaning the higher-capacity Zip drives can read any Zip disks
at their designated storage capacity or lower. For instance,
the Zip 750 drive can read all three sizes of Zip disks (although it can only
write to Zip 250 and Zip 750 disks), while the Zip 100 drive can only be used
with 100 MB Zip disks. Zip disks cannot be used in a conventional
floppy disk drive, and none of the Zip drives can read standard floppy disks.
Zip drives are most appropriate for users who need to back up large
files or transfer large files between PCs or other users that have a Zip drive.
Because Zip drives were one of the first high-capacity removable
storage solutions, they enjoy widespread use.
SuperDisk drives, originally made by Imation and more technically called
laser servo (LS) drives, are similar to Zip drives in that they accept disks --
called SuperDisks or LS-120 or LS-240 disks, depending on their capacity --
with larger capacities (120 or 240 MB) than standard floppy disks.
While SuperDisk drives are slower than Zip drives, they have the advantage of
being able to read from and write to standard floppy disks, in addition to
SuperDisks. A regular floppy drive, however, cannot read a SuperDisk.
SuperDisk drives are no longer being manufactured by Imation, although
other LS drives are available. It is expected that other types
of high-capacity media -- such as optical discs and flash memory media --
will eventually replace both the conventional floppy disk and high-capacity magnetic disks.
Hard Disk Drives
With the exception of computers designed to use only network storage devices
(such as network computers and some Internet appliances), virtually all PCs
come with a hard disk drive (commonly referred to as hard drive) that is used
to store most programs and data used with that PC. Hard drives are
typically located inside the system unit and are not designed to be removed,
unless they need to be repaired or replaced. In common practice,
the terms hard disk, hard disk drive, hard disk system, and hard drive are
Hard Drive Characteristics
- Similar to floppy drives, hard drives store data magnetically; their
disks are organized into tracks, sectors, and clusters; and they use
read/write heads to store and retrieve data. However, the
hard disks used with a hard drive are made out of metal and are permanently
sealed (along with the read/write heads and access mechanisms) inside the
hard drive. One drive may contain a stack of several hard disks,
as shown in
Hard drives are typically fixed-media systems in which
the storage media (the hard disks) are not removable from the storage
device (the hard drive); one exception is a hard drive that uses a removable
hard disk cartridge, as discussed later in this chapter.
Hard drives are faster than removable-media systems and can store a
great deal more data. The capacity of a typical internal
hard drive for today's desktop PCs ranges from 40 to 300 GB.
Internal hard drives for notebook computers are also getting larger --
up to 80 GB. Most hard drives for desktop PCs use 3 1/2-inch hard drives,
although a switch to 2 1/2-inch hard drives is expected in the near future;
most notebook computers use a 2 1/2 inch hard drive.
Even smaller hard disk systems are becoming available for systems requiring
tiny drives, such as the 1-inch Microdrive developed by IBM, who is now
partnered with Hitachi for their hard drive systems. Hitachi
recently announced that a 4 GB version of the Microdrive will be available
by the end of 2003. The increased capacity is due to a new storage
technology developed by IBM called Pixie Dust, which sandwiches three atoms
of the precious metal ruthenium between two magnetic layers.
This technology enables data to be stored at much higher densities on
magnetic media than previously possible.
Like floppy disks, hard disk surfaces are divided into tracks, sectors,
and clusters when formatted, but include many more of each.
A new hard drive is typically formatted for use at the factory before it
is sold, so it is ready for software and data as soon as it is installed.
Because reformatting a disk erases everything on the disk,
hard drives are rarely reformatted. This task is only performed
if errors are preventing the hard drive from operating properly and there
is no other option.
In addition to tracks, sectors, and clusters, hard drives use the concept
of a cylinder. A cylinder is the collection of one particular
track on each disk surface, such as the first track or the tenth track
on each disk surface. In other words, it's the area on all of
the hard disks inside the hard drive that can be accessed without moving
the read/write access mechanism, once it has been moved to the proper
position. For example, the four-disk system in
contains eight possible recording surfaces (using both sides of each disk),
so a cylinder on that system would consist of eight tracks, such as track
13 on all eight surfaces. Hard drives are commonly organized
into anywhere from a few hundred to a few thousand cylinders.
The number of tracks on a single disk is equal to the number of cylinders
in the disk system.
Most hard drives are hermetically sealed units. This precaution
keeps the disk surfaces completely free of contamination, enables the disks
to spin faster, and limits causes of operational problems.
Hard disks typically spin between 5,400 and 15,000 revolutions per
minute (rpm), depending on the type and size of the drive.
In addition to spinning faster than most other types of storage systems,
the hard disk constantly rotates when your computer is turned on instead
of only rotating when it needs to be accessed. This feature
eliminates the delay of waiting for the drive to come up to the correct
speed. (Most PCs can be set up to go to sleep and turn off
the hard drive after a specified period of inactivity to save power;
in this case, touching the keyboard or mouse starts the hard disks spinning again.)
To retrieve or store data, most hard drives have at least one read/write
head for each recording surface. These heads are mounted on
an access mechanism, similar to a floppy disk, this mechanism moves
the heads in and out among the tracks together. It positions
all the heads on the cylinder containing the track from which data is
to be read or to which data is to be written. It is important
to realize that a hard drive's read/write heads never touch the surface
of the hard disk at any time, even during reading and writing.
