DISK STORAGE 2

HALF HEIGHT:
PCs support both full-height and half-height bays for disk drives and other mass storage devices. The half-height models take up less space but usually have less storage capacity.

HALF CARD:
A hard disk drive and controller on an expansion card. Unlike most disk drives that are either external to the computer or fit in one of the disk drive bays, a hard card slips into an expansion slot. Hard cards are often faster than conventional disk drives, and easier to install. Their storage capacities, however, are more limited.

HARD DISK:
A magnetic disk on which you can store computer data. The term hard is used to distinguish it from a soft, or floppy, disk. Hard disks hold more data and are faster than floppy disks. A hard disk, for example, can store anywhere from 10 megabytes to several gigabytes, whereas most floppies have a maximum storage capacity of 1.4 megabytes.
A single hard disk usually consists of several platters. Each platter requires two read/write heads, one for each side. All the read/write heads are attached to a single access arm so that they cannot move independently. Each platter has the same number of tracks, and a track location that cuts across all platters is called a cylinder. For example, a typical 84 megabyte hard disk for a PC might have two platters (four sides) and 1,053 cylinders.
In general, hard disks are less portable than floppies, although it is possible to buy removable hard disks. There are two types of removable hard disks: disk packs and removable cartridges.

HARD DISK DRIVE:
The mechanism that reads and writes data on a hard disk. Hard disk drives for PCs generally have access times of about 15 milliseconds or less. Many disk drives improve their performance through a technique called caching.
The disk drives for hard disks are called hard disk drives or Winchester drives, Winchester being the name of one of the first popular hard disk drive technologies developed by IBM in 1973.
There are several interface standards for passing data between a hard disk and a computer. The most common are:
ST-506: The standard interface on all PC/XT and PC/AT computers. The ST-506 standard supports both MFM and RLL encoding formats.
SCSI (Small Computer System Interface): The standard interface for Apple Macintoshes, SCSI is also popular on UNIX systems and is available on many PC compatibles.IDE (Integrated Drive Interface): Not as fast as SCSI, but faster than ST-506.
Enhanced IDE: A new version of the IDE interface that supports data transfer rates comparable to SCSI.

HARD DISK TYPE:
A number that indicates important features of a hard disk, such as the number of platters and cylinders. Hard disk manufacturers have agreed on a numbering scheme so that it is possible to install and configure a new hard disk simply by entering the disk’s type number into the BIOS setup.

HEAD:
The mechanism that reads data from or writes data to a magnetic disk or tape. If the head becomes dirty, it will not work properly. This is one of the first things to check if your disk drive or tape drive begins to malfunction.
The head is sometimes called a read/write head. Double-sided floppy disk drives have two heads, one for each side of the disk. Hard disk drives have many heads, two for each platter.

HEAD CRASH:
A serious disk drive malfunction. A head crash usually means that the head has scratched or burned the disk. In a hard disk drive, the head normally hovers a few microinches from the disk. If the head becomes misaligned or if dust particles come between it and the disk, it can touch the disk. When this happens, you usually lose much of the data on the hard disk and will need to replace both the head and the disk. For this reason, it is important to operate disk drives, particularly hard disk drives, in as clean an environment as possible. Even smoke particles can cause a head crash.
Head crashes are less common for floppy disks because the head touches the disk anyway under normal operation.
Another term for head crash is disk crash.

HELICAL SCAN CARTRIDGE:
A type of magnetic tape that uses the same technology as VCR tapes. The term helical scan usually refers to 8 mm tapes, although 4 mm tapes (called DAT tapes) use the same technology. The 8 mm helical-scan tapes have data capacities from 2.5GB to 5GB.

HIGH DENSITY DISK:
A high-quality floppy disk capable of holding more data than a double-density disk. High-density 5¼-inch disks for PCs can hold 1.2MB (megabytes) of data. High-density 3½-inch disks can store 1.44MB.

