STORAGE. Memory Versus Storage

Transcription

1 STORAGE Memory Versus Storage Memory Memory, which is composed of one or more chips on the motherboard, holds data and instruction while they are being processed by the CPU. The contents of volatile memory, such as RAM, are lost when the power of the computer is turned off. The contents of nonvolatile memory, such as ROM, are not lost even when power is removed from the computer. Storage Storage, also called secondary storage, is nonvolatile, and holds items such as data, instructions, and information for future. A storage medium is the physical material on which items are kept. One commonly used storage medium is a disk, which is a round, flat piece of plastic or metal with a magnetic coating on which items can be written. A storage device is the mechanism used to record and retrieve items to and from a storage medium. Storage devices can function as sources of input and output. Reading refers to the process that a storage device transfers data, instruction, and information from a storage medium into memory. Writing refers to the process that a storage device transfers these items from memory to a storage medium. The speed of a storage device is defined by its access time, which is the minimum time it takes the device to locate a single item on a disk. Compared to memory, storage devices are slow. The access time of storage devices is measured in milliseconds (i.e., thousandths of a second) while that of memory is measured in nanoseconds (i.e., billionths of a second). The size, or capacity, or a storage device, is measured by the number of bytes (characters) it can hold. Page 1

2 Floppy Disks A floppy disk, or diskette, is a portable, inexpensive storage medium that consists of a thin, circular, flexible plastic disk with a magnetic coating enclosed in a square-shaped plastic shell. The most widely used floppy disk today is 3.5 inches wide. A floppy disk drive (FDD) is a device that can read from and write to a floppy disk. If a computer has only one floppy disk drive, the drive is usually designated drive A. If the computer has two floppy disk drives, the second one is usually designated drive B. Before anything can be written on a new floppy disk, it must be formatted. Formatting is the process of preparing a floppy disk or hard disk for reading and writing by organizing the disk into storage locations called tracks and sectors. A track is a narrow recording band that forms a full circle on the surface of the disk. A sector is a pie-shaped section on a track, and is capable of holding 512 bytes of data. Any sector that cannot be used due to a physical flaw on the disk is called a bad sector. A typical floppy disk stores data on both sides and has 80 tracks on each side of the recording surface with 18 sectors per track. For reading and writing purposes, sectors are grouped into clusters. A cluster consists of two to eight sectors and the exact number varies depending on the operating system used. A cluster is the smallest unit of space used to store date. Even if a file consists of only a few bytes, an entire cluster is used for storage. Although each cluster holds data from only one file, one file can be stored in many clusters. Today, most floppy disk are high density floppy disk, storing 2 (sides) x 80 (tracks) x 18 (sectors per track) x 512 (bytes per sector) = 1,474,560 bytes, or approximately 1.44 MB. The file allocation table (FAT) is a table of information used to locate files on a disk. Even if a user accidentally formats a disk, the disk can be unformatted with special software because the formatting process only erases the file location Page 2

3 information and redefines the file allocation table for these items; the actual files on the disk are not erased. Floppy disks have a write-protect notch, which is a small opening in the corner of the floppy disk with a tab that can slide to cover or expose the notch. If the write-protect notch is exposed, or open, nothing can be written on the disk. However, a floppy drive can read from a floppy disk whether the write-protect notch is open or closed. Some floppy disks have a second opening on the opposite side of the disk that does not have the small tab; this opening identifies the disk as a high-density floppy disk. On any 3.5-inch floppy disk, a piece of metal called the shutter covers an opening in the rigid plastic shell. When the floppy disk is inserted into a floppy disk drive, the drive slides the shutter to the side to expose a portion of both sides of the floppy disk s recording surface. The read/write head is the mechanism that actually reads items from or writes items on the floppy disk. The average access time for current floppy disk drives to locate an item on the disk is 84 milliseconds, or approximately 1/12 second. To care for a floppy disk, it should not be exposed to heat, cold, magnetic fields, and contaminants such as dust, smoke, or salt air. Several manufacturers have high-capacity floppy disk drives that use disks with capacities of 100 MB and greater. A SuperDisk drive is a high-capacity disk drive developed by Imation that reads from and writes on a 120 MB SuperDisk floppy disk. A HiFD TM (High-Capacity FD) drive is a high-capacity floppy disk drive developed by Sony that reads from and writes on a 200 MB HiFD TM floppy disk. Both the SuperDisk drive and the HiFD drive are downward compatible, which means that they can read from and write on standard 3.5-inch floppy disks. A Zip drive is a special high-capacity disk drive developed by Iomega that use a 3.5-inch Zip disk, which is slightly larger than and about twice as thick as a standard 3.5-inch floppy disk. A Zip disk can be 100 MB or 250 MB in storage capacity. Page 3

