DATA STORAGE 1

3480,3490: The IBM designation for families of half-inch magnetic tape drives typically used on mainframes and AS/400s. The 3480 drives use 18-track cartridges at 38000 bpi to yield 200MB. The 3490 uses built-in compression to obtain 400MB. The 3490e records 36 tracks and uses longer tape to hold 800MB. Tape libraries are available that hold from a handful to thousands of cartridges.The 3490 drives provide the fastest transfer rates in the tape backup business, generally at least 3MB/sec. Drives are available that use the ESCON and Fast SCSI-2 interfaces to obtain up to 20MB/sec. ACCESS TIME: The time a program or device takes to locate a single piece of information and make it available to the computer for processing. DRAM (dynamic random access memory) chips for personal computers have access times of 50 to 150 nanoseconds (billionths of a second). Static RAM (SRAM) has access times as low as 10 nanoseconds. The access time of memory should be fast enough to keep up with the CPU. If not, the CPU will waste a certain number of clock cycles, which makes it slower. Note, however, that reported access times can be misleading because most memory chips, especially DRAM chips, require a pause between back-to-back accesses. This is one reason why SRAM is so much faster than DRAM, even when the reported access times are equivalent; SRAM requires fewer refreshes, so the pause between back-to-back accesses is smaller. A more important measurement of a chip’s speed, therefore, is its cycle time, which measures how quickly two back-to-back accesses can be made. Access time is also frequently used to describe the speed of disk drives. Disk access times are measured in milliseconds (thousandths of a second), often abbreviated as ms. Fast hard disk drives for personal computers boast access times of about 9 to 15 milliseconds. This means that the drive can make about 500 data accesses per second. Note that this is about 200 times slower than average DRAM. The access time for disk drives (also called the seek time) refers to the time it actually takes for the read/write head to locate a sector on the disk. This is an average time since it depends on how far away the head is from the desired data. The performance of disk drives can be improved through special techniques such as caching and interleaving. AREAL DENSITY: The amount of data that can be packed onto a storage medium. Areal densities are generally measured in megabits per square inch. The term is useful for comparing different types of media, such as magnetic disks and optical disks. Current magnetic disks have an areal density of about 200 megabits per square inch. Optical disks, on the other hand, have areal densities of more than one gigabit per square inch. ATA: Short for AT Attachment, a disk drive implementation that integrates the controller on the disk drive itself. There are several versions of ATA, all developed by the Small Form Factor (SFF) Committee: ATA: Known also as ATAPI (AT Attachment Packet Interface) and IDE.ATA-2: Known as EIDE (Western Digital's implementation) and Fast-ATA (Seagate's implementation)ATA-3: Also known as Ultra ATA, supports data transfer rates of 33 MBps. BACKUP: To copy files to a second medium (a disk or tape) as a precaution in case the first medium fails. One of the cardinal rules in using computers is Back up your files regularly. Even the most reliable computer is apt to break down eventually. Many professionals recommend that you make two, or even three, backups of all your files. To be especially safe, you should keep one backup in a different location from the others. You can back up files using operating system commands, or you can buy a special-purpose backup utility. In general, backup programs are much faster than operating system shell commands because they use a DMA channel. In addition, the backup programs often compress the data so that backups require fewer disks. (n) (1) The act of backing up. A substitute or alternative. The term backup usually refers to a disk or tape that contains a copy of data. BAD SECTOR: A portion of a disk that cannot be used because it is flawed. When you format a disk, the operating system identifies any bad sectors on the disk and marks them so they will not be used. If a sector that already contains data becomes damaged, you will need special software to recover the data. Almost all hard disks come with bad sectors (sectors damaged during the manufacturing process), so do not be alarmed if a utility reports that your hard disk has bad sectors. However, additional bad sectors should occur only infrequently if your drive is functioning properly. Floppy disks should not have any bad sectors. BERNOULLI DISK DRIVE: Named after a Swiss scientist who discovered the principle of aerodynamic lift, the Bernoulli disk drive is a special type of floppy