Computer storage density

# Computer storage density

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Encyclopedia
Memory storage density is a measure of the quantity of information bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...

s that can be stored on a given length of track
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...

, area of surface
Surface
In mathematics, specifically in topology, a surface is a two-dimensional topological manifold. The most familiar examples are those that arise as the boundaries of solid objects in ordinary three-dimensional Euclidean space R3 — for example, the surface of a ball...

, or in a given volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....

of a computer storage medium
Computer storage
Computer data storage, often called storage or memory, refers to computer components and recording media that retain digital data. Data storage is one of the core functions and fundamental components of computers....

. Generally, higher density is more desirable, for it allows greater volumes of data to be stored in the same physical space. Density therefore has a direct relationship to storage capacity of a given medium. Density also generally has a fairly direct effect on the performance within a particular medium, as well as price.

## Examples

Hard drives store data in the magnetic polarization of small patches of the surface coating on a (normally) metal disk. The maximum areal density is defined by the size of the magnetic particles in the surface, as well as the size of the "head" used to read and write the data. The areal density of disk storage devices has increased dramatically since IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

introduced the RAMAC, the first hard disk
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...

in 1956. RAMAC had an areal density of 2,000 bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...

/in²
Square inch
A square inch is a unit of area, equal to the area of a square with sides of one inch. The following symbols are used to denote square inches:*square in*sq inches, sq inch, sq in*inches/-2, inch/-2, in/-2...

. Commercial hard drives in 2005 typically offer densities between 100 and 150 Gbit
Gigabit
The gigabit is a multiple of the unit bit for digital information or computer storage. The prefix giga is defined in the International System of Units as a multiplier of 109 , and therefore...

/in², an increase of about 75 million times over the RAMAC. In 2005 Toshiba
Toshiba
is a multinational electronics and electrical equipment corporation headquartered in Tokyo, Japan. It is a diversified manufacturer and marketer of electrical products, spanning information & communications equipment and systems, Internet-based solutions and services, electronic components and...

introduced a new hard drive using perpendicular recording
Perpendicular recording
Perpendicular recording is a technology for data recording on hard disks. It was first proven advantageous in 1976 by Shun-ichi Iwasaki, then professor of the Tohoku University in Japan, and first commercially implemented in 2005.-Advantages:Perpendicular recording can deliver more than three...

, which features a density of 179 Gbit/in². Toshiba's experimental systems have demonstrated 277 Gbit/in², and more recently Seagate Technology
Seagate Technology
Seagate Technology is one of the world's largest manufacturers of hard disk drives. Incorporated in 1978 as Shugart Technology, Seagate is currently incorporated in Dublin, Ireland and has its principal executive offices in Scotts Valley, California, United States.-1970s:On November 1, 1979...

has demonstrated a drive with a 421 Gbit/in² density. It is expected that perpendicular recording technology can scale to about 1 Tbit
Terabit
The terabit is a multiple of the unit bit for digital information or computer storage. The prefix tera is defined in the International System of Units as a multiplier of 1012 , and therefore...

/in² at its maximum.

Compact Disc
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...

s (CDs), another common storage media of the early 2000s, stores data in small pits in plastic surface that is then covered with a thin layer of reflective metal. The standard defines pits that are 0.83 micrometers long and 0.5 micrometers wide, arranged in tracks spaced 1.6 micrometers apart, offering a density of about 0.90 Gbit/in². DVD
DVD
A DVD is an optical disc storage media format, invented and developed by Philips, Sony, Toshiba, and Panasonic in 1995. DVDs offer higher storage capacity than Compact Discs while having the same dimensions....

disks are essentially a "product improved" CD, using more of the disk surface, smaller pits (0.64 micrometers), and tighter tracks (0.74 micrometers), offering a density of about 2.2 Gbit/in². Further improvements in HD DVD
HD DVD
HD DVD is a discontinued high-density optical disc format for storing data and high-definition video.Supported principally by Toshiba, HD DVD was envisioned to be the successor to the standard DVD format...

and Blu-ray offer densities around 7.5 Gbit/in² and 12.5 Gbit/in², respectively (for single-layer devices in both cases). When CDs were first introduced they had considerably higher densities (and overall capacity) than then-current hard drives, however hard drives have improved much more quickly than optical media, and by the time the latest blue-laser systems become widely available in 2007, the average hard drive will store somewhere between 500 and 750 GB
Gigabyte
The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is...

with densities between 150 and 250 Gbit/in².