IDE INTERFACE:
Abbreviation of either Intelligent Drive Electronics or Integrated Drive Electronics, depending on who you ask. An IDE interface is an interface for mass storage devices, in which the controller is integrated into the disk or CD-ROM drive. It is a low-cost alternative to SCSI interfaces.
The original IDE interface supports data transfer rates of about 3.3 megabytes per second and has a limit of 538 megabytes per device. However, a recent version of IDE, called enhanced IDE (EIDE) or Fast IDE, supports data transfer rates of about 12 megabytes per second and storage devices of up to 8.4 gigabytes. These numbers are comparable to what SCSI offers. To take advantage of EIDE, you need both an EIDE mass storage device and an EIDE interface installed in your computer.

INTERLEAVING:
Refers to the way sectors on a disk are organized. In one-to-one interleaving, the sectors are placed sequentially around each track. In two-to-one interleaving, sectors are staggered so that consecutively numbered sectors are separated by an intervening sector.
The purpose of interleaving is to make the disk drive more efficient. The disk drive can access only one sector at a time, and the disk is constantly spinning beneath the read/write head. This means that by the time the drive is ready to access the next sector, the disk may have already spun beyond it. If a data file spans more than one sector and if the sectors are arranged sequentially, the drive will need to wait a full rotation to access the next chunk of the file. If instead the sectors are staggered, the disk will be perfectly positioned to access sequential sectors.
The optimum interleaving factor depends on the speed of the disk drive, the operating system, and the application. The only way to find the best interleaving factor is to experiment with various factors and various applications.

JAZ DRIVE:
A removable disk drive developed by Iomega Corporation. The Jaz drive has a 12-ms average seek time and a transfer rate of 5.5Mbps. The removable cartridges hold 1GB of data. The fast data rates and large storage capacity make it a viable alternative for backup storage as well as everyday use.

LBA:
A removable disk drive developed by Iomega Corporation. The Jaz drive has a 12-ms average seek time and a transfer rate of 5.5Mbps. The removable cartridges hold 1GB of data. The fast data rates and large storage capacity make it a viable alternative for backup storage as well as everyday use.

LOSSLESS COMPRESION:
Refers to data compression techniques in which no data is lost. The PKZIP compression technology is an example of lossless compression. For most types of data, lossless compression techniques can reduce the space needed by only about 50%. For greater compression, one must use a lossy compression technique.

LOSSY COMPRESSION:
Refers to data compression techniques in which some amount of data is lost. Lossy compression technologies attempt to eliminate redundant or unnecessary information. Most video compression technologies, such as MPEG, use a lossy technique.

LOW-LEVEL FORMAT:
Hard disks must be formatted twice before they can be used. The first format, called a low-level or physical format, sets the interleave factor and prepares the disk for a particular type of disk controller. This is generally performed at the factory.

MO DRIVE:
A type of disk drive that combines magnetic disk technologies with CD-ROM technologies. Like magnetic disks, MO disks can be read and written to. And like floppy disks, they are removable. However, their storage capacity can be more than 200 megabytes, much greater than magnetic floppies. In terms of data access speed, they are faster than floppies and CD-ROMs, but not as fast as hard disk drives.

MASS STORAGE:
Refers to various techniques and devices for storing large amounts of data. The earliest storage devices were punched paper cards, which were used as early as 1804 to control silk-weaving looms. Modern mass storage devices include all types of disk drives and tape drives. Mass storage is distinct from memory, which refers to temporary storage areas within the computer. Unlike main memory, mass storage devices retain data even when the computer is turned off.
The main types of mass storage are:
floppy disks: Relatively slow and have a small capacity, but they are portable, inexpensive, and reliable.
hard disks: Very fast and with more capacity than floppy disks, but also more expensive. Some hard disk systems are portable (removable cartridges), but most are not.optical disks: Unlike floppy and hard disks, which use electromagnetism to encode data, optical disk systems use a laser to read and write data. Optical disks have very large storage capacity, but they are not as fast as hard disks. In addition, the inexpensive optical disk drives are read-only. Read/write varieties are very expensive.
tapes: Relatively inexpensive and can have very large storage capacities, but they do not permit random access of data.
Mass storage is measured in kilobytes (1,024 bytes), megabytes (1,024 kilobytes), and gigabytes (1,024 megabytes).