disk drive from Iomega Corporation that is faster and has greater storage capacity than traditional floppy drives. A Bernoulli drive is really a cross between a hard disk drive and floppy drive. Like the platters in hard disk drives, Bernoulli disks float between the read/write heads, so there is no actual contact between the disk and the heads. But the disk itself is flexible and removable like a floppy disk. Because the disk is flexible, it is less susceptible than a hard disk to head crashes. Bernoulli disk drives, however, are not as fast as hard disk drives. A Bernoulli disk drive is sometimes called a Bernoulli box. CAV: Short for Constant Angular Velocity, a technique for accessing data off CD-ROMs. With CAV, the disk rotates at a constant speed regardless of what area of the disk is being accessed. This differs from Constant Linear Velocity (CLV), which rotates the disk faster for inner tracks.The advantage of CAV is that it is much simpler to design and produce because the motor doesn't need to change speed. In addition, CLV runs into problems for very high-speed CD-ROMs because there's a brief latency whenever the drive needs to change the rotational speed. The drawback to CAV is that data on inner tracks is accessed more slowly than data on outside tracks. And unfortunately, most CDs are laid out with data on the inner tracks first. CD-I: A software and hardware standard developed jointly by Philips International and Sony Corporation for storing video, audio, and binary data on compact optical disks. It supports 552MB (megabytes) of binary data and specifies several different types of video and audio encoding formats. Unlike conventional CD-ROM drives, CD-I drives have a built-in microprocessor to handle many of the computing functions. It is sometimes referred to as the Green Book standard. Although there are some CD-I devices and titles, the format has not yet become widely accepted. CD-R DRIVE: Short for Compact Disk-Recordable drive, a type of disk drive that can create CD-ROMs and audio CDs. This allows users to “master” a CD-ROM or audio CD for publishing. Until recently, CD-R drives were quite expensive, but prices have dropped dramatically. A feature of many CD-R drives, called multisession recording, enables you to keep adding data to a CD-ROM over time. This is extremely important if you want to use the CD-R drive to create backup CD-ROMs. To create CD-ROMs and audio CDs, you'll need not only a CD-R drive, but also a CD-R software package. Often, it is the software package, not the drive itself, that determines how easy or difficult it is to create CD-ROMs. CD-R drives can also read CD-ROMs and play audio CDs. CD-ROM: Pronounced see-dee-rom, abbreviation of Compact Disc-Read-Only Memory. A type of optical disk capable of storing large amounts of data -- up to 1GB, although the most common size is 630MB (megabytes). A single CD-ROM has the storage capacity of 700 floppy disks, enough memory to store about 300,000 text pages. CD-ROMs are recorded by the vendor, and once recorded, they cannot be erased and filled with new data. To read a CD, you need a CD-ROM player. Almost all CD-ROMs conform to a standard size and format, so it is usually possible to load any type of CD into any ROM player. In addition, most CD-ROM players are capable of playing audio CDs, which share the same technology. CD-ROMs are particularly well-suited to information that requires large storage capacity. This includes color graphics, sound, and especially video. In recent years, as the prices of CD-ROM players have decreased, and the tools for creating new CD-ROM titles have improved, the CD-ROM industry has been expanding rapidly. To date, the most popular CD-ROM titles have been computer games and multimedia reference works. CD-ROM PLAYER: Also called a CD-ROM drive, a CD-ROM player is a device that can read information from a CD-ROM. CD-ROM players can be either internal, in which case they fit in a bay, or external, in which case they generally connect to the computer’s parallel port. Parallel CD-ROM players are easier to install, but they have several disadvantages: They’re somewhat more expensive than internal players, they use up the parallel port which means that you can’t use that port for another device such as a printer, and the parallel port itself may not be fast enough to handle all the data pouring through it. There are a number of features that distinguish CD-ROM players, the most important of which is probably their speed. CD-ROM players are generally classified as single-speed or some multiple of single-speed. For example, a 4X player access data at four times the speed of a single-speed player. Within these groups, however, there is some variation. Also, you need to be aware of whether the CD-ROM uses the CLV or CAV technology. The reported speeds of players that use CAV are