A number of technologies are attempting to surpass the densities of all of these media. IBM's Millipede memory is attempting to commercialize a system at 1 Tbit/in² in 2007 (800 Gbit/in² was demonstrated in 2005). This is about the same capacity that perpendicular hard drives are expected to "top out" at, and Millipede technology has so-far been losing the density race with hard drives. Development since mid-2006 appears to be moribund, although the latest demonstrator with 2.7 Tbit/in² seemed promising. A newer IBM technology, racetrack memory
Racetrack memory
Racetrack memory is an experimental non-volatile memory device under development at IBM's Almaden Research Center by a team led by Stuart Parkin. In early 2008, a 3-bit version was successfully demonstrated...

, uses an array of many small nanoscopic wires arranged in 3D, each holding numerous bits to improve density. Although exact numbers have not been mentioned, IBM news articles talk of "100 times" increases. Various holographic storage technologies are also attempting to leapfrog existing systems, but they too have been losing the race, and are estimated to offer 1 Tbit/in² as well, with about 250 GB
Gigabyte
The gigabyte is a multiple of the unit byte for digital information storage. The prefix giga means 109 in the International System of Units , therefore 1 gigabyte is...

/in² being the best demonstrated to date- for non-quantum holography systems.
Finally, molecular polymer storage has been shown to store 10 Tbit/in².

By far the densest type of memory storage experimentally to date is electronic quantum holography
Electronic quantum holography
Electronic quantum holography is a new technology devised by scientists to encode and read out data at unprecedented density....

. By superimposing images of different wavelengths into the same hologram, a Stanford research team was able to achieve a bit density of 35bit/electron (approximately 3 Exabytes/in^2.) This was demonstrated using electron microscopes and a copper medium as reported in the Stanford Report on January 28, 2009.

## Effects on performance

Increasing storage density of a medium almost always improves the transfer speed at which that medium can operate. This is most obvious when considering various disk-based media, where the storage elements are spread over the surface of the disk and must be physically rotated under the "head" in order to be read or written. Higher density means more data moves under the head for any given mechanical movement.

Considering the floppy disk as a basic example, we can calculate the effective transfer speed by determining how fast the bits move under the head. A standard 3½"
Inch
An inch is the name of a unit of length in a number of different systems, including Imperial units, and United States customary units. There are 36 inches in a yard and 12 inches in a foot...

floppy disk spins at 300 rpm, and the innermost track about 66 mm long (10.5 mm radius). At 300 rpm the linear speed of the media under the head is thus about 66 mm x 300 rpm = 19800 mm/minute, or 330 mm/s. Along that track the bit
Bit
A bit is the basic unit of information in computing and telecommunications; it is the amount of information stored by a digital device or other physical system that exists in one of two possible distinct states...

s are stored at a density of 686 bit/mm, which means that the head sees 686 bit/mm x 330 mm/s = 226,380 bit/s (or 28.3 KiB
Kibibyte
The kibibyte is a multiple of the unit byte for quantities of digital information. The binary prefix kibi means 1024; therefore, 1 kibibyte is . The unit symbol for the kibibyte is KiB. The unit was established by the International Electrotechnical Commission in 1999 and has been accepted for use...

/s).

Now consider an improvement to the design that doubles the density of the bits by reducing sample length and keeping the same track spacing. This would immediately result in a doubling of transfer speed because the bits would be passing under the head twice as fast. Early floppy disk interfaces were originally designed with 250 kbit/s transfer speeds in mind, and were already being outperformed with the introduction of the "high density" 1.44 MB
Megabyte
The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: bytes generally for computer memory; and one million bytes generally for computer storage. The IEEE Standards Board has decided that "Mega will mean 1 000...

(1,440 KiB) floppies in the 1980s. The vast majority of PCs included interfaces designed for high density drives that ran at 500 kbit/s instead. These too were completely overwhelmed by newer devices like the LS-120, which were forced to use higher-speed interfaces such as IDE.

Although the effect on performance is most obvious on rotating media, similar effects come into play even for solid-state media like Flash RAM or DRAM
Dram
Dram or DRAM may refer to:As a unit of measure:* Dram , an imperial unit of mass and volume* Armenian dram, a monetary unit* Dirham, a unit of currency in several Arab nationsOther uses:...