MEDIA:
Objects on which data can be stored. These include hard disks, floppy disks, CD-ROMs, and tapes.
In computer networks, media refers to the cables linking workstations together. There are many different types of transmission media, the most popular being twisted-pair wire (normal electrical wire), coaxial cable (the type of cable used for cable television), and fiber optic cable (cables made out of glass).
The form and technology used to communicate information. Multimedia presentations, for example, combine sound, pictures, and videos, all of which are different types of media.

MFM:
Abbreviation of modified frequency modulation, an encoding scheme used by some PC disk drives. A competing scheme, known as RLL (run length limited), produces faster data access speeds and can increase a disk's storage capacity by up to 50 percent.
Technically, any disk drive can use MFM or RLL. The one it uses depends on the disk controller.

MICROFLOPPY DISK:
A small floppy disk. Microfloppy disks come enclosed in hard, 3½ square-inch casings. Although smaller than older-style floppies, microfloppies have greater storage capacity.
PCs support two types of microfloppies:
· double-density microfloppies hold 720K.
· high-density microfloppies can store 1.44MB (megabytes).
For Macintosh computers, which have always used microfloppies, there are three sizes:
· Single-sided standard microfloppies hold 400K.
· Double-sided standard microfloppies hold 800 K.
· Double-sided, high-density microfloppies hold 1.44 megabytes.

MOUNT:
To make a mass storage device available. In Macintosh environments, for example, inserting a floppy disk into the drive is called mounting the floppy.
To install a device, such as a disk drive or expansion board.

MTBF:
Short for mean time between failures. MTBF ratings are measured in hours and indicate the sturdiness of hard disk drives and printers.
Typical disk drives for personal computers have MTBF ratings of about 40,000 hours. This means that at least half of the disk drives with such a rating will fail once in the first 40,000 hours of operation. Most working conditions are not ideal, so MTBF ratings can be considered as only approximate guidelines for judging the hardiness of disk drives. The fact that MTBF ratings exist at all, however, underscores the fact that every disk drive will eventually fail if run long enough.

OPTICAL DISK:
A storage medium from which data is read and to which it is written by lasers. Optical disks can store much more data -- up to 6 gigabytes (6 billion bytes) -- than magnetic media, such as floppies and hard disks. There are three basic types of optical disks:
CD-ROM: Like audio CDs, CD-ROMs come with data already encoded onto them. The data is permanent and can be read any number of times, but CD-ROMs cannot be modified.
WORM: Stands for write-once, read-many. With a WORM disk drive, you can write data onto a WORM disk, but only once. After that, the WORM disk behaves just like a CD-ROM.erasable: Optical disks that can be erased and loaded with new data, just like magnetic disks. These are often referred to as EO (erasable optical) disks.
These three technologies are not compatible with one another; each requires a different type of disk drive and disk. Even within one category, there are many competing formats, although CD-ROMs are relatively standardized.

OPTIMIZE:
In programming, to fine-tune a program so that it runs more quickly or takes up less space.
When applied to disks, the term means the same as defragment. See under fragmentation.

PARK:
To lock the read/write head of a hard disk drive in a safe position so that the disk will not be damaged while moving the drive. Parking the disk is particularly important for portable computers, which are moved frequently. The disk will automatically unpark itself once you turn the power on.
Most modern disk drives support automatic head parking, in which the drive automatically parks the head whenever the power is turned off.

