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disk drive

A disk drive is a randomly addressable and rewritable storage device. The term can be broadly interpreted to include optical drives and in earlier times, floppy drives. However, in popular usage, it has come to relate mainly to hard disk drives (HDDs).

Disk drives can either be housed internally within a computer or housed in a separate box that is external to the computer. They are found in PCs, servers, laptops and storage arrays, for example. They work by rotating very rapidly around a head or heads, which read and write data. They differ from solid state drives (SSDs), which have no moving parts and offer greater performance, but also cost more and generally offer less capacity.

Disk Drive Terms

Platter: Hard drives are normally composed of multiple disks called platters. These platters are stacked on top of each other. It is the platter that actually stores the data. It consists of a substrate coated with magnetic media. The substrate is there to act as a rigid support for the magnetic media. The magnetic layer is protected by a thin layer of carbon and a lubrication layer to prevent damage in case the head comes in contact with the platter surface. Typically, both sides of the platter have magnetic media on which to store data. Tens of thousands of tracks per inch can be laid down on these platters.

Spindle/Motor: The platters are attached at the center to a rod or pin called a spindle that is directly attached to the shaft of the motor that controls the speed of rotation.

Head-Actuator Assembly: This assembly consists of an actuator, the arms, the sliders and the read/write heads. The actuator is the device that moves the arms containing read/write heads across the platter surface in order to store and retrieve information. The head arms move between the platters to access and store data. At the end of each arm is a head slider, which consists of a block of material that holds the head and acts as an airfoil to keep it positioned at the precise height above the surface. The read/write heads convert the electronic 0s and 1s in the magnetic fields on the disks.

Logic Board: Logic boards consisting of chips, memory and other components control the disk speed and direct the actuator in all its movements. It also performs the process of transferring data from the computer to the magnetic fields on the disk.

Disk Drive Vendors

A couple of decades back, there were many different manufacturers of disk drives. These days, the field has whittled down mainly to Western Digital, Seagate, Maxtor, Fujitsu and Hitachi.

Disk Drive Technology

The two principal technologies at work in disk drives are Advanced Technology Attachment (ATA), also known as IDE or Integrated Drive Electronics, and Small Computer System Interface (SCSI). ATA drives are generally less expensive and more common, while the more expensive SCSI drives are used for higher-performance applications.

The two main types of HDDs used in storage today are SATA and SAS. SATA is short for Serial ATA. Basically, the architecture used in ATA disks hit the limit of its data transfer capability and was severely bottlenecking system performance. A different serial architecture used in SATA removed that restriction so that disk drive bandwidth could be steadily improved. These days SATA disk drives are widely deployed to store large quantities of data, or bulk storage.

SAS stands for Serial Attached SCSI. Like SATA, SAS provides a leap in performance over SCSI. While SAS disks are able to reach speeds of up to 15,000 rpm (about twice the speed of many SATA disks), they are more expensive. Hence SATA tends to be used for the largest volume of data, while SAS is often reserved for frequently accessed data.

The disks themselves have a number of features:

Tracks: A track is a concentric ring on the disk where data is stored.

Cylinders: On drives that contain multiple platters, all the tracks on all the platters that are at the same distance from the center are referred to as a cylinder. The data from all the tracks in the cylinder can be read by simply switching between the different heads, which is much faster than physically moving the head between the different tracks on a single disk.

Sectors: Tracks are further broken down into sectors, which are the smallest units of storage on a disk, typically 512 bytes. A larger number of sectors are recorded on the outer tracks, and progressively fewer toward the center. Data can also be read faster from the outer tracks and sectors than the inner ones.

Clusters: Sectors are grouped together into clusters. As far as the operating system is concerned, all the sectors in one cluster are a single unit.

Extents: A set of contiguous clusters storing a single file or part of a file is called an extent. It is best to keep the number of extents for any file to a minimum, as each extent requires a separate input/output (I/O) operation. Reducing the number of extents in a file is achieved using a process known as defragmentation. This greatly improves performance.

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