generally not accurate because they refer only to the access speed for outer tracks. Inner tracks are accessed more slowly. Two more precise measurements are the drive’s seek time and data transfer rate. The seek time, also called the access time, measures how long, on average, it takes the drive to access a particular piece of information. The data transfer rate measures how much data can be read and sent to the computer in a second. Aside from its speed, another important feature of a CD-ROM player is its compatibility with existing standards. If you plan to run CD-ROMs in a windows environment, you need a player that conforms to the MPC III standard. If you want to be able to view photographs stored on CD-ROM, make sure your player conforms to Kodak’s PhotoCD format. Finally, you should consider how the player connects to your computer. Most CD-ROMs connect via a SCSI bus. If your computer doesn’t already contain such an interface, you will need to install one. Other CD-ROMs connect to an IDE or Enhanced IDE interface, which is the one used by the hard disk drive; still others use a proprietary interface. CD-RW DISK: Short for CD-ReWritable disk, a new type of CD disk that enables you to write onto it in multiple sessions. One of the problems with CD-R disks is that you can only write to them once. With CD-RW drives and disks, you can treat the optical disk just like a floppy or hard disk, writing data onto it multiple times. The first CD-RW drives became available in mid-1997. They can read CD-ROMs and can write onto today's CD-R disks, but they cannot write on CD-ROMs. Many experts believe that they'll be a popular storage medium until DVD devices become widely available. CLUSTER: A group of disk sectors. The operating system assigns a unique number to each cluster and then keeps track of files according to which clusters they use. Occasionally, the operating system marks a cluster as being used even though the file that is supposedly using it no longer exists. This is called a lost cluster. You can free up disk space by deleting lost clusters, but you should first make sure that the clusters do not, in fact, contain valuable data. DOS and Windows keep track of clusters with the File Allocation Table (FAT). The size of each cluster depends on the disk's partition size. CLV: Short for Constant Linear Velocity, a method used by most CD-ROM players to access data. With CLV, the rotation speed of the disk changes based on how close to the center of the disk the data is. For tracks near the center, the disk rotates faster, and for data on the outside, the disk rotates slower. The purpose of CLV is to ensure a constant data rate regardless of where on the disk the data is being accessed. Because less data can fit on the inside tracks, the disk needs to rotate faster for these areas An alternative technology, which is becoming increasingly popular, is CAV (Constant Angular Velocity). CD: Known by its abbreviation, CD, a compact disk is a polished metal platter capable of storing digital information. The most prevalent types of compact disks or those used by the music industry to store digital recordings and CD-ROMs used to store computer data. Both of these types of compact disk are read-only, which means that once the data has been recorded onto them, they can only be read, or played. Another type of compact disk, generally called an erasable optical disk, can have its data erased and overwritten by new data. Currently, erasable optical storage is too slow to be used as a computer’s main storage facility, but as the speed improves and the cost comes down, optical storage devices are becoming a popular alternative to tape systems as a backup method. CYLINDER: A single track location on all the platters making up a hard disk. For example, if a hard disk has four platters, each with 600 tracks, then there will be 600 cylinders, and each cylinder will consist of 8 tracks (2 for each platter). DAT: Acronym for digital audio tape, a type of magnetic tape that uses an ingenious scheme called helical scan to record data. A DAT cartridge is slightly larger than a credit card and contains a magnetic tape that can hold from 2 to 24 gigabytes of data. It can support data transfer rates of about 2 MBps. Like other types of tapes, DATs are sequential-access media. The most common format for DAT cartridges is DDS (digital data storage). DATA RECOVERY: Salvaging data stored on damaged media, such as magnetic disks and tapes. There are a number of software products that can help recover data damaged by a disk crash or virus. In addition, there are companies that specialize in data recovery. Of course, not all data is recoverable, but data recovery specialists can often restore a surprisingly high percentage of the data on damaged media. DDS: Abbreviation of Digital Data Storage, the industry standard