PARTITION:
(v) To divide memory or mass storage into isolated sections. In DOS systems, you can partition a disk, and each partition will behave like a separate disk drive. Partitioning is particularly useful if you run more than one operating system. For example, you might reserve one partition for DOS and another for UNIX.In addition, partitioning on DOS and Windows machines can improve disk efficiency. This is because the FAT system used by these operating systems automatically assigns cluster size based on the disk size: the larger the disk, the larger the cluster. Unfortunately, large clusters can result in a wasted disk space. There is an entire sector of the software industry devoted to building utilities that let you partition your hard disk.
On Apple Macintosh computers, there are two types of partitioning: hard and soft. Hard partitioning is the same as DOS partitioning -- the disk is physically divided into different sections. Soft partitioning, on the other hand, does not physically affect the disk at all, but it fools the Finder into believing that the disk is partitioned. The advantage of this is that you can partition the disk without affecting the data on it. With hard partitioning, it is usually necessary to reformat the entire disk.
(n) A section of main memory or mass storage that has been reserved for a particular application.

PHASE CHANGE DISK:
A type of rewritable optical disk that employs the phase change recording method. Using this technique, the disk drive writes data with a laser that changes spots on the disk between amorphous and crystalline states. An optical head reads data by detecting the difference in reflected light from amorphous and crystalline spots. A medium-intensity pulse can then restore the original crystalline structure.Magneto-optical and dye-polymer technologies offer similar capabilities for developing re-writable optical disks.

PLATTER:
A round magnetic plate that constitutes part of a hard disk. Hard disks typically contain two, four, or eight platters. Each platter requires two read/write heads, one for each side.

QIC:
The abbreviation for quarter-inch cartridge. QIC tapes are among the most popular tapes used for backing up personal computers. QIC tapes are divided into two general classes: full-size and minicartridge. The full-size cartridges are often referred to as DC 6000 cartridges, and minicartridges are called DC 2000 cartridges.
The QIC-40 and QIC-80 standards are sometimes referred to as floppy tape standards because they are designed to use a personal computer's existing floppy disk drive controller instead of requiring a customized controller.
The various QIC standards are controlled by a consortium of manufacturers called the Quarter-Inch Cartridge Drive Standards, Inc. The term QIC, therefore, is used to refer both to the type of tape and to the standards -producing organization.

QUAD-SPEED CE-ROM DRIVE:
A CD-ROM drive designed to run four times as fast as original models. Often denoted as 4X CD players, they provide data transfer rates of 600K per second and data access times as low as 125 milliseconds (ms).

RAID:
Short for Redundant Array of Independent Disks, a category of disk drives that employ two or more drives in combination for fault tolerance and performance. RAID disk drives are used frequently on servers but aren’t generally necessary for personal computers.There are number of different RAID levels. The three most common are 0, 3, and 5:
Level 0: Provides data striping (spreading out blocks of each file across multiple disks) but no redundancy. This improves performance but does not deliver fault tolerance.
Level 3: Same as Level 3, but also reserves one dedicated disk for error correction data. It provides good performance and some level of fault tolerance.
Level 5: Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance.

RAM DISK:
Refers to RAM that has been configured to simulate a disk drive. You can access files on a RAM disk as you would access files on a real disk. RAM disks, however, are approximately a thousand times faster than hard disk drives. They are particularly useful, therefore, for applications that require frequent disk.
Because they are made of normal RAM, RAM disks lose their contents once the computer is turned off. To use a RAM disk, therefore, you need to copy files from a real hard disk at the beginning of the session and then copy the files back to the hard disk before you turn the computer off. Note that if there is a power failure, you will lose whatever data is on the RAM disk. (Some RAM disks come with a battery backup to make them more stable.)

READ-ONLY:
Capable of being displayed, but not modified or deleted. All operating systems allow you to protect objects (disks, files, directories) with a read-only attribute that prevents other users from modifying the object.

REMOVABLE HARD DISK:
A type of disk drive system in which hard disks are enclosed in plastic or metal cartridges so that they can be removed like floppy disks. Removable disk drives combine the best aspects of hard and floppy disks. They are as fast as hard disks and have the portability of floppy disks. The storage capacity of a typical removable hard disk can range from 60MB (megabytes) to 250MB, much more than the capacity of floppy disks.