4 Hard Disk A hard disk usually consists of several inflexible, circular disks, called platters, on which items are stored electronically. A platter is made of aluminum, glass, or ceramic and is coated with a material that allows items to be magnetically recorded on its surface. The hard disk in most desktop and laptop computers is housed inside the system unit, and is considered fixed disks because being not portable. Hard disks provide far larger storage capacities and much faster access times than floppy disks. Access time for today s hard disks ranges from five to eleven milliseconds. Hard disks undergo two formatting steps: A low-level format organizes both sides of each platter into tracks and sectors to define where items will be stored on the disk. After low-level formatting, a hard disk can be divided into partitions, or separate areas that function as separate hard disk. Partitioning makes a hard disk more efficient or allows for the installation of multiple operating systems. If a hard disk has only one partition, it is usually designated drive C by the computer. If the hard disk is divided into two partitions or more, the first partition is designated drive C; the second partition is designated drive D, and so on. A high-level format command is issued through the operating system to define, among other items, the file allocation table for each partition. A high-level format is carried out after low-level formatting and partitioning. Most hard disks have multiple platters stacked on top of one another and each platter has two read/write heads, one for each side. Because of the stacked arrangement of the platters, the location of the read/write heads is often referred to by its cylinder instead of its track. A cylinder is the location of a single track through all platters on a hard disk. If a hard disk has four platters (i.e., eight sides), each with 1000 tracks, then it will have 1000 cylinders with each cylinder consisting of eight tracks (two for each platter). The platters in the hard disk rotate at a speed of 5400 to 7200 revolutions per minute. Page 4

5 The spinning motion creates a cushion of air between the platter and its read/write head so that the read/write head floats above the platter instead of making direct contact with the platter surface. The distance between the read/write head and the platter is approximately two millionths of an inch. If contamination is introduced, the hard disk can have a head crash. A head crash occurs when a read/write head touches the surface of a platter, usually resulting in a loss of data or sometimes loss of the entire drive. Hard disks are sealed tightly to keep out contaminants. Some computers are able to improve the hard disk access time by using disk caching. Disk cache is a portion of memory that the CPU uses to store frequently accessed items. Disk cache works similarly to memory cache. When a program needs data, instructions, or information, the CPU checks the disk cache first. If the item is located in the disk cache, the CPU uses that item and completes the process. Otherwise, the CPU has to wait for the hard disk drive to locate and transfer the item from the disk to the CPU. Some disk caching system also attempt to predict what data, instructions, or information might be needed next and place them into cache before they are requested. Some hard disks are removable. A removable hard disk or disk cartridge is a disk drive in which hard disks are enclosed in plastic or metal cases so they can be removed from the drive. The Jaz disk by Iomega is a removable hard disk that can store up to two gigabytes of data, instructions, and information. Advantages of removable hard disks over fixed disk include A removable hard disk can transport a large number of files or to make backup copies of important files. For security measures, a removable hard disk can be locked up a the end of a work session, leaving no data on the computer. Networks, minicomputers, and mainframe computers often use disk packs. A disk pack is a collection of removable hard disks mounted in the same cabinet as the computer or enclosed in a large stand-alone cabinet. Page 5