If the read/write heads do touch the surface -- such as if the
PC is bumped while the hard drive is spinning or a foreign object gets onto
the surface of the disk, a head crash occurs, which may do permanent damage
to the hard drive. Because the heads are located extremely
close to the surface of the disk -- usually less than a millionth of an
inch above the surface -- the presence of a foreign object the width
of a human hair or even a smoke particle (about 2,500 and 100 millionths
of an inch, respectively) on a hard disk's surface is like placing a huge
boulder on a road and then trying to drive over it with your car
One never knows when a hard drive will crash -- there may be no
warning whatsoever -- and this is a good reason for keeping the drive backed
up regularly. Backing up a computer system is discussed in more
detail in Chapter 6. When hard drives containing critical data
become damaged, data recovery firms may be able to help out, as discussed in
the Inside the Industry box.
In order for a hard drive to read or write data, the following three events
must be carded out, all of which may add time to the total disk access time.
Move the read/write heads to the cylinder that contains (or will store)
the desired data—called seek time.
Rotate the disks into the proper position so that the read/write heads
are located over the part of the cylinder to be used—called rotational delay.
Read the data from the disk and transfer it to memory or transfer the
data to be written to the disk from memory and then store it on the disk --
called data movement time.
Typical hard disk access times are from 10 to 20 milliseconds.
To minimize disk access time, drives usually store related data on the same
cylinder. This strategy sharply reduces the seek-time component
and improves the overall access time.
In addition to being used with computers, hard drives are increasingly being
incorporated into consumer products, such as digital video recorders (DVRs)
like TiVo and game boxes like Xbox and PlayStation. Although
growth in the computer storage industry has been slowing, demand for storage
products for consumer applications is on the rise.
Partitioning and File Systems
- Partitioning a hard drive enables you to logically divide the physical
capacity of a single drive into separate areas called partitions.
You can then treat each of the partitions as an independent
disk drive, such as a C drive and a D drive, although they are physically
still one drive. At least one partition is created when a hard
drive is first formatted; you can change the number and sizes of the
partitions at a later time, although this action usually destroys
any data in the partitions being changed. Consequently,
you should back up your data located on that drive to another storage
medium before you repartition a hard drive, and then copy the data back
onto the repartitioned hard drive. Some operating systems have
a limit to the number of partitions that can be used.
Because older operating systems could only address hard drives up to
512 MB, hard drives larger than that limit had to use multiple partitions.
Most newer operating systems allow larger drives, but partitioning
a large drive can make it function more efficiently. This is
because operating systems typically use a larger cluster size with a
larger hard drive. When a large cluster size is used, disk
space is often wasted because even tiny files have to use up one entire
cluster of storage space. When a hard drive is partitioned,
each logical drive uses a smaller cluster size, since each logical drive
is smaller than the original drive. Windows computers using the
FAT32 file system are much more efficient than those using the original
FAT system since FAT32 systems allow cluster sizes to be as small as
4 KB each, which cuts down on wasted storage space. Windows
NT and Windows XP computers have the option of using the NTFS file
system, which can address much larger drives than either FAT or FAT32.
Another reason for partitioning a hard drive is to be able to use two
different operating systems on the same hard drive -- such as Windows and
Linux. You can then decide which operating system you will
run each time you turn on your computer. Creating the appearance
of having separate hard drives for file management, multiple users, or
other purposes is another common reason for partitioning a hard drive.
Some users choose to install their programs on one hard drive
(usually C) and store their data on a second drive (such as D).
This system of using separate logical drives for data and programs makes
locating data files easier, as well as enables users to back up all data
files simply by backing up the entire data drive (program files aren't
typically backed up as frequently as data files, if at all).
Operating systems and backing up data are discussed in more detail in Chapter 6.
- A cache (pronounced cash) is a place to store something temporarily.
For instance, in Chapter 3 we learned that cache memory
is a group of very fast memory chips located on or near the CPU that
are used to store the most frequently and recently used data and
instructions. Because transferring that data and instructions
from cache memory to the CPU is much faster than transferring them from
RAM or the hard drive, cache memory typically results in faster processing.
Disk caching is similar in concept—it is a strategy for
speeding up system performance by storing data or programs that might
be needed soon in a designated area of RAM to avoid having to retrieve
them from the hard drive when they are requested. Since
retrieving data from RAM is much faster than from the hard drive,
disk caching can speed up performance.
The location in RAM where disk caching takes place is called the disk
cache. When a hard drive uses disk caching (as most do today),
any time the hard drive is accessed the computer copies the requested
program and data, as well as extra programs or data located in neighboring
areas of the hard drive (such as the entire track or cylinder), to the disk
cache. The theory behind disk caching assumes that neighboring
data will likely have to be read soon anyway (research indicates that there
is an 80 to 90% chance the next request will be for data located adjacent
to the data last read), so the computer can reduce the number of times
the hard drive is accessed by copying that data into RAM early.
When the next data is requested, the computer system checks the disk-cache
area first, to see if the data it needs is already there.
If it is, the data is retrieved for processing; if not, the computer
retrieves the requested data from the disk (see
Disk caching saves not only time but also wear and tear
on the hard drive. In portable computers, it can also extend battery life.
Hard Drive Standards
- Hard drives connect, or interface, with a computer using one of several
different standards. These standards determine performance
characteristics, such as the density with which data can be packed onto
the disk, the speed of disk access, how large the disk can be, and the way
the disk drive interfaces with other hardware. Some of the most
common interfaces are discussed next.
