Secondary Storage Devices
Magnetic Tapes
A recording medium consisting of a thin tape with a coating of a fine magnetic
material, used for recording analogue or digital data. Data is stored in frames across the
width of the tape. The frames are grouped into blocks or records which are separated
from other blocks by gaps.
Magnetic tape is a serial access medium, similar to an audio cassette, and so data (like
the songs on a music tape) cannot be quickly located.
However large amounts of information can be stored within magnetic tape. This
characteristic has prompted its use in the regular backing up of hard disks.
Magnetic Disk
The primary computer storage device. Like tape, it is magnetically recorded and
can be re-recorded over and over. Disks are rotating platters with a mechanical arm that
moves a read/write head between the outer and inner edges of the platter's surface. It
can take as long as one second to find a location on a floppy disk to as little as a couple
of milliseconds on a fast hard disk.
Tracks and Spots
The disk surface is divided into concentric tracks (circles within circles). The thinner the
tracks, the more storage. The data bits are recorded as tiny magnetic spots on the
tracks. The smaller the spot, the more bits per inch and the greater the storage.
Sectors
Tracks are further divided into sectors, which hold a block of data that is read or written
at one time; for example, READ SECTOR 782, WRITE SECTOR 5448. In order to
update the disk, one or more sectors are read into the computer, changed and written
back to disk. The operating system figures out how to fit data into these fixed spaces.
Modern disks have more sectors in the outer tracks than the inner ones because the
outer radius of the platter is greater than the inner radius Tracks are concentric circles
on the disk, broken up into storage units called "sectors." The sector, which is typically
512 bytes, is the smallest unit that can be read or written.
SAN (Storage Area Network)
A SAN (Storage Area Network) is a network specifically dedicated to the task of
transporting data for storage and retrieval. SAN architectures are alternatives to storing
data on disks directly attached to servers or storing data on Network Attached Storage
(NAS) devices which are connected through general purpose networks.
In order to meet the demands of the storage system, enterprises apply SAN to increase
the system efficiency and capacity expansion. According to SNIA (Storage Networking
Industry Association), SAN is:
1. The purpose of the SAN is transmitting data between storage systems and
storage systems or storage systems and client servers. The SAN fabric contains
physical connections from storage systems to client, and then storage
management devices, servers, and network devices. However, SAN is usually
defined as block I/O services provider.
2. The storage system contains storage components, devices, computer
equipments, software applications, and network devices.
SAN is able to attach with various kinds of storage devices, such as disk-array
subsystems, CD towers, magnetic tape drivers and libraries, and provides data I/O
services via hub or switches through network connections.
The Internet Fibre Channel Protocol (iFCP) transports Fibre Channel Layer 4 FCP on IP
networks.
Advantages of SAN
By integrating storage devices, SAN increases the storage space usability and cost
efficiency.
SAN is the high-speed storage sharing system.
SAN increases the network bandwidth and reliability of data I/O.
SAN is separated from the regular network system, and has an ability to expand
the storage capacity.
SAN reduces the cost of the storage management since it simplifies the system
fabric and devices management.
Optical Disk
An optical disk is impressed with a series of spiral pits in a flat surface.
A master disk is burnt by high-intensity laser beams in bit-patterns from which
subsequent copies are formed which can be read optically by laser.
The optical disk is a random access storage medium; information can be easily read
from any point on the disk. A standard CD-ROM can store up to 650Mb of data, with
14,500 tracks per inch (tpi).
CD-ROM stands for Compact Disk - Read Only Memory. It is now possible to have CD-
ROMs where extra tracks of information can be written onto them by the user. These
are called read/writable CD-ROMs and these are becoming a popular and cheap
method for storage.
Magneto-Optical
The magneto-optical (MO) drive is a popular way to back up files on a personal
computer. As the term implies, an MO device employs both magnetic and optical
technologies to obtain ultra-high data density. A typical MO cartridge is slightly larger
than a conventional 3.5-inch magnetic diskette, and looks similar. But while the older
type of magnetic diskette can store 1.44 megabytes (MB) of data, an MO diskette can
store many times that amount,anging from 100 MB up to several An MO system
achieves its high data density byusing a laser and a magnetic read/write head in
combination. Boththe laser and the magnet are used to write data onto thediskette. The
laser heats up the diskette surface so it can beeasily magnetized, and also to allow the
region of magnetizationto be precisely located and confined. A less intense laser isused
to read data from the diskette. Data can be erased and/oroverwritten an unlimited
number of times, as with a conventional3.5-inch diskette.
Examples of magneto-optical drives are the Fujitsu DynaMO, a 230 MB drive
used in the PowerPC Apple Powerbook, a note book computer, and the Pinnacle
Micro Vertex, a 2.6 GB drive.
DVD
DVD is an abbreviation of Digital Versatile Disc- read only memory. DVD-ROM
is an optical disc storage media format that can be used for data storage, which
includes movies with high video and sound quality. DVD-ROM is a non-volatile optical
storage medium similar to CD-ROM, which contains computer data that cannot be
erased or rewritten. These DVDs are read-only disks that have storage capacity for 133
minutes of high quality video, in format, and audio full- length feature film. The discs are
pressed in a similar fashion as the CDs. The reflective surface is gold or silver colored.
DVD-ROM can be accessed using a DVD-ROM drive attached to the personal
computer. DVD-ROM drives are backward compatible, i.e. they are also capable of
reading CD-ROMs and audio CDs as well. The DVD-ROM supports disks with
capacities of 4.7 GB to 17 GB and access rates of 600 KBps to 1.3 MBps. A standard
DVD disc store up to 9.4 GB of data. DVD-ROMs are of same size as a compact disc,
but holds data about 7 times more. DVD can store that much of data because both the
sides of a disc are used, with sophisticated data compression technologies.
DVD-ROM is a variation of CD-ROM that is being used in place of CD-ROMs in many
personal computers. All DVD-ROMs contain a file system, UDF, which is an extension
of the ISO 9660 Standard used for data CDs.
Memory Card
A memory card (sometimes called a flash memory card or a storage card) is a small
storage medium used to store data such as text, pictures, audio, and video, for use on small,
portable or remote computing devices. Most of the current products use flash memory, although
other technologies are being developed. There are a number of memory cards on the market,
including the SD card (secure digital card), the CF card (CompactFlash card), the SmartMedia
card, the Memory Stick, and the MultiMediaCard (MMC). These cards are of varying sizes, and
each is available in a range of storage capacities that typically corresponds directly to the price.
The CompactFlash card is about the size of a matchbook, while the MultiMediaCard and Secure
Digital card are each about the size of a postage stamp. The latter two are expected to reach
storage capacities up to 1 gigabyte (GB) by the end of 2002.
Most available cards have constantly powered nonvolatile memory, which means that
data is stable on the card, is not threatened by a loss of power source, and does not
need to be periodically refreshed. Because memory cards are solid state media, they
have no moving parts, and therefore, are unlikely to suffer mechanical difficulties.
Earlier removable storage media, such as the PC card, the smart card, and similar
cards used for game systems, can also be considered to be memory cards. However,
the newer cards are smaller, require less power, have higher storage capacity, and are
portable among a greater number of devices. Because of these features, memory cards
are influencing the production of an increasing number of small, lightweight and low-
power devices.
Memory cards offer a number of advantages over the hard disk drive: they're much
smaller and lighter, extremely portable, completely silent, allow more immediate access,
and are less prone to mechanical damage. In comparison, however, the hard disk still
offers a compelling advantage: currently, a memory card (for example, CompactFlash)
with a 192MB capacity typically costs more than a hard drive with a capacity of 4OGB.