6 The flow of data, instructions, and information to and from a disk is managed by a specially purpose chip and its associated electronic circuits called the disk controller. A controller for a hard disk is called a hard disk controller (HDC). Two types of hard disk controllers for personal computers are IDE and SCSI. IDE (Integrated Drive Electronics) controllers support up to four hard disks and can transfer data, instructions, and information to the disk at rates of up to 66 MB per second. IDE controllers are also referred to as ATA (AT attachment), which integrates the controller into the disk drive. SCSI (Small Computer System Interface) controllers support multiple disk drives, as well as other peripherals such as scanners, high-capacity disk drives, compact disk drives, tape drives, and printers. As many as 30 SCSI devices can be daisy chained together. SCSI controllers are faster than EIDE controllers, providing up to 160 MB per second transfer rates. Raid (Redundant Array of Independent Disks) is a type of hard disk system that connects several smaller disks into a single unit that acts like a single large hard disk. RAID is more reliable than traditional disks and is often used with network and Internet servers. Reliability is improved with RAID through the duplication of data, instructions, and information. Duplication is implemented in different ways, depending on the storage design, or level, used. The simplest RAID storage design is level 1, called mirroring, which has one backup disk for each disk. Levels beyond level 1 use a technique called striping, which splits data, instructions, and information across multiple disks in the array. Striping improves disk access times, but does not offer data duplication. Some RAID levels combine both mirroring and striping. To prevent loss of items stored on a hard disk, preventative maintenance such as defragmenting or scanning the disk for errors should be carried out regularly. Page 6

7 Compact Discs A compact disc (CD) is a flat, round, portable, metal storage medium that usually is 4.75 inches in diameter and less than one-twentieth of an inch thick. Compact discs store items by using microscopic pits (indentations) and land (flat areas) that are in the middle layer of the disc. A high-powered laser light creates the pits. A low-powered laser light reads items from the compact disc by reflecting light through the bottom of the disc. The reflected light is converted into a series of bits that the computer can process. Land causes light to reflect, which is read as binary digit 1. Pits absorb the light and this absence of light is read as binary digit 0. A compact disc stores items in a single track that spirals from the center of the disc to the edge of the disc. As with a hard disk, this single track is divided into evenly sized sectors in which items are stored. The drive designation of a compact disc drive usually follows alphabetically after that of the hard disk (i.e., if the hard disk is drive C, then the compact disc is drive D). A jewel box is the protective case in which a compact disc should be stored. With proper care, a compact disc may last up to 50 years. Guidelines for the proper care of compact discs include: Do store the disc in a jewel box when not in use. Do hold a disc by its edges. Do not touch the underside of the disc. Do not write on the label side of the disc. Do not stack discs. Do not expose the disc to excessive heat or sunlight. Do not eat, smoke, or drink near a disc. Two basic types of compact discs designed for use with computers are CD-ROM and DVD-ROM. CD-ROM A CD-ROM (Compact Disc Read-Only Memory) is a compact disc that uses the same laser technology as audio CDs for recording music. A CD-ROM, however, can contain text, graphics, video, and sound. The contents of standard CD-ROMs are written, or recorded, by the manufacturer, and can only be read and used. They cannot be erased or modified by the user. Page 7

8 A CD-ROM drive or CD-ROM player reads items on a CD-ROM. A CD-ROM drive can also be used to play back audio CD because they use the same laser technology. A CD-ROM can hold up to 700 MB of data, instructions, and information. The speed of a CD-ROM drive is measured by its data transfer rate, which is the time it takes the drive to transmit data, instructions, and information from the drive to another device. The original CD-ROM drive was a single-speed drive with a data transfer rate of 150 KB per second. All subsequent CD-ROM drives have been measured relative to this first CD-ROM drive and use an X to denote the original transfer rate of 150 KB per second. e.g., A 40X CD-ROM drive has a data transfer rate of 40 x 150 = 6,000 KB or 6MB per second. Most standard CD-ROMs are single-session, which means that all items must be written to the disc at the time it is manufactured. Some CD-ROM variations are multisession, which means that additional data, instructions, and information can be written to the disc at a later time. PhotoCD A PhotoCD is a compact disc that only contains digital photographic images saved in the PhotoCD format, which is developed by Eastman Kodak. CD-R CD-R (Compact Disc-Recordable) is a technology that allows a user to write on a compact disc using his or her own computer. A user can write on the disc in stages writing on part of it one time and writing on another part at a later time. However, the user can write on each part only once and cannot erase it. CD-R is a write once, read many technology. CD-R software and a CD-R drive are required in order to write on a CD-R. CD-RW A CD-RW (Compact Disc-Rewritable) is an erasable disc that can be written on multiple times. Page 8

9 CD-RW acts like a floppy or hard disk, allowing data, instructions, and information be written and rewritten onto it multiple times. CD-RW software and a CD-RW drive are required in order to write and rewrite on a CD-RW. One problem with CD-RW is that they cannot be read by all CD-ROM drives. DVD-ROM A DVD-ROM (Digital Video Disc-ROM) is an extremely high capacity compact disc capable of storing from 4.7 GB to 17 GB. A DVD-ROM s quality also far surpasses that of a CD-ROM. A DVD-ROM drive or DVD player is required to read a DVD-ROM. Although the size and shape of a DVD-ROM and a CD-ROM are similar, a DVD-ROM uses one of three storage techniques to increase storage capacity: The first technique involves making the disc more dense by packing the pits closer together. A second technique involves using two layers of pits, which doubles the capacity of the disc. Finally, some DVD-ROMs are double-sided, which means that they can be removed and turned over to read the other side. Recordable and rewritable versions of DVD-ROM are also available. A DVD-R (DVD-Recordable) can be written once and read (play) for many times. With a DVD-RAM, items can be erased and recorded on it multiple times. Tapes Magnetic tape is a magnetically coated ribbon of plastic capable of storing large amounts of data and information at a low cost. It is one of the first storage media used with mainframe computers. Tape storage requires sequential access, which refers to reading or writing data consecutively. Floppy disks, hard disks, and compact discs all use direct access, or random access, which means that a particular data item or file can be located immediately, without having to move consecutively through items stored in front of it. Tapes are used most often for long-term storage and backup. Page 9

10 A tape drive is used to read from and write data and information onto a tape. A tape cartridge is a small, rectangular, plastic housing for tape. Tape cartridges containing one-quarter-inch wide tape are slightly larger than audiocassette tapes. Three common types of tape drives are QIC (Quarter-inch cartridge), DAT (Digital audio tape), and DLT (Digital linear tape). PC Cards A PC Card is a thin, credit card-sized device that fits into a PC Card expansion slot on a laptop or other personal computer. PC Cards are available in three types: Type I and Type II cards are used to add memory or communications capabilities to a computer. The thicker Type III cards are used to house hard disks. Some digital cameras also use a picture card or compact flash card, to store pictures. Other Types of Storage Smart Cards A smart card, which is similar in size to a credit card or ATM card, stores data on a thin microprocessor embedded in the card. An intelligent smart card contains a CPU and has input, process, output and storage capabilities. A memory card has only storage capabilities. When the smart card is inserted into a specialized card reader, the information on the smart card is read and, if necessary, updated. Applications of smart cards include: Storing a prepaid dollar amount (e.g., a prepaid telephone calling card). Storing patient records and other health-care information. Tracking information of customers and employees. Electronic money (e-money), or digital cash, is a means of paying for goods and services over the Internet. Microfilm and Microfiche Microfilm and microfiche are used to store microscopic images of documents on roll of sheet film. Microfilm uses a 100- to 215-foot roll of film. Microfiche uses a small sheet of film, usually about for inches by six inches. Page 10

11 The images are recorded onto the film using a device called a computer output microfilm (COM) recorder. The stored images are so small that they can be read only with a microfilm or microfiche reader. Applications of microfilm and microfiche include: Storing back issues of newspapers, magazines, and genealogy records in libraries. Archiving inactive files in large organizations such as banks. Advantages of using microfilm and microfiche include: It greatly reduces the amount of paper used. It is inexpensive. It has the longest life of any storage medium. Enterprise Storage System and Data Warehouses An enterprise storage system is a strategy that focuses on the availability, protection, organization, and backup of storage in a company. A storage area network (SAN) is a high-speed network that connects storage devices. A data warehouse centralizes the computing environment, in which large megaservers store data, information, and programs, and less powerful client devices connect to the megaservers to access these items. Page 11