MAGNETIC DISKS
Magnetic disks are the most popular direct-access secondary storage device. They are also the most popular online secondary storage device.
A magnetic disk is a thin circular plate/platter made of metal or plastic and coated on both sides with a recording material that can be magnetized such as iron oxide. Data are recorded on a disk in the form of tiny invisible magnetized and non-magnetized spots (representing 1s and 0s) on the coated surfaces 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 are stacked together in a sealed contamination-free container.
Like magnetic tapes, magnetic disks can also be erased and reused indefinitely. Old data on a disk are erased automatically by recording new data in the same area. However, stored data can be read many times without affecting the data.
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. Based on these differences, there are many different types of magnetic disks available today. However, all of them may be broadly classified into two types — floppy disks and hard disks. Floppy disks are individually packaged in protective envelopes or plastic cases, whereas hard disks may be packaged individually or in multiples, in cartridges or contamination-free containers. Depending on the type of packaging, hard disks are further classified into Zip/Bernoulli disks, disk packs, and Winchester disks. The four types of magnetic disks are described below.
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 gives handling protection to the disk surface. Moreover, it has a special liner, which provides a wiping action to remove dust particles, which are harmful for the disk surface and the 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, and are now being produced in various sizes and capacities by many manufacturers.
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 disks, as and when they are needed, for reading/writing data on to them. A floppy disk can be very easily loaded into or unloaded from a floppy-disk drive, just by slipping the disk inside the drive’s slot, or by pressing the eject button on the drive. The disks are loaded into the disk drive along with the jacket cover, and data are read/written through an aperture in 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. Manufacturers guarantee about 2 million reads of a floppy disk track. In case of currently used floppy-disk drives, the rotational speed of a floppy disk is of the order of 300 to 400 rpm, and the data transfer rate is of the order of 10 to 30 Kilobytes/second.
31/2 –INCH FLOPPY DISK
It is the most commonly used floppy disk. It consists of a 31/2 –inch diameter disk encased in a square, hard-plastic jacket cover. The jacket cover has a cutout (aperture) for read/write head to make contact with disk surface. This aperture is covered with a sliding metal piece. When a floppy disk is inserted into a floppy-disk drive for use, the metal piece slides back to expose the disk’s surface to the drive’s read/write head.
All 31/2 –inch floppy disks are of double-sided type that record data on both disk surfaces. However, they come in three different capacities — double density, high density, and very high density. The double-density 31/2 –inch diskettes have 40 tracks, 18 sectors/track, and 512 bytes/sector, giving a total disk storage capacity or 2 (for two surfaces) x 40 x 18 x 512 = 7,37,280 bytes = 720 KB (approximately). The high-density 31/2 –inch diskettes have 80 tracks, 18 sectors/track, and 512 bytes/sector, giving a total disk storage capacity of 2 x 80 x 18 x 512 = 14,74,560 bytes = 1.4 MB (approximately). The very high-density 31/2 –inch diskettes have 80 tracks, 36 sectors/track, and 512 bytes/sector, giving a total disk storage capacity of 2 x 80 x 36 x 512 = 29,49,120 bytes = 2.88 MB (approximately).
HARD DISKS
Hard disks are the primary on-line secondary storage device for most computer systems today. They are made of rigid metal (frequently aluminum) platters and come in many sizes ranging from 1 to 14-inch diameter.
Types of hard disks
Depending on how they are packaged, hard disks are normally categorized into three types:
1. Zip/Bernoulli Disk. It consists of a single hard disk platter encased in a plastic cartridge. The disk is commonly of 31/2 –inch size having storage capacity of about 100MB. The storage capacity slightly varies depending on the formatting style used by a computer system with which it is used. Its disk drive, called zip drive, may be of portable or fixed type. The fixed type is part of computer system permanently connected to it. The portable type can be brought and connected to a computer system for the duration of use and can then be disconnected and taken away. A zip disk can be easily loaded/unloaded into a zip drive just as we insert/remove a floppy disk in a floppy disk drive or a video cassette in a VCR.
2. Disk Pack. It consists of multiple (two or more) hard disk platters mounted on a single central shaft. All the disks revolve together at the same speed. Its disk drive has a separated read/write head for each usable disk surface (recall that when multiple disks are used together in a disk device, the upper surface of the topmost disk and the lower surface of the bottommost disk are sometimes not used). Its disk drive is of interchangeable type and allows loading/unloading of different disk packs as and when they are to be used. When not in use, a disk pack is stored off-line in a plastic case. This gives virtually unlimited storage capacity to disk packs.
3. Winchester Disk. A Winchester disk consists of multiple (two or more) hard disk platters mounted on a single central shaft. However, unlike a disk pack drive, a Winchester disk drive is of fixed type. That is, its hard disk platters and disk drive are sealed together in a contamination-free container and cannot be separated from each other. Hence, Winchester disks have limited capacity. However, for the same number of disk platters of the same size, Winchester disks can have larger storage capacity than disk packs due to following reasons:
a) As both disk platters and disk drive are permanently sealed together, all surfaces of all disk platters (including the upper surface of the topmost platter and the lower surface of the bottommost platter) are used for data recording in a Winchester disk. That is, for a Winchester disk with four platters there are eight usable surfaces as opposed to six surfaces in case of a disk pack with four platters.
b) The contamination-free environment allows Winchester disks to employ greater precision of data recording and accessing, resulting in greater data storage density than interchangeable disk packs. Winchester disks were so named after 30-30 Winchester rifle because the early Winchester disk systems had two 30-MB disks. Storage capacity of today’s Winchester disks ranges from a few tens of megabytes to a few gigabytes (109 bytes).
