Secondary Storage Devices
Magnetic Tape: Magnetic tape is the most popular storage medium for large data, which are sequentially accessed and processed.
The magnetic tape medium is a plastic ribbon, which is usually ½ inch or ¼ inch wide and 50 to 2000 feet long. It is coated with a magnetizable material, such as iron oxide or chromium dioxide. Data are recorded on the tape in the form of tiny invisible magnetized spots (representing 1s and 0s) on the coated surface of the tape. The tape ribbon is itself stored in reels or a small cartridge or cassette.
Like audio or videotape, the magnetic tape used in computer system can also be erased and reused indefinitely. Old data on a tape are automatically erased, as new data are recorded in the same area. However, the information stored can be read many times, without affecting the stored data.
Magnetic Disk: Magnetic disk is the most popular storage medium for direct access secondary storage. Due to their random access capability, magnetic disks are the most popular on-line secondary storage device.
A magnetic disk is a thin, circular plate/platter made of metal or plastic, which is usually coated on both sides with a magnetizable recording material, such as iron oxide. Data are recorded on the disk in the form of tiny invisible magnetized or non-magnetized spots on the coated surface of the disk. A standard binary code, usually 8-bit EBCDIC is used for recording data. The disk itself is stored in a specially designed protective envelope or cartridge, or several of them may be stacked together in a sealed contamination-free container.
Like magnetic tapes, magnetic disks can also be erased, as new data are recorded in the same area. However, the information stored can be read many times, without affecting the stored data.
Storage Organization: For data recording, the surface of a disk is divided into a number of invisible concentric circles, called tracks. The track is further subdivided into sectors, pie shaped segments. If there are such eight such pie-shaped segments, each track will be divided into eight parts, and each of these portions of track is called a sector.
A sector typically contains 512 bytes. A sector is the smallest unit with which any disk can work. That is, disk drives are designed to access (read/write) only whole sector at a time. Even if the computer needs to change only one byte, out of the 512 bytes stored, it rewrites the entire sector.
Often, multiple disks are stacked and together to create large capacity disk-storage system. In this case, a set of magnetic disks is fixed to a central shaft. , one below to other, to form a disk pack. The disk pack is sealed and mounted on a disk drive, which consists of a motor to rotate the disk pack about its axis. The disk drive should access arms assembly, which has separate read/write heads for each surface of the disk pack, on which data can be recorded. Normally upper surface of the top disk and the lower surface of the bottom disk , are not used in a disk pack, because these surfaces may be easily scratched.
Storage capacity of a disk system = Number of recording surfaces X
X Number of tracks per surface
X Number of sectors per track
X Number of bytes per sector
= 18 X 2655 X 125 X 512 = 3,05,85,60,000 bytes = 3 X 109 bytes (appr) = 3 GB
Access Mechanism: Data are recorded on the tracks of a spinning disk surface, and read from the surface by one or more read/write heads. Most disks drives have a single head for each disk surface. However some faster disk systems use multiple heads on each access arm to service a number of adjacent tracks simultaneously.
Types of Magnetic Disks: All magnetic disks are round platters. They come in different sizes, different types of packaging and can be made of rigid metal or flexible plastic. They are classified into two types – floppy disks and hard disks.
Floppy Disks Zip/Bernoulli Disks
Floppy Disks: A floppy disk is a round, flat piece of flexible plastic, coated with magnetic oxide. It is encased in a square plastic or vinyl jacket cover. The jacket cover has a cutout (aperture) for the read/write head to make contact with the disk surface. The jacket gives handling protection to the disk surface. It has a special liner, which provides a wiping action to remove dust particles, which are harmful for the disk surface and read/write head. Floppy disks are so called because they are made of flexible plastic plates, which can bend, not hard plates. They are also known as floppies or diskettes. They were introduced by IBM in 1972. The 31/2-inch floppy disk is the most commonly used floppy disk today. Its diameter is 31/2-inch. The high density diskettes have 80 tracks, 18 sectors/tracks, and 512 bytes/sector, giving total diskette capacity of 2 X 80 X 18 X 512 = 14,74,560 bytes = 1.4 MB (approximately).
Floppy-disk drive: A floppy-disk drive is a device which is used to read/write data from /to floppy disks. The drive has a spindle, which rotates the disk, and read/write heads, which can move in and out to position the read/write heads on any track of the disk surface. It is of the interchangeable magnetic disks type. That is it allows the loading and unloading of magnetic disk, as and when they are needed for reading/writing data on to them. A floppy disk can be easily loaded or unloaded form, a floppy-disk drive, just by slipping the disk inside the drives slots, or by pressing the eject button on the drive. The disks are entered in the disk drive along with the jacket cover and data are read/written through an aperture on the jacket. Unlike the hard-disk drives, the read write heads of a floppy disk drive make direct contact with the disk surface during the process of reading or writing. Hence floppy disks are worn out with constant use.
