Disk Storage | Research & Encyclopedia Articles

Disk Storage

Disk storage refers to a broad range of devices in which information is placed on disks in digital form (more specifically, as bits that consist of the binary digits "1" and "0"). For many computer systems, especially for smaller computers such as personal computers (PCs), disk storage is the only method for retaining information (or data) after the computer is shut down. There are three types of disk storage technology in common use today: magnetic disks, optical disks, and magneto-optical disks. Although the three types of disks utilize differing technologies to store information, they all share some fundamental attributes. First, each type of disk must be within an appropriate disk drive before information can be read from, or written to, the disk (in this context, "reading" means to retrieve information and "writing" means to store information). Some disk drives are designed to house disks permanently (i.e., the disk(s) within them are non-removable); other disk drives are designed so that their disks can be removed. Another shared characteristic is non-volatile data storage, meaning that any data stored on a disk is retained even if its disk drive unit is shut off. This is an important feature of data storage using disks, because there are other types of computer memory (such as the computer's primary memory, or RAM) that are volatile, meaning that any stored data is lost when the computer is turned off. A third common attribute of all types of disks is that disks must be spun, or rotated, in order for information to be retrieved and stored. Within the disk drive, the disk is turned on a spindle (i.e., an axle) by an electric motor. A read/write head (a device that can read and/or write data) passes over the disk as it spins. By moving the head from the center to the rim of the disk, any part of the disk can be accessed. This means that disk storage provides direct access (also called random access) to data; if the computer requests particular data from disk storage, the disk drive can quickly position a head directly over the needed data. This process is in contrast to the serial access of data. An example of serial data storage is magnetic tape: the read head is stationary, and data passes it in a sequential (or serial) fashion as the tape winds and unwinds from one reel to the other. Because of direct access, disk storage provides much quicker access to data than devices like tape.

Magnetic disks consist of a plastic or metal platter coated with tiny particles that can be magnetized. All magnetic disk drives are capable of changing the information stored on their disk(s). To write (i.e., store) data to the disk, the magnetic read-write head produces a magnetic field that orients the magnetic particles on the disk's surface. To read data from the disk, the same read-write head registers the magnetic field produced by the particles. Some disk drives are designed to allow removal of the disks that they use. There are several types of removable magnetic disks. A couple of the most common are floppy disks, which are inserted into a floppy disk drive, and called "floppies" because their disks are flexible; and removable cartridges, which have a hard outer shell encasing a rigid magnetic disk (or disks). Non-removable magnetic disks are (like cartridge disks) rigid, and are used in hard drives or fixed disk drives. Hard drives usually have multiple disks and multiple read-write heads to maximize capacity and access speed (the time required for the disk drive to locate and retrieve information from the disk). Hard disk drives and cartridge drives can access their data much more quickly than can floppy disk drives.

Optical discs ("disc" is often used instead of "disk" when referring to optical media) utilize a laser to read the data stored on them. One type of optical disc is CD-ROM (Compact Disk-Read Only Memory); CD-ROM discs are basically the same as the CDs (Compact Discs) used to record music. But unlike audio CDs, CD-ROMs are intended for use with computer systems. CD-ROMs are made of rigid plastic coated with a thin layer of aluminum. Data is recorded on the aluminum surface of CD-ROMs in the form of microscopic pits. This "pitting" process is irreversible, and any data on a CD-ROM cannot be modified (i.e., a CD-ROM drive cannot write data to a CD-ROM disc, hence the term "read only memory"). Another type of optical disc is DVD (Digital Video (or Versatile) Disc). DVD comes in two versions, one of which is computer-compatible, called DVD-ROM. A DVD looks just like a CD, but can hold many times more information. CD-ROMs and DVD-ROMs, which are removable discs, can generally store much more data than can removable magnetic disks (such as floppies or cartridges). Their major drawback is that a user cannot record new data on CD-ROMs or DVD-ROMs. This limitation is overcome by CD-R (Compact Disc-Recordable), which employs a special type of CD disc upon which data can be recorded once by a CD-R drive. The data stored on a CD-R disc can be read by standard CD-ROM drives (the same sort of situation applies for special DVD-R discs, which can be stored to once and then read by standard DVD-ROM drives). Even more versatile optical technologies are CD-RW (Compact Disc-Rewriteable) and DVD-RW, which allow for multiple re-writes of data onto special CD and DVD discs, respectively.

Magneto-optical discs employ a combination of optical and magnetic technology. The read-write head on magneto-optical drives possess both optical and magnetic components. The drive writes data to a disc by using a laser to heat tiny crystals in the disc, which can then be oriented by a magnetic field to encode information. Data is read from a disc by shining a low-intensity laser on the disc surface and detecting the polarization of the reflected light (which varies with the alignment of the crystals). Magneto-optical disks are removable and, like magnetic disks, new data can be recorded over existing data many times.

When information is recorded on any type of disk, it is placed there under a particular encoding scheme, or format. By using different data formats the same information can be recorded in different ways, even using the same type of disk. An important example of how formatting effects data storage is that of data compression (or data packing). Data compression is a way of reducing the number of bits that would otherwise be needed to represent a given amount of information (i.e., compressed data requires less disk space than it does uncompressed). Data is often automatically compressed and uncompressed as it is written and read from disk storage, thereby increasing (sometimes several-fold) the effective storage space of the disk.