ADVANTAGES AND LIMITATIONS OF MAGNETIC DISKS
Advantages
• Unlike magnetic tapes, which support sequential access of data, magnetic disks support direct access of data. Hence, they are more suitable for a wider range of applications.
• Due to its random access property, magnetic disks are often used simultaneously by multiple users as a shared device. For example, Winchester disks and disk packs are often used as on-line secondary storage devices, in which case they store data of multiple users of the computer system. A tape is not suitable for such type of usage, due to its sequential-access property.
• Magnetic disks are suitable for both on-line and off-line storage of data. For example, Winchester disks and disk packs are often used as on-line secondary storage devices, whereas floppy disks and zip disks are used as off-line secondary storage devices. In fact, the high-capacity Winchester disks have made it possible for today’s most personal computer users to enjoy the convenience of having all data and software readily accessible at all times.
• Except for fixed type Winchester disks, the storage capacity of other magnetic disks is virtually unlimited, because as magnetic disks is virtually unlimited, because as many disks as required can be used for storing very large data sets.
• Due to their low cost and high data recording densities, the cost per bit of storage is low for magnetic disks. An additional cost benefit is that magnetic disks can be erased and reused many times.
• Floppy disks and zip disks are compact and light in weight. Hence, they are easy to handle and store. Very large amount of data can be stored in a small storage space.
• Due to their compact size and light weight, floppy disks and zip disks are also easily portable from one place to another. They are often used for transferring data and programs from one computer to another, which are not linked together.
• Any information desired from a disk storage can be accessed in a few milliseconds, because it is a direct access storage device. This is not possible in case of a tape storage, which is a sequential access storage device.
• Data transfer rate for a magnetic disk system is normally higher than a tape system.
• Magnetic disks are less vulnerable to data corruption due to careless handling or unfavorable temperature and humidity conditions than magnetic tapes.
Limitations
• Although magnetic disks may be used for both types of applications (random as well as sequential data processing applications), for applications of the latter type, use of magnetic disks may be less efficient than magnetic tapes.
• It is more difficult to maintain the security of information stored on magnetic disks, which are used as shared, on-line secondary storage devices, as compared to information stored on magnetic tapes or on other types of magnetic disks.
• For Winchester disks, a disk crash or drive failure often results in the loss of entire data stored on it. It is not easy to recover the lost data. Hence, suitable backup procedures are suggested for data stored on Winchester disks.
• Some types of magnetic disks, such as disk packs and Winchester disks, are not so easily portable like magnetic tapes.
• On a cost-per-bit basis, the cost of magnetic disks is low, but the cost of magnetic tapes is even lower.
• They must be stored in a dust-free environment.
USES OF MAGNETIC DISKS
Magnetic disks are typically used for one or more of the following purposes:
• For applications that are based on random data processing.
• As a shared, on-line secondary storage device. Winchester disks and disk packs are often used for this purpose.
• As a backup device for off-line storage of data, so that if by accident, the data on an on-line storage device is corrupted, it can be retrieved from the backup storage. Floppy disks, zip disks, and disk packs are often used for this purpose.
• Archiving of data which are not used frequently, but which may be used once in a while. Floppy disks, zip disks, and disk packs are often used for this purpose.
• Transferring of data and programs from one computer to another, which are not linked together. Floppy disks and zip disks are often used for this purpose.
• Distribution of software by vendors. Originally sold software or software updates are often distributed by vendors on floppy disks and zip disks.
MEMORY STORAGE DEVICES
With the advent of efficient manufacturing technologies, price of memory chips has fallen considerably. As a result, a new breed of secondary storage devices has been introduced that not only provides storage but also applications on top like music player, etc.
FLASH DRIVE (PEN DRIVE)
Flash drive is a compact device of the size of a pen, comes in various shapes and stylish designs (such as pen shape, wallet shape etc.), and may have different added features (such as with a camera, with a built-in MP3/WMA/FM Radio play back for music on the go, etc.). It enables easy transport of data from one computer to another.
It is a plug-and-play device that simply plugs into a USB (Universal Serial Bus) port of a computer. The computer detects it automatically as removable drive. Now one can read, write, copy, delete, and move data from the computer’s hard disk drive to the flash drive or from the flash drive to the hard disk drive. One can even run applications, view videos, or play MP3 files from it directly. Once done, it can be simply plugged out of the USB port of the computer and kept into the pocket for being carried anywhere. A flash drive does not require any battery, cable, or software, and is compatible with most PCs, desktop, and laptop computers with USB 2.0 port. All these features make it ideal external data storage for mobile people to carry or transfer data from one computer to another.