RLL:
Abbreviation of run length limited, an encoding scheme used to store data on some PC hard disks. Although RLL produces fast data access times and increases a disk's storage capacity, it is not as prevalent as another encoding scheme called MFM (modified frequency modulation).
Technically, any disk drive can use either encoding method. The one used depends on the disk controller.

SCSI:
Abbreviation of small computer system interface. Pronounced scuzzy, SCSI is a parallel interface standard used by Apple Macintosh computers, some PCs, and many UNIX systems for attaching peripheral devices to computers. All Apple Macintosh computers starting with the Macintosh Plus come with a SCSI port for attaching devices such as disk drives and printers.
SCSI interfaces provide for faster data transmission rates (up to 40 megabytes per second) than standard serial and parallel ports. In addition, you can attach many devices to a single SCSI port, so that SCSI is really an I/O bus rather than simply an interface.Although SCSI is an ANSI standard, there are many variations of it, so two SCSI interfaces may be incompatible. For example, SCSI supports several types of connectors.
While SCSI is the only standard interface for Macintoshes, PCs support a variety of interfaces in addition to SCSI. These include IDE, enhanced IDE and ESDI for mass storage devices, and Centronics for printers. You can, however, attach SCSI devices to a PC by inserting a SCSI board in one of the expansion slots. Many high-end new PCs come with SCSI built in. Note, however, that the lack of a single SCSI standard means that some devices may not work with some SCSI boards.
The following varieties of SCSI are currently implemented:
SCSI: Uses an 8-bit bus, and supports data rates of 4 MBps
Fast SCSI: Uses an 8-bit bus, and supports data rates of 10 MBps.
Ultra SCSI: Uses an 8-bit bus, and supports data rates of 20 MBps.
Fast Wide SCSI: Uses a 16-bit bus and supports data rates of 20 MBps.
Ultra Wide SCSI: Uses a 16-bit bus and supports data rates of 40 MBps. Also called SCSI-3.

SECTOR:
The smallest unit that can be accessed on a disk. When you format a disk, the operating system divides it into tracks and sectors. The tracks are concentric circles around the disk and the sectors are segments within each circle. For example, a formatted disk might have 40 tracks, with each track divided into 10 sectors. The operating system and disk drive keep tabs on where information is stored on the disk by noting its track and sector number.
Generally, every track has the same number of sectors, but some high-capacity hard disk drives use a technique called zoned-bit recording in which tracks on the outside of the disk contain more sectors than those on the inside. A sector that cannot be used due to a physical flaw on the disk is called a bad sector.

SEEK TIME:
Refers to the time a program or device takes to locate a particular piece of data. For disk drives, the terms seek time and access time are often used interchangeably.

SEQUENTIAL ACCESS:
Refers to reading or writing data records in sequential order, that is, one record after the other. To read record 10, for example, you would first need to read records 1 through 9. This differs from random access, in which you can read and write records in any order.Some programming languages and operating systems distinguish between sequential-access data files and random-access data files, allowing you to choose between the two types. Sequential-access files are faster if you always access records in the same order. Random-access files are faster if you need to read or write records in a random order.
Devices can also be classified as sequential access or random access. For example, a tape drive is a sequential-access device because to get to point q on the tape, the drive needs to pass through points a through p. A disk drive, on the other hand, is a random-access device because the drive can access any point on the disk without passing through all intervening points.

SINGLE-DENSITY DISK:
A low-density floppy disk. All modern floppies are double-density or high-density.

ST-506 INTERFACE:
A standard interface for connecting hard disk drives to PCs. Newer standards, such as enhanced IDE and SCSI, support faster data transfer rates. ST-506 is sometimes referred to as MFM, which is the most prevalent encoding scheme used on ST-506 disk drives. ST-506 also supports the RLL encoding format.

SUPER DRIVE:
The common name for the FDHD (floppy disk, high density) disk drive that comes with all models of the Apple Macintosh computer. The SuperDrive can read and write to all three Macintosh disk sizes (400K, 800K, and 1.2MB) as well as the two PC 3½-inch disk sizes (720K and 1.44MB).