With EIDE, for enhanced integrated drive electronics, the hard drive controller --
the chip that controls the flow of data to and from the hard drive --
is built into the drive. SCSI, for small computer system
interface and pronounced "skuzzy," hard drive controller chips are
either attached directly to the motherboard or are located on a SCSI
interface card to which the drive is connected. Both EIDI and
SCSI are very fast and can support multiple hard drives. EIDE
has a variety of different specifications, such as ATA, Fast ATA, Fast
IDE, or ATA-2, ATA/100, and serial ATA. EIDE drives are
typically less expensive than SCSI drives; consequently, EIDI drives are
found more often in desktop PCs. SCSI is usually faster for server
operations with multiple users. In addition to being used with
hard disk drives, SCSI interfaces can also be used to connect some scanners,
CD drives, and DVD drives.
Fibre Channel is a newer storage standard that is expected to become widely
used with network storage systems, as well as in other high-capacity business
storage applications. Fibre Channel storage devices connect to the
host computer using a special Fibre Channel interface card and have the
advantage of reliability, flexibility, and very fast data delivery --
up to two gigabits per second. Because it is more expensive than
other standards and is geared for long-distance, high-bandwidth applications.
Fibre Channel is not expected to be widely used with PCs and low-end
servers, at least not in the near future. It is, however,
expected to eventually replace SCSI for high-end storage systems.
Some external hard drives today follow none of these standards; instead
they connect to the PC using USB or Fire Wire standards through a USB or FireWire port.
Portable Hard Drive Systems
- While most hard drive systems are designed to be internal devices permanently
located inside the system unit, portable hard drives are available.
Portable hard drives fall into two basic categories:
those in which the entire drive is transported from one location to another,
and those in which a cartridge containing the hard disk is removed from
the hard drive and transported. When the entire drive is portable --
essentially an external hard drive -- the drive is typically attached to the
PC through a USB, FireWire, or PC card port (see
Common capacities for external hard drives are 20 to 160 GB.
Portable hard drive systems using removable hard disk cartridges use a
hard drive that remains attached to the PC in conjunction with hard disk
cartridges that can be inserted into and removed from the drive, similar
to a floppy disk system. Also similar to a floppy drive, the
hard drives used with hard disk cartridges can be internal or external,
but external devices are more widely used. Hard disk cartridges
can usually store about 20 GB, although larger storage capacities are
expected in the near future. Most removable hard disk cartridges
are proprietary, so they can only be used with their respective drives.
Both types of portable hard drive systems are useful for storing and
backing up very large files, transporting large files from one PC to
another, and for exceptionally secure facilities -- such as government and
research labs -- that require all hard drives to be locked up when not
in use. They are also commonly used for complete system
backups. Although their portability has its advantages,
portable hard drives generally perform more slowly than conventional
fixed internal hard drives.
Storage Systems for Large Computer Systems and Networks
- Hard drive systems for large computer systems (such as those containing
mainframe computers and midrange servers) implement many of the same
standards and principles as PC-based hard drives, but on a much larger
scale. Instead of finding a single hard drive installed within
the system unit, you are most likely to find a storage server -- a separate
piece of hardware containing multiple high-speed hard drives -- connected to
the computer system. Large storage servers, such as the one shown
, contain racks of hard drives capable of storing a total of
30 TB or more. These types of storage systems -- also referred
to as enterprise storage systems -- usually use fast Fibre Channel connections.
In addition to being used as stand-alone storage for large
computer systems, storage servers may also be used in network attached
storage (NAS), storage area network (SAN), and RAID storage systems.
Network Attached Storage (NAS) and Storage Area Networks (SANs)
- Storage servers are increasingly being used to provide storage for
computer networks. With the huge amounts of data that many
companies need to manage and store today -- for instance, Yahoo!
needs to store on an on-going basis more than a petabyte of data
generated by Yahoo! e-mail users -- network-based storage has become
One possibility is the network attached storage (NAS) device.
NAS devices are high-performance storage servers that
are individually connected to a network to provide storage for the
computers on that network. Storage area networks (SANs)
also provide storage for a network, but consist of a separate network
of hard drives or other storage devices.
That storage area network is, in turn, attached to the main network.
difference between NAS and SANs is whether the storage devices act as
individual network nodes, just like PCs, printers, and other devices
on the network (NAS), or whether they are located in a completely
separate network of storage devices that is accessible by the main
However, in terms of functionality, the
distinction between NAS and SANs is blurring, since they both provide
storage services to the network.
Both NAS and SAN systems are scalable, so new devices can be added as
more storage is needed and devices can be added or removed without
disrupting the network.
- RAID (redundant arrays of independent disks) is a method of storing data on two
or more hard drives that work in combination to do the job of a larger
drive. Although RAID can be' used to increase performance,
it is most often used to protect critical data on a storage server.
Because RAID usually involves recording redundant
(duplicate) copies of stored data, the copies can be used, when
necessary, to reconstruct lost data. This helps to
increase the fault tolerance—the ability to recover from an
unexpected hardware or software failure, such as a system crash—of a storage system.
There are six different RAID designs or levels (0 to 5) that use
different combinations of RAID techniques. For example
RAID level 0 uses disk striping, which spreads files over several
disk drives (see the leftmost part of
Although striping improves performance, since multiple drives can
be accessed at one time to store or retrieve data, it doesn't provide fault tolerance.
Another common RAID technique is disk mirroring, in which data is
written to two duplicate drives simultaneously (see the rightmost
). The objective of disk mirroring is
to increase fault tolerance -- if one of the disk drives fails, the
system can instantly switch to the other drive without any loss of
data or service. RAID level 1 uses disk mirroring.
Levels beyond level 1 use some combination of disk striping and disk
mirroring, with different types of error correction provisions.
Because using RAID is significantly more expensive than just using
a traditional hard ( drive storage system, it has been reserved for
use with network and Internet servers. However, recently
RAID has become more popular with PC users looking for increased
performance. One recent test by PC World magazine showed
that two RAID-connected drives completed some tasks in 40% less
time than one drive of the same type. To implement RAID
on a desktop PC, a RAID expansion card must be used.
OPTICAL DISC SYSTEMS
- Optical discs (such as CDs and DVDs) store data optically -- using laser beams --
instead of magnetically, like floppy and hard disks. Lasers can write
and read data at densities much higher than magnetic technology, so the storage
capacity of optical discs is much higher than magnetic disks of the same physical size --
usually from 650 MB on up.
Optical discs are made out of plastic with a reflective metallic or otherwise
light-sensitive coating. Data can be stored on one or both sides of
an optical disc, depending on the disc. Most optical discs are 4 1/2 inches
in diameter, although smaller discs are sometimes used. To keep data
organized, optical discs are divided into tracks and sectors like magnetic disks,
but use a single grooved spiral track beginning at the center of the disc
), instead of a series of concentric tracks.
Because lasers can be very precise, the track can be quite narrow and the spiral
can be very tight -- when measured from end to end, the total length of the track
on a typical CD is over 3 miles.
The number of sectors used varies depending on the size and type of disc, but
a standard 650 MB CD has over 330,000 sectors. Because the track
starts at the center of the disc, optical discs can be made into a variety of
sizes and shapes -- such as a heart, triangle, custom shape, or the hockey-rink
shape commonly used with business card CDs -- the track just stops when it reaches
the outer edge of the disc. Standard shapes are molded and less expensive;
custom shapes—such as those that match a key product or service being sold (such as
a soda can, musical instrument, saw blade, candy bar, or house) -- are custom cut
and are more costly. The practice of using optical discs to replace
ordinary objects, such as the business card discs shown in
becoming more common. For a closer look at business card CDs, see the
How it Works box (on page 151 of your textbook).
CD and DVD discs are read by CD and DVD drives. The speed of a CD or
DVD drive is rated as a number followed by the "x" symbol to indicate how fast
the drive is compared to the first version of that drive. For instance,
a 52x CD drive is 52 times faster than the original CD drive, and a 4x DVD drive
is four times faster than the original DVD drive. Most optical discs
have a title and other text printed only on one side and are inserted into the
drive with the printed side facing up (the data is stored on the bottom, non-printed
side of the disc). When inserting a CD or DVD, be careful not to get
dirt, fingerprints, scratches, or anything else that might hinder light reflectivity
on the disc's reflective recording surface. The advantages of CDs and DVDs
include their large capacity -- typically 650 or 700 MB per CD and 4.7 GB per DVD,
although double-sided DVDs currently hold 9.4 GB and are expected eventually to reach
17 GB -- and their small size. (Because DVD technology uses smaller
pits and the tracks are closer together, DVDs hold more data than CDs.)
Another advantage is that optical discs last longer and are more durable than magnetic
media, although the discs should be handled carefully and stored in a protective jewel
case when not in use to prevent scratches and fingerprints from getting on the disc.
Optical discs are the standard today for software delivery; they are
also commonly used for storing and transporting high-capacity music and video files.
There are a variety of types of CDs and DVDs. Some of the most important
types and characteristics of CDs and DVDs are discussed next.
- Read-Only Discs: CD-ROM and DVD-ROM Discs
- CD-ROM (compact disc read-only memory) discs were the first optical discs
of wide acceptance. Because they are read-only, the data on CD-ROM
discs cannot be erased, changed, or added to. Data on a CD-ROM is
stored by burning tiny depressions (called pits) into the disc's surface with
a high-intensity laser beam; the parts of the disc that aren't changed are
called lands. The disc is read by a lower-intensity laser beam
inside the CD-ROM drive; based on the reflection of light from the disc as
it hits the pits and lands, the Is and Os can be determined (see
Because the storage process permanently alters the surface of the
CD-ROM, the data cannot be erased and no data can be added to the disc.
DVD-ROM (digital versatile disc read-only memory) discs are similar to CD-ROM
discs, but they are newer and have a higher storage capacity. While
CD-ROM discs typically hold 650 MB, DVD-ROMs can contain from 4.7 GB to 17 GB,
depending on the number of recording layers and disc sides being used.
The DVD was initially developed to store the full contents of a standard two-hour
movie, but is now also used for prerecorded music, videos, and software.
DVD-ROM discs are designed to be read by a DVD-ROM drive.
CD-ROM drives can usually play audio CDs, in addition to data CDs. DVD-ROM
drives can typically play data and audio CDs, data CDs, DVD-ROM discs, and DVD movies.
For a look at an emerging issue—copy protection for CDs and other digital media --
see the Trend box (on page 155 of yourtextbook).
- Recordable Discs: CD-R, DVD-R, and DVD+R Discs
- Recordable discs can be written to, but the discs cannot be erased and reused.
Recordable CDs are referred to as CD-R discs; recordable DVDs are
called DVD-R or DVD+R discs, depending on the standard being used (different optical
disc and drive manufacturers support different standards). CD-R, DVD-R,
and DVD+R discs are recorded in CD-R, DVD-R, and DVD+R drives, respectively; CD-R
discs can be read by most types of CD and DVD drives, and DVD-R or DVD+R discs
can be read by most DVD drives. Recordable CDs are commonly used for
backing up files, sending large files to others, and creating custom music CDs
from MP3 files legally downloaded from the Internet or from songs on CDs the
user owns. DVD-Rs can be used for similar purposes when more storage
space than is available on a CD-R disc is needed, as well as for storing home
movies and other video applications since video requires a tremendous amount
of storage space. As shown in Figure 4-17, recordable CDs and DVDs
look very similar to their read-only counterparts. Standard-sized 4 1/2-inch
CD-R discs hold 700 MB, 3-inch mini CD-R discs hold about 200 MB, business-card-sized
CD-R discs hold 50 MB, and DVD-R or DVD+R discs can store 4.7 GB per side.