for digital audio tape (DAT) formats. The latest format, DDS-3, specifies tapes that can hold 24GB (the equivalent of over 40 CD-ROMs) and support data transfer rates of 2 MBps. DEFRAGMENT: To optimize a disk by unfragmenting files. DENSITY: How tightly information is packed together on a storage medium (tape or disk). A higher density means that data are closer together, so the medium can hold more information. Floppy disks can be single-density, double-density, high-density, or extra-high-density. To use a double-density, high-density, or extra-high-density disk, you must have a disk drive that supports the density level. Density, therefore, can refer both to the media and the device. The table shows the storage capacities of double- and high-density floppies on the PC and the Apple Macintosh. Note that the only difference between double-density and high-density disks is that the high-density disks were found to be higher quality during the testing process. High-quality disks are sold as high-density, and lower-quality disks are sold as double-density. It is often possible, therefore, to format a double-density disk as a high-density disk, but this practice is not encouraged. DISK: A round plate on which data can be encoded. There are two basic types of disks: magnetic disks and optical disks. A magnetic disk is like a phonograph record, except that the data is encoded as microscopic magnetized needles on the disk's surface rather than as grooves in the vinyl. Also, you can record and erase data on a magnetic disk any number of times, just as you can with a cassette tape. Magnetic disks come in a number of different forms: floppy disk: A typical 5¼-inch floppy disk can hold 360K or 1.2MB (megabytes). Microfloppies (3½-inch floppies) normally store 720K, 1.2MB or 1.44MB of data. hard disk: Hard disks can store anywhere from 20MB to more than 3GB. Hard disks are also from 2 to 20 times faster than floppy disks.removable cartridge: Removable cartridges are hard disks encased in a metal or plastic cartridge, so you can remove them just like a floppy disk. Removable cartridges are very fast, often faster than fixed hard disks. A typical cartridge has a capacity of about 80MB. Optical disks record data by burning microscopic holes in the surface of the disk with a laser. To read the disk, another laser beam shines on the disk and detects the holes by changes in the reflection pattern. Optical disks have a much larger data capacity than magnetic disks, but they are slower. Optical disks come in three forms: CD-ROM: Most optical disks are read-only. When you purchase them, they are already filled with data. You can read the data from a CD-ROM, but you cannot modify, delete, or write new data.WORM: Stands for write-once, read-many. WORM disks can be written on once and then read any number of times; however, you need a special WORM disk drive to write data onto a WORM disk. erasable optical (EO): EO disks can be read to, written to, and erased just like magnetic disks. The machine that spins a disk is called a disk drive. Within each disk drive is one or more heads (often called read/write heads) that actually read and write data. Accessing data from a disk is not as fast as accessing data from main memory, but disks are cheaper and more stable. Unlike RAM, disks hold on to data even when the computer is turned off. Consequently, disks are the storage medium of choice for most types of data. Another storage medium is the magnetic tape. But tapes are used only for backup and archiving because they are sequential-access devices (to access data in the middle of a tape, the tape drive must pass through all the preceding data). A new disk, called a blank disk, has no data on it. Before you can store data on a blank disk, however, you must format it. In DOS systems, disks are formatted with the FORMAT command. When you execute the FORMAT command, the operating system allocates space for directories and other bookkeeping information. Every operating system formats disks somewhat differently, so a disk formatted by one operating system usually cannot be read by another operating system. DISK CACHE: A portion of RAM used to speed up access to data on a disk. The RAM can be part of the disk drive itself (sometimes called a hard disk cache) or it can be general-purpose RAM in the computer that is reserved for use by the disk drive (sometimes called a soft disk cache). Hard disk caches are more effective, but they are also much more expensive. In both cases, a disk cache works by storing the most recently accessed data in the RAM cache. When a program needs to access new data, the operating system first checks to see if the data is in the cache before reading it from the disk. Because computers can access data from RAM much faster than from a disk, disk caching can significantly increase performance. Many