SECONDARY STORAGE

It is consist of a read/write mechanism and a storage medium. Device controller provides interface and Provide long-term storage of programs and data. In this type of memory the cost per bit of storage is low. However, the operating speed is slower than that of the primary storage. Huge volume of data are stored here on permanent basis and transferred to the primary storage as and when required. Most widely used secondary storage devices are magnetic disk and optical disk.

Magnetic Disk

•         Circular platter (metal/plastic) coated with magnet sable material

o   Iron oxide (rust!) - Older drives, thick layer, low density, soft

o   Thin film - newer drives, durable, thinner and higher density

•         Different Range of packaging

•          Non-removable disk

o   Permanently mounted in the drive

o   Removable disks

o   Can be removed from drive and replaced with another disk

o   Provides unlimited storage capacity

o   Easy data transfer between systems

Non-removable magnetic Disk:

HARD DRIVES

Most Hard drives have the following common features:

•   the disk and read/write heads are enclosed in a sealed airtight unit;

•    the disk(s) spin at a high speed, one such speed may be 7200 revolutions per minute;

•         the read/write head do not actually touch the disk surface;

•         the disk surface contains a magnetic coating;

•      The data on disk surface (platter) are arranged in the series of concentric rings. Each ring is called a track, is subdivided into a number of sectors, each sector holding a specific number of data elements called bytes or characters.

•      The smallest unit that can be written to or read from the disk is a sector. The storage capacity of the disk can be determined as the number of tracks, number of sectors, byte per sector and number of read/write heads.

Characteristics of Storage Devices

•         Drive Speed

•         Access Time

•         Rotation Speed

•         Tracks and Sectors

•         Bad Blocks

•         Sector Interleave

•         Drive Speed

•         Bandwidth

•         Access latency

Partitioning and Formatting:

•         FAT, FAT16, FAT32

•         NTFS

•         Inode

Hard Drive Interface:

•         IDE

•         SCSI

•         EIDE

•         Ultra DMA

•         ATA/66

REMOVABLE DRIVES

·         Floppy Drives

·         Zip Drive

·         Jaz Drive

·         Cartridge Drive

Floppy Drives

A small removable disk made up of plastic coated with magnetic recording material. The disk rotates at 360RPM. Floppies can be accessed from both the sides of the disk.

Characteristics of Floppy Drives

•         Average data transfer rate

•         Size Capacity

•         Tracks and Sectors

Zip Drive:

 The Zip drive is a special high-capacity disk drive that uses a 3.5-inch Zip disk which can store 100MB of data. It allows an easy and rapid shift of the data from desktop to laptop.

Jaz Drive:

The Jaz drive size is 2GB and its use in graphic design and publishing, 3D CAD/CAM, enterprise management systems and entertainment authorizing markets by giving them unlimited space for dynamic digital content. It has an impressive sustained transfer rate of 8.0 MB/s - fast enough to run applications or deliver full-screen, full-motion video.

Cartridge Drive:  

A cartridge is a protective case or covering, used to hold a disk, magnetic tape, a printer ribbon or toner. The contents are sealed inside a plastic container so that they cannot be damaged.

Optical Storage Disks

Optical storage has been a popular form of storage media due to its low cost, ease in manufacturing and portable size. Optical storage media is generally available on a standard size disc measuring 12 cm in diameter.CD-ROM can be used as an optical storage device. Optical disks store data by changing the reflective properties of a plastic disk. Binary computer data (0s and 1s) are represented by the way the disk reflects light when a low power laser is shown at it. A 0 stored on a disk reflects light differently to a 1 stored on a disk. Like floppy disks, optical disks can be moved from one computer to another. They have much larger storage capacities than floppy disks but cannot store as much data as a hard disk. Data can be read from an optical disk more quickly than from a floppy disk but hard disks are much quicker. As with a hard disk the drive head in an optical drive can move directly to any file on the disk so optical disks are direct access.

There are four types of optical disks that are currently in use:

·         CD-ROM

·         WORM

·         CD-R

·         CD-RW

·         DVD

·         HD-DVD

·         Blu-ray

CD-ROM (Compact Disk - Read Only Memory):

This is by far the most widely used type of optical disk. A CD-ROM disk can store up to 650Mb of data. The data is written onto the CD-ROM disk before it is sold and cannot be changed by the user. Because of this CD-ROMs are often described as Write Once Read many times (WORM) disks. CD-ROMs are used for applications such as distributing software, digital videos or multimedia products.

CD-R (Compact Disc - Recordable):

 A CD-R disk can store up to 650Mb of data. A CD-R disk is blank when it is supplied. The user can write data to it just once. After data has been written to the disk it cannot be changed. A special CD-R drive which contains a higher powered laser than a CD-ROM drive is required to write to the disk. CD-Rs are often used for making permanent backups of data and distributing software when only a small number of copies are required.

CD-RW (Compact Disc - Read / Write):

A CD-RW disk can store up to 650Mb of data. CD-RW disks can be read from and written to just like a hard disk. CD-RWs can be used for any application that a hard disk can be used for but the time taken to access data is much longer than that for a hard disk.

DVD (Digital Versatile Disk or Digital Video Disk):

DVD is the new standard for optical disks. By using a shorter wavelength laser, storing data on both sides of the disk and having more than one layer of data on each side of a disk DVD disks are able to store much more data than CD disks. The DVD standard includes disk capacities up to 18Gb. Current DVD disks store far less than this. There are two types of DVD disks. DVD-ROM disks can be read from but not written to whilst DVD-RAM disks can be read from and written too.

 HD-DVD (High Definition DVD):

HD-DVD offered enough storage space for full-length, high-definition movies on a single disc. HD-DVD discs were capable of holding 15 GB per layer with a maximum of two layers. It was released by Toshiba HD-DVD drives use a blue wavelength laser that is able to read smaller pits on the optical media.

 Blu-ray:

Blu-ray is the newest high-definition optical format. Developed by Sony, Blu-ray discs offer 25 GB per layer with up to two layers per disc. Blu-ray discs have a protective coating that reduces the number of scratches and makes the disc durable. Like HD-DVD, Blu-ray offers a full-length, high-definition movie on one disc. Blu-ray recordable, or BD-R, drives are becoming more common in home computers. The BD-R drives allow for up to 50 GB of storage on one disc. Blu-ray drives use the same type of laser as HD-DVD drives to allow for greater storage on the optical disc.

Magneto-Optical DISK

Magneto-optical disk drive is a computer storage device which utilizes both an optical laser and magnetic field to record data on a special removable optical disk. This recording technology has a number of benefits for the user including high reliability and low cost per megabyte of storage. Magneto Optical disks are coated with a special material which can be magnetized, but only at a relatively high temperature of approximately 300 degrees, called the Curie point. Data is recorded by changing the magnetic field of the spot being heated by the laser. When the spot cools, it retains the orientation of the magnetic field, which represents a data bit. A magneto-optical drive is a kind of optical disc drive capable of writing and rewriting data upon a magneto-optical disc. Both 130 mm (5.25 in) and 90 mm (3.5 in) form factors exist. Magneto-optical, appear as hard disk drives to the operating system and do not require a special file system. They can be formatted as FAT, HPFS, NTFS, etc.

Process of Magneto-Optical DISK:

      Magneto-optical discs are hybrid storage medium. In reading, spots with different directions of magnetization give different polarization in the reflected light of a low-power laser beam. In writing, every spot on the disk is first heated by a strong laser beam and then cooled under a magnetic field, magnetizing every spot in one direction, to store all 0s and 1s.