Storing data on a recordable disc is similar to the concept illustrated in
Figure 4-16, but the discs contain a light-sensitive dye or chemical
embedded beneath layers of protective plastic instead of a reflective metallic
layer. The recording laser inside the CD-R or DVD-R drive is less powerful
than the one used to create read-only discs, but still makes permanent marks on
the disc to represent Os and Is. The process of recording data onto
an optical disc is called burning. To bum a CD-R or DVD-R disc, special
software is needed. Many commercial programs are available, and burning
capabilities are also included in many recent operating systems, such as Windows XP.
- Rewritable Discs: CD-RW, DVD-RW, DVD_RW, DVD-RAM, and Blue Laser Discs
- The newer rewritable discs can be recorded on, erased, and overwritten just like
a magnetic disk. The most common types of rewritable optical media
are CD-RW, DVD-RW, and DVD+RW discs. CD-RW discs are written to
using a CD-RW drive and can be read by most CD and DVD drives.
DVD-RW discs and DVD+RW discs are recorded using a DVD-RW drive or DVD+RW
drive, respectively, and can be read by most DVD drives. An
additional rewritable DVD format is DVD-RAM, which requires the DVD disc
to be located inside a cartridge (see Figure 4-17) in order for the disc
to be used.
The newest recordable and rewritable technologies use blue lasers instead
of infrared (CDs) or red (DVDs) lasers to store data more compactly on the
disc. Blue laser discs based on this technology—developed by
Sony and called Blu-ray—can hold 23.3 GB per disc. A
similar, but competing, format developed by Toshiba and NEC is called
the Advanced Optical Disc format and is capable of storing up to 36 GB
of data on a dual-layer disc. In contrast, CD-RW discs hold
700 MB, DVD+RW and DVD-RW discs hold 4.7 GB per side, and DVD-RAM discs
typically hold between 2.6 and 9.4 GB, depending on the speed of the disc
and the number of sides used.
To record and erase rewritable optical discs, phase-change technology is
most often used. With this technology, the recordable CD or DVD
disc is coated with a special metal alloy compound that has two different
appearances once it's been heated and then cooled, depending on the
temperature reached during the heating process. With one
temperature, the surface is reflective; with a higher temperature, it's
not. Before any data is written to a disc, the disc is completely
reflective. To record onto the disc, pits are burned into the surface
by creating non-reflective areas; unburned areas (lands) remain reflective.
Just as with other CDs and DVDs, these pits and lands are interpreted as 1 s and 0s
when the disc is read. To erase the disc, the appropriate temperature
is used to change the areas to be erased back to their original reflective state.
It is important to realize that the DVD industry has not yet reached a single
standard, so there are competing formats that are not necessarily compatible
with each other. Luckily, many DVD drive manufacturers are introducing
new drives that support more than one standard, such as one drive from Sony that
is compatible with DVD+RW, DVD+R, DVD-RW, DVD-R, CD-RW, and CD-R discs.
Because of the format controversy, recordable and rewritable DVD technology has
taken off more slowly than originally anticipated. However, just as
CD-R and CD-RW drives have virtually replaced CD-ROM drives, it is expected that,
eventually, rewritable DVD drives will replace CD drives. About 4
million recordable DVD drives were in use in 2002; that number is expected to
exceed 37 million by 2005.
OTHER TYPES OF STORAGE SYSTEMS
- Other types of storage systems include magneto-optical discs, flash memory
media, magnetic tape, remote storage, and smart cards. A possibility
for the future is holographic storage.
- Magneto-Optical Discs
- There are a few types of storage systems that use a combination of
magnetic and optical technology—the magneto-optical (M-0) disc is one
of the most common. Magneto-optical drives read special M-0
discs, which are usually optical discs inside a rectangular cartridge,
similar in appearance to the DVD-RAM disc shown in Figure 4-17.
M-0 discs are available in both 3^-inch and 514-inch sizes and can store up
to 9.1 GB per disk.
- Flash Memory Media
- Unlike magnetic and optical storage systems whose drives have moving parts,
flash memory media consists of chips and other circuitry that don't move within
the drive as it's being accessed -— called a solid-state storage system.
Because flash memory devices and media are very small, use much
less power than conventional drives, and are resistant to shock and vibration
since they have no moving parts, they are especially appropriate for use
with digital cameras, digital music players, handheld PCs, notebook computers,
smart phones, and other types of portable devices (see
Today, flash memory is found in the form of rewritable sticks, cards, or
drives. Some computers and many mobile devices contain at least
one flash memory port; when an appropriate port is not built into the device,
a flash memory card reader or adapter can be used. Typically,
flash memory media is purchased blank, but some flash-memory-card-based software
is available, such as games, encyclopedias, language translators and more.
Although flash memory media is relatively expensive per gigabyte,
its convenience and universal acceptance makes it an appealing storage option
for many purposes.
Flash Memory Sticks
- Flash memory sticks were introduced by Sony initially for use with
their digital music players. Since then, however, flash
memory use has expanded to digital cameras, PCs, printers, and other
applications. Some newer computers come with a memory stick
port built in; if not, an external reader can be used. Flash
memory sticks are about the size of a stick of gum (see
hold from 32 MB to 1 GB each; a 128 MB card cost about $50.