Hard Disks: Hard Disks are the primary on-line secondary storage device for most computer systems today. Unlike floppy disks, which are made of flexible or Mylar, hard disks are made of rigid metal (frequently aluminium).
Types of hard disks: Depending on how they are packaged, hard disks are normally categorized into the following three types:
- Because both the disk platters and the disk drives are sealed in a contaminated-free container, and do not require to be separated later, all the surfaces of all the disk platters (including the upper surface of the topmost platter and lower surface of the bottommost platter) are used for data recording in case of Winchester disks. That is, for a Winchester disk with four platters, there are eight surfaces on which data can be recorded.
- The contamination-free environment allows Winchester disks to employ much greater precision of data recording and accessing, resulting in greater density of data storage than the interchangeable disk packs.
Uses of Magnetic Disks:
Optical Disk: Optical disks are random access medium for high capacity secondary storage, because it can store extremely large amounts of data in a limited space.
An optical-disk storage system consists of a rotating disk, which is coated with a thin metal or some other material that is highly reflective. Laser beam technology is used for recording/reading of data on the disk. Due to the use of laser beam technology, optical disks are also known as laser disks or optical laser disks.
Storage Organization: an optical disk has one track, which starts ate the outer edge and spirals inward to the center. This spiral track is ideal for reading large blocks of sequential data, such as music. However it makes for slower random access time data than the concentric tracks used by magnetic disks, whose sectors are can be located faster, because they are always found on a given track at a fixed distance from center.
Like a track on a magnetic disk, the track of an optical disk is split up into sectors, but with optical disks, each sector has the same length, regardless of whether it is located near the disk’s center or away from the center. This type of organization allows data to be packed at maximum density over the entire disk. However, it also requires a more complicated drive mechanism, because the rotation speed of the disk must vary inversely with radius; the drive must slow down the disk’s rotation speed to read sectors towards the outside of the disk, and speed it up to read sectors towards the center of the disk.
Storage capacity of an optical disk = Number of sectors X Number of bytes per sector
Access Mechanism: Optical disks use laser beam technology for recording/ reading of data on the disk surface. That is the read/write head is used in magnetic storage is replaced by two laser beam sources. One laser beam is used to write for the recording surface by etching microscopic pits on the disk surface, and another laser beam is used to read the data from the light-sensitive recording surface. That is data recording is done by focusing the stronger laser beam on the surface of the spinning disk. The laser beam is turned on and off. At a varying rate, due to which tiny pits are burnt into the metal coating of the disk along with its tracks. To read the data less powered laser beam is focused on the disk surface. This beam is strongly reflected by the coated surface (known as land), and weakly reflected by the burnt surface (known as pit), producing pattern of on and off reflection, which are converted into electronic signals of binary 1s and 0s by a sensor. Note that, mechanical read/write access arm is not needed in this case, because a light beam can be easily deflected to the desired place on the optical disk.
Access Time: Data accessing of Optical disks is slower than magnetic disk though no read/write access arm is there. Optical drives are slower than magnetic disk because of following reasons:
Optical Disk Drive: An optical disk drive has to be mounted on an optical disk drive, before it can be used for reading or writing of information. An optical disk drive contains al the mechanical, electrical and electronic components for holding an optical disk, and for readings or writing of information on to it. That is, it contains the tray on which the disk is kept, the read/write laser beams assembly, and the motor to rotate the disk.
Types of Optical Disks: The most popular types of optical disks are CD-ROM and WORM disks.
CD-ROM: CD-ROM stands for Compact Disk-Read-Only Memory. CD-ROM disk is a shiny, silver color metal disk of 51/4-inch (12 cm) diameter. It has storage capacity of about 650 Megabytes. It is so called because of its enormous storage capacity on a compact-size disk, and because it is read-only storage medium. That is, these disks come pre-recorded, and the information stored on them cannot be altered.
WORM Disk: WORM Disk stands for write-once, read-many. WORM disks allow the users to create their own CD-ROM disks by using the CD-recordable (CD-R) drive, which can be attached to a computer as a regular peripheral device. WROM disks, which look like standard CD-ROM disks, are purchased blank and encoded using a CD-R drive. The information recorded on a WORM disk by CD-R drive can be read by any ordinary CD-ROM drive. As the name implies the data can be written only once on a WORM disk, but can be read many times.
Uses of Optical Disks: