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Chapter 4 Storage Learning Objectives Learning Objectives Learning Objectives Explain the difference between storage systems and Chapter 4 memory. Name several general properties of storage systems. Storage Identify the two primary types of magnetic disk systems and describe how they work. Learning Objectives, cont’d. Overview Discuss the various types of optical disc systems This chapter covers: available and how they differ from each other and from magnetic systems. Common characteristics of storage systems List at least three other types of storage systems. Magnetic disk storage Summarize the storage alternatives for a PC, Optical disc storage including which storage systems should be included in all PCs and when the other systems would be appropriate. Magnetic tape and other types of storage systems Properties of Storage Systems: Properties of Storage Systems: Physical Parts Nonvolatility Property Involve two physical parts: a storage device and a Storage media are nonvolatile: storage medium. When power to the device is shut off, data stored Can be internal (configured into the system unit), or on the medium remains. external (separate devices). This is in contrast to most types of memory, which Media must often pass by a read/write head in the are volatile. storage device to be read from or written to. 1 Properties of Storage Systems: Properties of Storage Systems: Removable vs. Fixed Media Random vs. Sequential Access Fixed media: typically faster and less expensive Sequential access: records in a file can be retrieved only in the same sequence in which they are physically stored (like tape) Removable media: unlimited capacity and can be easily transported and secured Direct access (random access): records can be retrieved in any sequence, independent of physical storage (most disks and optical media) Properties of Storage Systems: Logical vs. Physical Representation Logical file representation refers to the user’s view of the way data is stored. Physical file representation is the actual physical way the data is stored on the storage media as viewed by the computer. Magnetic Disk Systems Magnetic disks are the most important storage medium in computers today. Two popular types: floppy disks hard disks Data is written by magnetizing particles on the disks in a certain way to represent 1s and 0s. 2 Floppy Disks Physical properties Most are 3½ inches in diameter and hold 1.44 megabytes. Floppy Disks, cont’d. Floppy Disks, cont’d. Tracks, sectors, and clusters Using floppy disks The disk’s file directory keeps track of the Must be inserted into the proper drive in the contents of the disk. proper direction Should not be removed when the disk is being accessed Superdiskettes Zip drives, SuperDisk drives, HiFD drives Hard Disks Physical properties Permanently sealed inside the hard drive—allows faster speeds than removable systems Sizes: 3½-inch on PCs, 2½-inch on notebooks or smaller (IBM microdrive – size of a quarter) Rotation: 5,400-15,000 revolutions per minute (RPM) Types: internal or external 3 Hard Disks, cont’d. Hard Disks, cont’d. Reading and writing data Hard disk addressing (like on diskette plus) Obstacles on a hard-disk system can damage the Disk cylinder—the collection of tracks located in surface and read/write head. the same location on a set of hard disk surfaces Movable access mechanism positions read/write heads over the surface of the platters Head crash occurs if the head comes in contact with the surface of the disk Hard Disks, cont’d. Hard Disks, cont’d. Disk access time factors: Disk standards seek time – time to position read/write heads over ATA/IDE – attachment/integrated drive electronics the cylinder Controller is built into drive. rotational delay – time to rotate disk into proper SCSI – small computer system interface – position controllers are built into computers or on a special data movement time – time for data transfer SCSI card. ATA are less expensive and easier to configure. SCSI are faster. Disk cache Fibre Channel – extremely fast (up to 2 Gbps) Strategy for speeding up system performance Hard Disks, cont’d. Hard Disks, cont’d. Hard disk systems for notebooks Removable hard disk Can use either internal or removable hard drives systems Offer large storage capacities and portability 4 Hard Disks, cont’d. RAID Hard disk systems for large computer systems RAID – Redundant Array of Independent Disks May use a system consisting of multiple hard Increases performance drives; RAID is a recent trend Increases fault tolerance (ability of system to recover from hardware or software failure) RAID levels from 0 to 5 RAID 