Flash Memory Cards
- Flash memory cards are the primary removable storage media for handheld
PCs, digital cameras, portable entertainment products, and mobile devices.
Flash memory cards come in a variety of formats and are typically
inserted into a flash memory port located on the PC or device.
Some flash memory ports accept only one type of memory media; others
can read from and write to several types. Just as with flash
memory sticks, external card readers are available that read one or more
memory media formats. Typically, these readers' plug into a
PC card slot or USE port. The main types of flash memory cards
are listed next; some are illustrated in
Of the following flash media formats, CompactFlash and Secure Digital (SD) are
the most widely used at the present time.
CompactFlash cards are widely used with digital cameras,
handheld and portable PCs, digital music players, printers, and other
portable devices. CompactFlash card capacity ranges from 32 MB to 4 GB.
Secure Digital (SD) cards are one of the most commonly used
cards for handheld PC and smart phone storage. Also used
with digital cameras, digital music players, and digital camcorders,
the capacity of the stamp-sized SD card ranges from 32 MB to 1 GB.
MiniSD cards are a smaller version of Secure Digital cards
that became available in 2003. Geared primarily for use
with mobile and smart phones, capacity currently ranges from 16 MB to 256 MB.
MultiMedia cards (MMC) are most commonly used with digital
music players and digital camcorders. Because they are
the same size as SD cards, some devices can use these two types of
cards interchangeably. Capacity ranges from 16 MB to 128 MB.
SmartMedia cards are frequently used with digital cameras,
although other devices may accept them as well. Larger
in physical size than the other types of flash media, SmartMedia
cards can hold from 8 MB to 128 MB of data.
xD cards (sometimes called xD Picture cards) are one of
the newest formats and are designed primarily for digital camera
use; capacity of these cards range from 32 MB to 512 MB.
Flash Memory Drives
- Flash memory drives (sometimes also called USB mini-drives, removable
flash drives, or key drives}, are self-contained storage systems that
consist of flash memory media and the drive hardware necessary to write
to and read from that media. Because they have no moving parts,
flash memory drives are much more resistant to shock and vibration than
conventional drives and are therefore appropriate for harsh environments,
as well as for transporting data from one place to another in a briefcase
or pocket. They also have a longer expected life than removable
magnetic media. Although larger flash memory drives exist to
replace conventional hard drives in situations where the PC will be
subjected to jarring movements, strong vibrations, or other unstable
conditions that might harm a conventional hard drive, most flash memory
drives are designed to be very portable and so are small enough to fit
in a pocket or be carried on a keychain (see Figure 4-20).
To read from or write to a flash memory drive, it is plugged into a PC's
USB port and then it is automatically assigned a drive letter by the
computer, like any other type of drive attached to a PC.
Files can be read from or written to the drive until it is unplugged
from the USB port. Some flash memory drives have their flash
memory media permanently sealed inside; others use standard flash memory
cards and can be opened to replace the drive with a new memory card when
the original is full or if it becomes damaged. Flash memory
drives today are available in capacities from 32 MB to 2 GB.
- Magnetic Tape Systems
- Magnetic tape consists of plastic tape coated with a magnetizable substance
that is polarized to represent the bits and bytes of digital data, similar
to magnetic disks. Tape was once a prominent storage medium for
computer systems, but because of its sequential-access property it has since
been replaced by magnetic disks, optical discs, and flash memory media for
day-to-day use. It is still used today for backup and archival
purposes because it is a very inexpensive medium.
Most computer tapes today are in the form of cartridge tapes (similar to a
video or audio tape), instead of the older reel-to-reel format.
Tapes are read by tape drives, which can be either an internal or external
piece of hardware (an internal tape drive is shown in Figure 4-21).
Tape drives contain one or more read/write heads over which the
tape passes to allow the drive to read or write data. Just as with
other magnetic storage technologies, the Is and Os stored on magnetic tape are
There are a variety of sizes and formats of cartridge tapes, such as digital
audio tape (DAT), quarter-inch-cassette (QIC), Travan, digital linear tape
(DLT), advanced intelligent tape (AIT), Super advanced intelligent tape
(S-AIT), and linear tape-open (LTO). Sizes and formats of
tapes are not generally interchangeable, but since magnetic tapes are
most often used for backup with a specific tape drive, this incompatibility
is usually not a problem. A typical tape cartridge holds between
4 GB to 240 GB, although some can hold up to 1 TB. When a larger
capacity is needed, some tape drives are designed to be used with multiple
tape cartridges, increasing the potential storage capacity to well over 2 TB.
- Remote Storage Systems
- Remote storage refers to using a storage device that is not connected directly to
your PC system; instead, the device is accessed through a local network or
the Internet. Using remote storage devices and media works similarly
to using local storage (the storage devices and media that are directly
attached to your PC); you just need to select the appropriate remote storage
device (typically a hard drive attached to a network server), and then you can
store data on or retrieve data from it.
When the remote device is accessed through a local network, it is
sometimes referred to as network storage; the term online storage most
commonly refers to storage accessed via the Internet. Individuals
and businesses can use online storage Web sites to transfer files between two
computers, to share files with others, and for backup in case of a fire or other
disaster. For some Internet appliances, network computers, and mobile
communications devices with little or no local storage capabilities, online
storage is especially important.