cache systems also attempt to predict what data will be requested next so they can place that data in the cache ahead of time. Although caching improves performance, there is some risk involved. If the computer crashes (due to a power failure, for example), the system may not have time to copy the cache back to the disk. In this case, whatever changes you made to the data will be lost. Usually, however, the cache system updates the disk frequently so that even if you lose some data, it will not be much. A special type of disk cache, called a write-thru cache, removes the risk of losing data because it only caches data for read operations; write operations are always sent directly to the disk. DISK CONTROLLER: A chip and associated circuitry that is responsible for controlling a disk drive. There are different controllers for different interfaces. For example, an IDE interface requires an IDE controller and a SCSI interface requires a SCSI controller. On Macintosh computer systems, the disk controller is built into the motherboard. On PCs, the disk controller is sometimes housed on a separate card. DISK DRIVE: A machine that reads data from and writes data onto a disk. A disk drive resembles a stereo turntable in that it rotates the disk very fast. It has one or more heads that read and write data. There are different types of disk drives for different types of disks. For example, a hard disk drive reads and writes hard disks, and a floppy drive accesses floppy disks. A magnetic disk drive reads magnetic disks, and an optical drive reads optical disks. Disk drives can be either internal (housed within the computer) or external (housed in a separate box that connects to the computer). DISK MIRRORING: A technique in which data is written to two duplicate disks simultaneously. This way if one of the disk drives fails, the system can instantly switch to the other disk without any loss of data or service. Disk mirroring is used commonly in on-line database systems where it's critical that the data be accessible at all times. DISK OPTIMIZER: A program that makes a disk more efficient by defragmenting the disk. Fragmentation occurs naturally when a disk is used often. DISK STRIPING: A technique for spreading data over multiple disk drives. Disk stripping can speed up operations that retrieve data from disk storage. The computer system breaks a body of data into units and spreads these units across the available disks. Systems that implement disk striping generally allow the user to select the data unit size or stripe width. Disk striping is available in two types. Single user striping uses relatively large data units, and improves performance on a single-user workstation by allowing parallel transfers from different disks. Multi-user striping uses smaller data units and improves performance in a multi-user environment by allowing simultaneous (or overlapping) read operations on multiple disk drives. Disk striping stores each data unit in only one place and does not offer protection from disk failure such a RAID technology. DOUBLE-DENSITY DISK: A floppy disk that has twice the storage capacity of a single-density floppy. Single-density disks are now practically obsolete. Double-density 5¼-inch disks for PCs can hold 360K of data. Double-density 3½-inch disks can hold 720K. Although high-density disk drives can format both high-density and double-density disks, double-density disks formatted by a high-density drive may not be readable by a double-density drive. Similarly, high-density drives may not be able to read disks that have been formatted by a double-density disk drive. DOUBLE-SIDED DISK: A floppy disk with both sides prepared for recording data. You can store twice as much data on a double-sided disk, but you need to use a double-sided disk drive. Nearly all modern disks and disk drives are double-sided. DRIVE BAY: An area of reserved space in a personal computer where hard or floppy disk drives (or tape drives) can be installed. The number of drive bays in a computer determines the total number of internal mass storage devices it can handle. DVD: Short for digital versatile disk or digital video disk, a new type of CD-ROM that holds a minimum of 4.7GB (gigabytes), enough for a full-length movie. Many experts believe that DVD disks, called DVD-ROMs, will eventually replace CD-ROMs, as well as VHS video cassettes and laser discs. Currently, however, DVD is more promise than reality. There are only a few DVD devices currently on the market and even fewer DVD-ROM titles. The DVD specification supports disks with capacities of from 4.7GB to 17GB and access rates of 600KBps to 1.3 MBps. One of the best features of DVD drives is that they are backward-compatible with CD-ROMs. This means that DVD players can play old CD-ROMs, CD-I disks, and video CDs, as well as new DVD-ROMs. DVD players