As the name implies, it is based on flash memory storage technology. Recall that flash memory is non-volatile, Electricity Erasable Programmable Read Only Memory (EEPROM) chip. It is a highly durable solid-state storage having data retention capability of more than 10 years.
Available storage capacities are 8MB, 64MB, 128MB, 256MB, 1GB, 2GB, 4GB, and 8GB. A pen drive of 16MB capacity has 5600 times more storage capacity than a 1.4 MB floppy disk.
It has a main body and usually a port connector cover. The cover is removed or port connector is pushed out when the drive is to be plugged into the USB port of a computer. The main body usually has a write protect tab, a read/write LED (Light Emitting Diode) indicator, and a strap hole. Some manufacturers also provide software to be used with the drive.
MEMORY CARD (SD/MMC)
Similar to flash drive, flash memory based cards are available as removable storage device in different types of electronic equipment. Some of the most popular ones are Secure Digital (SD) and Multimedia Card (MMC). Some other manufacturers have their own proprietary cards based on flash memory. Storage capacity of these cards range from 8MB to 2GB.
In addition to computers, these cards are used in various types of digital devices such as digital camera and cell phone. This also facilitates easy transfer of data from these devices to a computer for storage in the computer’s hard disk or for further processing by the computer. For example, photographs taken by a digital camera can be transferred to a computer for storage or editing. Each of these cards has its own interface and specific design features for use with certain types of devices.
MASS STORAGE DEVICES
Mass storage devices are storage systems having storage capacity of several trillions of bytes of data. They use multiple units of a storage media (such as multiple disks, multiple tapes, multiple CD-ROMs, etc.) as a single secondary storage device. Commonly used types of mass storage devices are:
1. Disk array using a set of magnetic disks.
2. Automated tape library using a set of magnetic tapes.
3. CD-ROM jukebox using a set of CD-ROMs.
They are described below.
The average access times of mass storage devices are measured in seconds, instead of milliseconds, because a transport mechanism must first move to retrieve the storage media (such as disk, tape, or CD-ROM) on which the desired data is stored. It requires several seconds to locate the storage media specified first and then few milliseconds to transfer the data from it to memory. However, a mass storage device has huge storage capacity and a very small cost per bit stored.
Relatively slow access times limit the use of mass storage devices in many applications. However, they are cost-effective alternative to on-line magnetic tape or disk storage in applications requiring huge storage capacity and in which rapid access to data is not essential. When used for off-line storage devices are often referred to as archival storage because of the very large volumes of historical or backup data that they can store.
DISK ARRAY
A disk array, which first appeared on the market in 1993, is a set of hard disks, hard disk drives, and a controller, mounted in a single box. All the disks of form a single large storage. A disk array is commonly known as a RAID (Redundant Array of Inexpensive Disks). The term inexpensive comes from the fact that each of the medium-sized hard disks in the disk array is much less expensive than a single large hard disk.
Although RAID systems were originally developed to provide large secondary storage capacity with enhanced performance, today, they are also becoming popular due to enhanced reliability. Enhanced reliability is achieved in RAID units by taking advantage of the presence of multiple disks.
With more and more organizations relying on information technology to run their business, unscheduled interruptions have more serious consequences. As a result, today, many organizations are turning to the RAID as a safer way to store large amounts of data.
AUTOMATED TAPE LIBRARY
An automated tape library is a set of magnetic tapes, magnetic tape drives, and a controller, mounted in a single unit. The unit has one or more tape drives to read and write data on the tapes in the tape library. In case of units having multiple tape drives, it is possible to read/write data from/to multiple magnetic tapes simultaneously, resulting in faster data transfer rates. Multiple drives also lead to improved reliability of the storage unit, because if one of the drives fails, the unit can continue to function with other drives at a slower speed. The unit also has robotic arms to retrieve the appropriate tape from the tape from the tape library, and mount it on one of the tape drives for processing. The tape is automatically returned to the
Library at the end of the job. A large tape library can accommodate up to several hundred high capacity magnetic tapes, bringing the storage capacity of the storage unit to several terabytes (1 terabyte is equal to one trillion bytes).
Automated tape libraries are typically used for data archiving, and as an on-line data backup device for automated backup. They are mainly found in large computer centers.
CD-ROM JUKEBOX
A CD-ROM jukebox is a set of CD-ROM drives, and a controller, mounted in a single unit. The unit has one or more CD-ROM drives to read data from the CD-ROM disks in the jukebox. In case of units having multiple CD-ROM drives, it is possible to read data from multiple CD-ROM disks simultaneously, resulting in faster data transfer rates. Multiple drives also lead to improved reliability of the storage unit, because if one of the drives fails, the unit can continue to function with other drives at a slower speed. The unit also has robotic arms to retrieve the appropriate CD-ROM and mount it on one of the CD-ROM jukebox can accommodate up to several hundred CD-ROM disks, bringing the storage capacity of the storage unit to several terabytes.
CD-ROM jukeboxes are typically used for archiving read-only data, which needs to be accessed on-line, but where access time is not very critical. For example, on-line museums, on-line digital libraries and on-line encyclopedia are a few systems, which may make use of this mass storage device in an effective manner.
