Areal density (computer storage)

Areal density is a measure of the quantity of information bits that can be stored on a given length of track, area of surface, or in a given volume of a computer storage medium. 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.

Storage device classes

Magnetic disk media

Hard disk drives store data in the magnetic polarization of small patches of the surface coating on a 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 introduced the IBM 350 disk storage, the first hard disk drive in 1956 at an areal density of 2,000 bit/in². In 2015 Seagate introduced a hard drive at a density of 1.34 Tbit/in², about 600 million times that of the first disk drive. It is thought that current recording technology can continue to scale beyond 1 Tbit/in².^[1] New technologies are under development and are expected to continue magnetic areal density progress.

Optical disc media

Compact discs (CDs) store data in small pits in a 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 disks are essentially a higher-density 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. Single-layer HD DVD and Blu-ray disks offer densities around 7.5 Gbit/in² and 12.5 Gbit/in², respectively.

When introduced in 1982 CDs had considerably higher densities than hard disk drives, but hard disk drives have since advanced much more quickly and eclipsed optical media in both areal density and capacity per device.

Magnetic tape media

The first magnetic tape drive, the Univac Uniservo, recorded at the density of 128 bit/in on a half-inch magnetic tape, resulting in the areal density of 256 bit/in².^[2] In 2015, IBM and Fujifilm claimed a new record for the magnetic tape areal density of 1.23 Gbit/in²,^[3] while LTO-6, the highest-density production tape shipping in 2015, provides an areal density of 0.84 Gbit/in².^[4]

Research

A number of technologies are attempting to surpass the densities of existing media.

IBM's Millipede memory was 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, 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/in² being the best demonstrated to date- for non-quantum holography systems.

Other experimental technologies offer even higher densities. Molecular polymer storage has been shown to store 10 Tbit/in².^[5] By far the densest type of memory storage experimentally to date is electronic quantum holography. By superimposing images of different wavelengths into the same hologram, a Stanford research team was able to achieve a bit density of 35 bit/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.^[6]

In 2012, DNA was successfully used as an experimental data storage medium, but required a DNA synthesizer and DNA microchips for the transcoding. As of 2012, DNA holds the record for highest-density storage medium.^[7]

Effects on performance

With the notable exception of NAND Flash memory, increasing storage density of a medium is generally associated with improved 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" 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 bits 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/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 (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. In this case the performance is generally defined by the time it takes for the electrical signals to travel though the computer bus 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 that lead to Moore's Law, the resistance is reduced and less power is needed to operate the cells. This, in turn, means that less electric 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 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 DIMMs 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 the average selling price for both of the major HDD manufacturers has been $45–75 US since 2007.^[8] 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. 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 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 retailed for $1199.00, or $30/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.^[9] As of 2011, 2TB drives are selling for less than $70, or 3.5¢/GB, an improvement of 1 million times since 1989, and 280 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.^[10]

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).^[10]

See also

• Bit cell – the length, area or volume required to store a single bit
• Patterned media
• Shingled magnetic recording (SMR)

References