cannot, however, read CD-R disks.DVD uses MPEG-2 to compress video data. DVD-RAM: A new type of rewritable compact disc that provides much greater data storage than today's CD-RW systems. The specifications for DVD-RAMs are still being hammered out by the DVD Consortium. ERASABLE OPTICAL DISK: A type of optical disk that can be erased and loaded with new data, just like magnetic disks. In contrast, most optical disks, called CD-ROMs, are read-only. Although the technology is still young, erasable optical disks seem destined to become the future medium of choice. A single 5¼-inch optical disk can store 650 megabytes of data, about 1,000 times more than a typical floppy disk. And unlike hard disks, optical disks are portable. The data -access speed of optical disks varies considerably, from about 20 to 200 milliseconds. Though comparable to floppy disks, they are not yet as fast as magnetic hard disk drives. FAT 32: A new version of the file allocation table (FAT) available in Windows 95 OSR 2. FAT32 increases the number of bits used to address clusters and also reduces the size of each cluster to 4KB. This result is that it can support larger disks (up to 2 terabytes) and better storage efficiency. FDHD: Short for floppy drive, high density, and pronounced fud-hud. FDHD refers to 3½-inch disk drives for Macintosh computers that can accept double-density or high-density 3½-inch floppy disks. FDHDs can also read DOS-formatted floppy disks, which enables Macintosh computers and PCs to share data. FDHD drives are often called SuperDrives. FIBRE CHANNEL: A data transfer architecture developed by a consortium of computer and mass storage device manufacturers and now being standardized by ANSI. The most prominent Fibre Channel standard is Fibre Channel Arbitrated Loop (FC-AL). FC-AL was designed for new mass storage devices and other peripheral devices that require very high bandwidth. Using optical fiber to connect devices, FC-AL supports full-duplex data transfer rates of 100MBps. FC-AL is compatible with, and is expected to eventually replace, SCSI for high-performance storage systems. In addition, FC-AL serves as the physical specification for Gigabit Ethernet. FLOPPY DISK: A soft magnetic disk. It is called floppy because it flops if you wave it (at least, the 5¼-inch variety does). Unlike most hard disks, floppy disks (often called floppies or diskettes) are portable, because you can remove them from a disk drive. Disk drives for floppy disks are called floppy drives. Floppy disks are slower to access than hard disks and have less storage capacity, but they are much less expensive. And most importantly, they are portable. Floppies come in two basic sizes: 5¼-inch: The common size for PCs made before 1987. This type of floppy is generally capable of storing between 100K and 1.2MB (megabytes) of data. The most common sizes are 360K and 1.2MB.3½-inch: Floppy is something of a misnomer for these disks, as they are encased in a rigid envelope. Despite their small size, microfloppies have a larger storage capacity than their cousins -- from 400K to 1.4MB of data. The most common sizes for PCs are 720K (double-density) and 1.44MB (high-density). Macintoshes support disks of 400K, 800K, and 1.2MB. FORMAT: To prepare a storage medium, usually a disk, for reading and writing. When you format a disk, the operating system erases all bookkeeping information on the disk, tests the disk to make sure all sectors are reliable, marks bad sectors (that is, those that are scratched), and creates internal address tables that it later uses to locate information. You must format a disk before you can use it. Note that reformatting a disk does not erase the data on the disk, only the address tables. Do not panic, therefore, if you accidentally reformat a disk that has useful data. A computer specialist should be able to recover most, if not all, of the information on the disk. You can also buy programs that enable you to recover a disk yourself. The previous discussion, however, applies only to high-level formats, the type of formats that most users execute. In addition, hard disks have a low-level format, which sets certain properties of the disk such as the interleave factor. The low-level format also determines what type of disk controller can access the disk (e.g., RLL or MFM). Almost all hard disks that you purchase have already had a low-level format. It is not necessary, therefore, to perform a low-level format yourself unless you want to change the interleave factor or make the disk accessible by a different type of disk controller. Performing a low-level format erases all data on the disk.To specify the properties, particularly visible properties, of an object. For example, word processing applications allow you to format text, which involves specifying the font, alignment, margins, and other properties.