Hard disks

Hard disks store data on rigid aluminium or ceramic platters coated with a magnetic oxide material. Their primary advantages over floppy disks are speed and capacity. With some hard disks, you can access information 100 times faster than with a floppy disk. In a hard disk, the disk and the read/ write head are enclosed in a sealed container. By sealing the entire unit, the major source of contaminants — atmospheric pollution — is eliminated. Also, the read/write head can be made much smaller and can ride much closer to the surface of the disk. The closer the read/ write head can be to the surface of the disk, the more data it can read from or write to the disk. The high-speed spinning of a hard disk (5000 to 7200 rpm) literally creates a miniature gust of wind at the surface of the disk. The read/write heads use that wind to float on a cushion of air. Because hard disks are rigid, groups of disks can be stacked on the same spindle. Such an assembly requires multiple read/write heads for each surface; these are attached to access arms that move the heads to the desired track. As with floppy disks, the number of tracks and the density vary from disk to disk.

To a personal computer user, the speed and capacity advantages of hard disks over floppy disks offer access to program and data files without the nuisance of swapping floppy disks. In addition, some programs are extremely slow and limited without access to a hard disk whereas others require the use of a hard disk. The disadvantages are that most hard disks must be backed up regularly and are more sensitive to shock than floppy disks.

Disk cartridges are removable disks that are sealed in a container similar to a videotape cartridge. These disk cartridges offer the same amount of storage as many of the non-removable disks and can be inserted and removed in the same manner as floppy disks. They can be used as the primary storage medium or as a backup medium. A major drawback of any fixed disk is the need to make backup copies of files. Backing up is simplified in systems that have either two cartridge drives or one hard drive and one cartridge drive.

Disk arrays combine and synchronize multiple disk drives into one self-contained unit. The advantages include faster data access and higher data throughput than an equivalent-sized single disk drive because switching from a read/write head on one disk to one on another disk can be accomplished electronically. Disk arrays can also store redundant data on several disk drives, and automatically recover data should one or more drives in the array fail.

Each disk track is divided into regions called sectors. In effect, a sector is equivalent to a physical record on a disk. The operating system’s job is to keep a directory to show which physical records are in use by which files. If a record contains 122 bytes and you are going to store your file sequentially on a floppy disk that has 512-byte sectors, each 512-byte physical record can contain four complete 122-byte logical records with 24 leftover bytes. If, for example, your file is to contain 100 records, then 25 sectors will be required to store your file.

Knowing these figures allows you to calculate roughly the size of your files; that information, combined with an estimate of how many records are going to be in the file, allows you to estimate the disk space requirements for your application.

Storage of a direct file is different. If you described a direct file whose record length totalled 122 bytes, some disks would store one logical record per sector. This makes it easier and faster for the disk drive to know where to access records in a direct manner. Logical records whose length exceeds the sector size are often stored in adjacent sectors. For example, if the logical record size is 600 bytes and the sector size is 5 12 bytes, two adjacent sectors will be required per record.

3.4 Give English equivalents.

Безпосереднім чином, розмір сектору, надлишкові дані, таким же чином, щільність доріжок, повітряна подушка, головні переваги, тривалість доступу, головка читання/запису, перевага у швидкодії, поверхня диску, логічний запис, жорсткий диск, сектор, набір дисків, дисковод, записувати на диск, ємність/обсяг, високошвидкісне обертання, доріжок на дюйм, носій середовище.

3.5 Make up phrases or words by combining hard and disk with the words below. Give the meaning of each phrase or word in Ukrainian.

Copy sell drive drugs magnetic labour worker hard disk optical internal сompact capacity currency ware time directory  

3.6 Answer the questions.

1. What is the main function of a hard disk?

2. Which unit is used to measure hard disk capacity?

3. What are the advantages of hard disks over floppy disks?

4. In what sectors is a file stored in a fragmented disk?

5. In what sectors is a file stored in a defragmented disk?

6. What are disk cartridges used for?

4.1 Read and translate the following words and phrases.

Magnetic media, density limitations, high-density magnetic disks, incompatibility problems, compression technique, to achieve eras ability, write and erase times, an optical guidance system, chip technology, magnetic write head conventional magnetic disks, a direct access retrieval.

4.2 Learn the key-words and word-combinations:

CD ROM(compact disk read-only memory) - постійна пам’ять на компакт-диску

DVI(digital video interactive) - цифровий відеоформат

WORM(write-once, read mostly) - оптичний диск(разового запису і багаторазового читання)

magnetic medium – магнітний носій

density limitation – обмеження за щільністю

concentric (adj) – концентричний

access speed – швидкість доступу

complement (v) – доповнювати

compression technique – техніка (метод) ущільнення

real-time(adj) – реального часу

response time – час реакції/відповіді

seek time – тривалість установлення (головки)

reverse polarity – змінити полярність (на протилежну)

file server – файловий процесор

storage unit – запам’ятовувальний пристрій (блок)

indelible (v) – такий, що не стирається

large-scale integration – велика інтеграція

4.3 Read the text and note in the table the points for and against the three main types of optical disks. Then make notes about their use.

Дата добавления: 2015-01-03