TAPE:
A magnetically coated strip of plastic on which data can be encoded. Tapes for computers are similar to tapes used to store music. Some personal computers, in fact, enable you to use normal cassette tapes.Storing data on tapes is considerably cheaper than storing data on disks. Tapes also have large storage capacities, ranging from a few hundred kilobytes to several gigabytes. Accessing data on tapes, however, is much slower than accessing data on disks. Tapes are sequential-access media, which means that to get to a particular point on the tape, the tape must go through all the preceding points. In contrast, disks are random-access media because a disk drive can access any point at random without passing through intervening points.
Because tapes are so slow, they are generally used only for long-term storage and backup. Data to be used regularly is almost always kept on a disk. Tapes are also used for transporting large amounts of data.Tapes come in a variety of sizes and formats.Tapes are sometimes called streamers or streaming tapes.

TAPE DRIVE:
A device, like a tape recorder, that reads data from and writes it onto a tape. Tape drives have data capacities of anywhere from a few hundred kilobytes to several gigabytes. Their transfer speeds also vary considerably. Fast tape drives can transfer as much as 20MB (megabytes) per minute.The disadvantage of tape drives is that they are sequential-access devices, which means that to read any particular block of data, you need to read all the preceding blocks. This makes them much too slow for general-purpose storage operations. However, they are the least expensive media for making backups.

TPI:
Short for tracks per inch, the density of tracks on a disk. For example, double-density 5.25-inch floppies have a TPI of 48, while high-density floppies record 96 TPI. High-density 3.5-inch diskettes are formatted with 135 TPI.

TRACK:
A ring on a disk where data can be written. A typical floppy disk has 80 (double-density) or 160 (high-density) tracks. For hard disks, each platter is divided into tracks, and a single track location that cuts through all platters is called a cylinder. Hard disks can have anywhere from 400 to more than 1000 cylinders. Each track is further divided into a number of sectors. The operating system and disk drive remember where information is stored by noting its track and sector numbers.
The density of tracks (how close together they are) is measured in terms of tracks per inch (TPI).

ULTRA ATA:
The newest version of the AT Attachment (ATA) standard, which supports burst mode data transfer rates of 33.3 MBps. To take advantage of these high speeds, your computer must also be equipped with Ultra DMA, a protocol that supports faster data transfer rates to and from hard disk drives.

ULTRA DMA:
A protocol developed by Quantum Corporation and Intel that supports burst mode data transfer rates of 33.3 MBps. This is twice as fast as the previous disk drive standard for PCs, and is necessary to take advantage of new, faster Ultra ATA disk drives.The official name for the protocol is Ultra DMA/33.

VOLUME:
A fixed amount of storage on a disk or tape. The term volume is often used as a synonym for the storage medium itself, but it is possible for a single disk to contain more than one volume or for a volume to span more than one disk.

WINCHESTER DISK DRIVE:
Another term for hard disk drive. The term Winchester comes from an early type of disk drive developed by IBM that stored 30MB (megabytes) and had a 30-millisecond access time; so its inventors called it a Winchester in honor of the.30-caliber rifle of the same name. Although modern disk drives are faster and hold more data, the basic technology is the same, so Winchester has become synonymous with hard.

WORM:
Refers to an optical disk technology that allows you to write data onto a disk just once. After that, the data is permanent and can be read any number of times. A single WORM disk can store from 600MB (megabytes) (megabytes) to over 3GB (gigabytes) of data. Unlike CD-ROMs, there is no single standard for WORM disks, which means that they can only be read by the same type of drive that wrote them. This has hampered their acceptance, although they have found a niche market as an archival media.

ZIP DRIVE:
A high-capacity floppy disk drive developed by Iomega Corporation. Zip disks are slightly larger than conventional floppy disks, and about twice as thick. They can hold 100MB of data. Because they're relatively inexpensive and durable, they have become a popular media for backing up hard disks and for transporting large files.