It is becoming increasingly common for individuals to want to share files --
particular digital photographs -- through the Internet. Some Web
sites dedicated to online storage offer the service for free to individuals;
others charge a small fee, such as $5 per month for up to 75 MB of storage space
(business accounts typically cost more).
Although some sites allow access to anyone, most online storage sites are
set up to have file access only by password to limit access to just yourself
and anyone else you give your password to. Some sites allow you
to e-mail links to others to download specific files in your online collection
without having to supply a password. Other online storage sites
contain an automatic back up option in which the files in designated folders
on your PC are uploaded to your online account at regular specified intervals.
Two examples of online storage sites are shown in
- Smart Cards
- A smart card is a credit-card-sized piece of plastic that contains some
computer circuitry, typically a processor, memory, and storage (see
Although the storage capacity of a smart card is fairly small --
usually from a few kilobytes to a few megabytes -- it can be used to hold specific
pieces of information that may need to be updated periodically.
Typically, smart cards are used for payment or identification purposes.
For example, a smart card can store a prepaid amount of digital
cash for purchases using a smart card-enabled vending machine or PC; loyalty
system information (frequent flyer points, for example); identification
data for accessing facilities or computer networks; or an individual's
medical history and insurance information for fast treatment and hospital
admission in an emergency.
Smart cards are also increasingly being used for national ID cards and
student ID cards (see the Campus Close-Up box on page 162 of your textbook).
Although these applications have used conventional magnetic
stripe technology in the past, the microprocessor in a smart card protects the
integrity of the data on the card (in contrast with the straight data
storage capabilities of a flash memory card or CD, for example), and
data stored in the card's memory can be added or modified as needed.
For an even higher level of security, some smart cards
today contain biometric data—such as a fingerprint—to ensure the authenticity
of the user (biometrics is discussed in more detail in Chapter 8).
Many debit and credit cards today are also smart cards.
To use a smart card, it must be inserted into a card reader built into or
attached to a PC, vending machine, or other item. Some keyboards
now have a built-in smart card reader to facilitate secure e-commerce
applications, such as online shopping. Once a smart card has been
accepted, the transaction -- such as making a purchase or unlocking a door --
can be completed. While many smart card readers require direct
contact between the smart card and the reader, contactless smart card systems
using wireless technology allow the card to be read when it is within a
particular distance of the reader without physical contact.
E-commerce is covered in more detail in Chapter 11.
One new smart card application is a combination smart card/magnetic disk.
This emerging product, such as the StorCard, has a flexible
magnetic disk housed inside a tiny cavity created between the top and bottom
layers of the smart card. A proprietary reader is able to access
the disk via a shutter, similar to a floppy disk, to read from and write to
the card. Current capacity is about 100 MB. The smart
card capabilities of this product enable the data on the card to be encrypted
or otherwise protected using smart card technology.
- Holographic Storage
- Storing information in three dimensions is far from a new idea.
DVDs use multiple layers to store more data on the same size disc as a CD,
and 3D memory chips are in the works. One very promising technology
for 3D storage systems being researched by such companies as IBM, Lucent
Technologies, and Imation is holographic storage.
Holographic storage systems use multiple laser beams to store data in three
dimensions, in order to store more data on the disc. Data is stored
in a "page" format, in which all data on each page is stored and retrieved
together. Because a million or more bits can be located on each
page and thousands of pages can be stored in material no larger than a small
coin, holographic systems offer the possibility of compact storage media
holding many terabytes of information. The additional advantages
of no moving parts and simultaneous access of all information stored in a
page give this technology the potential for very rapid access.
Some predictions include a data-throughput rate of at least 1 billion bps
(bits per second).
Potential applications for holographic data storage systems include high-speed
digital libraries and image processing for medical, video, and military purposes --
applications for which data needs to be stored or retrieved quickly in large
quantities, but rarely changed. Rewritable holographic storage is
an expected improvement in the future.
COMPARING STORAGE ALTERNATIVES
- Storage alternatives are often compared by weighing a number of product
characteristics and cost factors. Some of these product characteristics
include speed, compatibility, storage capacity, convenience, and the portability
of the media.
Keep in mind that each storage alternative normally involves trade-offs.
For instance, most systems with removable media are slower than those with
fixed media, and external drives are typically slower than internal ones.
Although cost is a factor when comparing similar devices, it is
often not the most compelling reason to choose a particular technology.
For instance, although the flash memory drives are very expensive
per GB, many users find them essential for transferring files between work and home
or for taking presentations on the road.
For drives that use a USB interface, the type of USE port is also significant.
For instance, a typical flash memory drive designed for the original
USE 1.1 port transfers data at up to 1.5 MB per second; USB 2.0 flash memory drives
are about 40 times faster.
With so many different storage alternatives available, it's a good idea to research
which devices and media are most appropriate for your personal situation.
In general, most users today need a hard drive (for storing programs
and data), some type of CD or DVD drive (for installing programs, backing up files,
and sharing files with others), and a floppy drive (for sharing small files with
others). Some users may choose to include an additional drive for a
particular type of high-capacity removable media, such as Zip disks, if they only
need to use the disks in their PC or a PC they know has a drive compatible with that medium.
Users who plan to transfer music, digital photos, and other multimedia data on a
regular basis between several different devices -- such as a PC, digital camera,
handheld PC, and printer -- will want to select and use the flash memory media
that is most compatible with the devices they are using and obtain the necessary
adapter for their PC. Some of the most common types of portable
storage media are compared in
- Properties of Storage Systems
- Storage systems make it possible to save programs, data, and processing results for later use. They provide non-volatile storage, so when the power is shut off, the data stored on the storage medium remains intact. This differs from memory, which is volatile. The most common types of storage media are magnetic disks and optical discs, which are read by the appropriate type of drive. Drives can be internal or external.
