VDict - Definition of disk drives

Computing (FOLDOC) dictionary (also found in English - English (Wordnet), English - Vietnamese)

disk drive

hardware, storage (Or "hard disk drive", "hard drive",

"floppy disk drive", "floppy drive") A peripheral device

that reads and writes hard disks or floppy disks. The

drive contains a motor to rotate the disk at a constant rate

and one or more read/write heads which are positioned over the

desired track by a servo mechanism. It also contains the

electronics to amplify the signals from the heads to normal

digital logic levels and vice versa.

In order for a disk drive to start to read or write a given

location a read/write head must be positioned radially over

the right track and rotationally over the start of the right

sector.

Radial motion is known as "seeking" and it is this which

causes most of the intermittent noise heard during disk

activity. There is usually one head for each disk surface and

all heads move together. The set of locations which are

accessible with the heads in a given radial position are known

as a "cylinder". The "seek time" is the time taken to

seek to a different cylinder.

The disk is constantly rotating (except for some floppy disk

drives where the motor is switched off between accesses to

reduce wear and power consumption) so positioning the heads

over the right sector is simply a matter of waiting until it

arrives under the head. With a single set of heads this

"rotational latency" will be on average half a revolution

but some big drives have multiple sets of heads spaced at

equal angles around the disk.

If seeking and rotation are independent, access time is seek

time + rotational latency. When accessing multiple tracks

sequentially, data is sometimes arranged so that by the time

the seek from one track to the next has finished, the disk has

rotated just enough to begin accessing the next track.