0 – disk stripping (no fault tolerance) RAID 1 – disk mirroring Optical Discs CD fabrication Laser beams write and read data packed at very tight The CD fabrication machine uses a high-powered storage densities, several times finer than that of a laser to etch the bump pattern into photoresist typical magnetic disk. material coated onto a glass plate Through an elaborate imprinting process, this pattern is pressed onto acrylic discs The discs are then coated with aluminum (or another metal) to create the readable reflective surface Finally, the disc is coated with a transparent plastic layer that protects the reflective metal from nicks, scratches and debris. General CD Structure CD pits 5 CDs CD-ROM Drive CD-ROM (compact disk, read-only memory) Tiny depressions on the disk are called pits CDs are permanently etched when data is stored on the disc. Recordable (CD-R) discs can be written to, but not erased and rewritten. Rewritable (CD-RW) CDs can be used in the same way as magnetic disks—data can be stored and erased as needed. CD-Rs CD-R (recordable) CD-Rs, don't have any pits or flat areas at all Instead, they have a smooth reflective metal layer, which rests on top of a layer of photosensitive dye. When the disc is blank, the dye is translucent: Light can shine through and reflect off the metal surface. But when you heat the dye layer with concentrated light of a particular frequency and intensity, the dye turns opaque: It darkens to the point that light can't pass through. CD-RW (rewritable) CD-RW (rewritable) cont’d CD-RW discs have taken the idea of writable CDs a step further, building in an erase function so you can record over old data you don't need anymore These discs are based on phase-change technology 6 DVDs DVD types DVD (Digital Versatile Disk) Stores 4.7 Gb per side per layer (up to 17Gb on two-sided double-layer disks) DVDs are read-only and will replace VHS as the movie media of choice DVD-5 (Single-side, single-layer) DVD-10 (double-sided) DVD-9 (single-sided, dual-layer) DVD-17 (double-sided, dual-layer) 7 DVD fabrication DVD Structure Once the clear pieces of polycarbonate are formed, a Pits on DVDs are half as wide as on CDs. thin reflective layer is sputtered onto the disc, Distance between pits is half as large as on CDs. covering the bumps The microscopic dimensions of the bumps make the Aluminum is used behind the inner layers, but a spiral track on a DVD extremely long semi-reflective gold layer is used for the outer layers, If you could lift the data track off a single layer of a allowing the laser to focus through the outer and DVD, and stretch it out into a straight line, it would onto the inner layers be almost 7.5 miles long! After all of the layers are made, each one is coated That means that a double-sided, double-layer DVD with lacquer, squeezed together and cured under would have 30 miles of data! infrared light DVDs Recordable DVDs history Recordable DVDs (2 standards + and -) Pioneer released 3.95G DVD-R(A) 1.0 drives allow to record once in October 1997 (about 6 months late) for $17,000 Rewritable DVDs (DVD-RAM or DVD+/-RW) allow users to record on DVD discs multiple times New 4.7G DVD-R(A) 1.9 drives appeared in limited quantities in May 1999 for $5,400 Pioneer's DVR-A03 DVD-R(G) drive was released in May 2001 for under $1000 Magneto-Optical Discs Magnetic Tape Systems M-O discs use a combination of magnetic and optical Magnetic tape is used primarily for backup technologies. purposes. Can store up to 5.2 GB per disc. The device that reads tapes is called a tape drive. Most tape media are in the form of cartridge tapes, though detachable-reel tapes exist as well. 8 Online Storage Online storage is usually storage on a server accessible over the Internet or another network. Online storage can be used as a primary storage medium or for backup or distribution purposes. Smart Cards Flash Memory Devices Smart card: credit-card-sized piece of plastic that Flash memory sticks contains some type of computer circuitry. Size of a stick of gum; hold from 4 to 64 MB each Holds less than a few megabytes Flash memory cards Commonly used to store prepaid amounts of Standards include CompactFlash, SmartMedia, digital cash, or personal information Secure Digital (SD), and MultiMedia Card (MMC) Flash memory drives Smart card readers are attached to a PC or built into Have no moving parts—are more shock-proof and a cell phone, keyboard, or other device. portable than conventional drives Comparing Storage Alternatives Speed, expense, portability, and compatibility Most PC users require Hard drive CD or DVD drive Floppy drive 9.
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