All storage systems involve two physical parts: A storage device and a storage medium. In addition to being non-volatile, storage devices can record data either on removable media, which provide access only when inserted into the appropriate storage device, or fixed media, in which the media is permanently located inside the storage device. Removable media provide the advantages of unlimited storage capacity, transportability, safer backup capability, and security. Fixed media have the advantages of higher speed, lower cost, and greater reliability.
Two basic access methods characterize secondary storage systems: Sequential and random access. Sequential access allows a computer system to retrieve the records in a file only in the same order in which they are physically stored. Random access (or direct access) allows the system to retrieve records in any order.
Files (sometimes called documents) stored on a storage medium are given a filename and can be organized into folders. This is referred to as logical file representation. Physical file representation refers to how the files are physically stored on the storage medium by the computer.
- Magnetic Disk Systems
- Magnetic disk storage is most widely available in the form of hard disks and floppy disks. Computer systems commonly include floppy disk storage because it provides a uniform removable storage system at a low cost. Each side of a floppy disk holds data and programs in concentric tracks encoded with magnetized spots representing Os and 1 s.
Sector boundaries divide a floppy disk surface into pie-shaped pieces. The part of a track crossed by a fixed number of contiguous sectors forms a cluster. The disk's file directory or file allocation table (FAT), which the computer system maintains automatically, records where files stored on the disk are physically located. To use a floppy disk, you insert it into a floppy disk drive.
Today, floppy disks are facing challenges from other removable media with much higher storage capacities, such as Zip disks, SuperDisks, CDs, DVDs, and flash memory media.
Hard disk drives are the main storage medium for most PCs. They offer faster access than floppy disks and much greater storage capacity. A hard drive contains one or more hard disks permanently sealed inside along with an access mechanism. A separate read/write head corresponds to each disk surface, and the access mechanism moves the heads in and out among the tracks to read and write data. All tracks in the same position on all surfaces of all disks in a hard drive form a disk cylinder. Hard drives can be divided into multiple partitions (logical drives) to reduce cluster size or to facilitate multiple users or operating systems.
Three events determine the time needed to read from or write to most disks: seek time, rotational delay, and data movement time. The sum of these three time components is called disk access time. A disk cache strategy, in which the computer fetches program or data contents in neighboring disk areas and transports them to RAM whenever disk content is retrieved, can speed up access time.
Three disk standards—EIDE, SCSI, and Fibre Channel—dominate the hard drive market, although some external drives can connect via a USB port. If portability is required, portable hard drives, in which either the entire drive or a removable hard drive cartridge can be moved to another PC, are available. Hard drives for notebook PCs can be internal, external, or in a PC card format.
Disk drives on larger computers implement many of the same standards as PC-based hard drives. Instead of finding a single set of hard disks inside a hard drive permanently installed within a system unit, however, a storage system separate from the system unit often encloses several removable racks of hard disk drives, sometimes called an enterprise storage system. Network attached storage (NAS) and storage area networks (SANs) are commonly used to provide storage for a business network. RAID technology can be used on larger systems to increass fault tolerance and performance.
- Optical Disc Systems
- Optical discs store data optically using laser beams much more densely than magnetic disks. They are divided into tracks and sectors like magnetic disks, but use a single grooved spiral track instead of concentric tracks. Optical discs are available in a wide variety of CD and DVD formats and are read by CD or DVD drives.
CD-ROM discs come with data already stored on the disc. Data is represented by pits and lands permanently burned into the surface of the disk. CD-ROM discs cannot be erased or overwritten—they are read-only. DVD-ROM discs are similar to CD-ROM discs, but they hold much more data (4.7 GB instead of 700 MB). Recordable discs (CD-R, DVD-R, and DVD+R discs) and rewritable disks (CD-RW, DVD-RW, DVD+RW, DVD-RAM, and blue laser discs) can all be written to, but only recordable discs can be erased and rewritten to, similar to a floppy disk or hard drive.
Recordable CDs and DVDs store data by burning permanent marks onto the disc, similar to CD-ROM and DVD-ROM discs; rewritable discs typically use phase-change technology to change the reflectivity of the disc to represent Is and Os. It is expected that, eventually, some form of DVD disc will eventually replace CDs as the optical disc standard.
- Other Types of Storage Systems
- Other types of storage systems include magneto-optical (MO) discs, which use a combination of magnetic and optical technology and magnetic tape, which stores data on plastic tape coated with a magnetizable substance. Magnetic tapes are usually enclosed in cartridges and are inserted into a tape drive to be used.
Flash memory media are a rapidly growing new storage alternative. They are used with digital cameras, portable PCs, and other portable devices, as well as with desktop PCs. Plash memory can be in the form of flash memory sticks, flash memory cards, or flash memory drives. Remote storage—using a storage device that is not directly a part of your PC system—typically involves using a network storage device or an online storage service.
Online storage services enable users to share files with others over the Internet, access files while on the road, and backup documents. Smart cards are credit-card-sized pieces of plastic that contain a chip or other circuitry usually used to store data or a monetary value.
Holographic storage, which uses multiple laser beams to store data in three dimensions, is a possibility for the future.
- Comparing Storage Alternatives
- Most PCs today include a hard drive, floppy disk drive, and some type of CD or DVD drive. The type of optical drive and any additional storage devices are often determined by weighing a number of product characteristics and cost factors. These characteristics include speed, compatibility, capacity, removability, and convenience.