. In this case the performance is generally defined by the time it takes for the electrical signals to travel though the computer bus
Computer bus
In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...

to the chips, and then through the chips to the individual "cells" used to store data (each cell holds one bit).

One defining electrical property is the resistance of the wires inside the chips. As the cell size decreases, through the improvements in semiconductor fabrication
Semiconductor fabrication
Semiconductor device fabrication is the process used to create the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photolithographic and chemical processing steps during which electronic circuits are gradually created on a wafer...

Moore's Law
Moore's law describes a long-term trend in the history of computing hardware: the number of transistors that can be placed inexpensively on an integrated circuit doubles approximately every two years....

, the resistance is reduced and less power is needed to operate the cells. This, in turn, means that less electrical current is needed for operation, and thus less time is needed to send the required amount of electrical charge into the system. In DRAM in particular the amount of charge that needs to be stored in a cell's capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

also directly affects this time.

As fabrication has improved, solid-state memory has improved dramatically in terms of performance. Modern DRAM chips had operational speeds on the order of 10 ns or less. A less obvious effect is that as density improves, the number of DIMM
DIMM
A DIMM or dual in-line memory module, comprises a series of dynamic random-access memory integrated circuits. These modules are mounted on a printed circuit board and designed for use in personal computers, workstations and servers...

s needed to supply any particular amount of memory decreases, which in turn means less DIMMs overall in any particular computer. This often leads to improved performance as well, as there is less bus traffic. However, this effect is generally not linear.

## Effects on price

Storage density also has a strong effect on the price of memory, although in this case the reasons are not so obvious.

In the case of disk-based media, the primary cost is the moving parts inside the drive. This sets a fixed lower limit, which is why most modern hard drives bottom out around \$100 US retail, and have for many years now. That said, the price of high-end drives has fallen rapidly, and this is indeed an effect of density. In this case the only way to make a higher capacity drive is to use more platters, essentially individual hard drives within the case. As the density increases the number of platters needed to supply any given amount of storage falls, leading to lower costs due to the reduction of mechanical parts inside. So while a low-end drive still costs about \$100 (although increasing rapidly in overall storage), the price for a large drive is falling rapidly as they become mechanically simpler. It is worth observing dollars per GB for Hard drives.

The fact that overall price has remained fairly steady has led to the common measure of the price/performance ratio
Price/performance ratio
In economics and engineering, the price/performance ratio refers to a product's ability to deliver performance, of any sort, for its price. Generally speaking, products with a higher price/performance ratio are more desirable, excluding other factors....

in terms of cost per bit. In these terms the increase in density of hard drives becomes much more obvious. IBM's RAMAC from 1956 supplied 5 MB for \$50,000, or \$10,000 per megabyte. In 1989 a typical 40 MB hard drive from Western Digital
Western Digital
Western Digital Corporation is one of the largest computer hard disk drive manufacturers in the world. It has a long history in the electronics industry as an integrated circuit maker and a storage products company. Western Digital was founded on April 23, 1970 by Alvin B...

retailed for \$1199.00, or \$36/MB. Drives broke the \$1/MB in 1994, and in early 2000 were about 2¢/MB. By 2004 the 250 GB Western Digital Caviar SE listed for \$249.99, approaching \$1/GB, an improvement of 36 thousand times since 1989, and 10 million since the RAMAC. As of 2011, 2TB drives are selling for less than \$70, or 3.5¢/GB, an improvement of 1 million times since 1989, and 28 million since the RAMAC. This is all without adjusting for inflation, which adds another factor of about seven times since 1956.

Solid-state storage has seen similar dramatic reductions in cost per bit. In this case the primary determinant of cost is yield, the number of working chips produced in a unit time. Chips are produced in batches printed on the surface of a single large silicon wafer, which is then cut up and non-working examples are discarded. To improve yield, modern fabrication has moved to ever-larger wafers, and made great improvements in the quality of the production environment. Other factors include packaging the resulting wafer, which puts a lower limit on this process of about \$1 per completed chip.

The relationship between information density and cost per bit can be illustrated as follows: a memory chip that is half the physical size means that twice as many units can be produced on the same wafer, thus halving the price of each one. As a comparison, DRAM was first introduced commercially in 1971, a 1 kbit part that cost about \$50 in large batches, or about 5 cents per bit. 64 Mbit parts were common in 1999, which cost about 0.00002 cents per bit (20 microcents/bit).