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Density is a measure of the quantity of information bits that can be stored on a given pysical space of a computer storage medium. There are three types of density: length (linear density) of track, area of the surface (areal density), or in a given volume (volumetric density).
Generally, higher density is more desirable, for it allows more 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 affects the performance within a particular medium, as well as price.
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. In 1956 the first hard drive, the IBM350, had an areal density of 2,000 bit/in2. Since then, the increase in density has matched Moore's Law, reaching 1Tbit/in2 in 2014.[2] In 2015, Seagate introduced a hard drive with a density of 1.34Tbit/in2,[3] more than 600 million times that of the IBM 350. It is expected that current recording technology can "feasibly" scale to at least 5Tbit/in2 in the near future.[3][4] New technologies like heat-assisted magnetic recording (HAMR) and microwave-assisted magnetic recording (MAMR) are under development and are expected to allow increases in magnetic areal density to continue.[5]
Optical disc media
Optical discs store data in small pits in a plastic surface that is then covered with a thin layer of reflective metal. Compact discs (CDs) offer a density of about 0.90Gbit/in2, using pits which are 0.83 micrometers long and 0.5 micrometers wide, arranged in tracks spaced 1.6 micrometers apart. 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 2.2Gbit/in2. Single-layer HD DVD and Blu-ray disks offer densities around 7.5Gbit/in2 and 12.5Gbit/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 UnivacUniservo, recorded at the density of 128bit/in on a half-inch magnetic tape, resulting in the areal density of 256bit/in2.[6] In 2015, IBM and Fujifilm claimed a new record for the magnetic tape areal density of 123Gbit/in2,[7] while LTO-6, the highest-density production tape shipping in 2015, provides an areal density of 0.84Gbit/in2.[8]
Research
A number of technologies are attempting to surpass the densities of existing media.
IBM aimed to commercialize their Millipede memory system at 1Tbit/in2 in 2007 but development appears to be moribund. A newer IBM technology, racetrack memory, uses an array of many small nanoscopic wires arranged in 3D, each holding numerous bits to improve density.[9] Although exact numbers have not been mentioned, IBM news articles talk of "100times" increases.
Holographic storage technologies are also attempting to leapfrog existing systems, but they too have been losing the race, and are estimated to offer 1Tbit/in2 as well, with about 250GB/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 10Tbit/in2.[10] 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, in 2009 a Stanford research team achieved a bit density of 35 bit/electron (approximately 3 exabytes/in2) using electron microscopes and a copper medium.[11]
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[update], DNA holds the record for highest-density storage medium.[12] In March 2017, scientists at Columbia University and the New York Genome Center published a method known as DNA Fountain which allows perfect retrieval of information from a density of 215 petabytes per gram of DNA, 85% of the theoretical limit.[13][14]
Effects on performance
With the notable exception of NAND Flash memory, increasing storage density of a medium typically 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.
For example, we can calculate the effective transfer speed for a floppy disc by determining how fast the bits move under the head. A standard 3½-inch floppy disk spins at 300rpm, and the innermost track is about 66mm long (10.5mm radius). At 300rpm the linear speed of the media under the head is thus about 66mm× 300rpm = 19800mm/minute, or 330mm/s. Along that track the bits are stored at a density of 686bit/mm, which means that the head sees 686bit/mm× 330mm/s = 226,380bit/s (or 28.3KB/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 double the transfer speed because the bits would be passing under the head twice as fast. Early floppy disk interfaces were designed for 250kbit/s transfer speeds, but were rapidly outperformed with the introduction of the "high density" 1.44MB (1,440KB) floppies in the 1980s. The vast majority of PCs included interfaces designed for high density drives that ran at 500kbit/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 through 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 led 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 10ns 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.
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 US$45–75 since 2007.[15] That said, the price of high-capacity drives has fallen rapidly, and this is indeed an effect of density. The highest capacity drives use more platters, essentially individual hard drives within the case. As the density increases, the number of platters can be reduced, leading to lower costs.
Hard drives are often measured in terms of cost per bit. For example, the first commercial hard drive, IBM's RAMAC in 1957, supplied 3.75MB for $34,500, or $9,200 per megabyte. In 1989, a 40MB hard drive cost $1200, or $30/MB. And in 2018, 4 Tb drives sold for $75, or 1.9¢/GB, an improvement of 1.5 million since 1989 and 520 million since the RAMAC. This is without adjusting for inflation, which increased prices nine-fold from 1956 to 2018.
Hard drive cost per GB over time
date
capacity
cost
$/GB
1957
3.75 MB
$34,500
$9.2 million/GB
1989
40 MB
$1,200
$30,000/GB
1995
1 GB
$850
$850/GB
2004
250 GB
$250
$1/GB
2011
2 TB
$70
$0.035/GB
2018
4 TB
$75
$0.019/GB
2023
8 TB
$175
$0.022/GB
Solid-state storage has seen a similar drop in cost per bit. In this case the cost is determined by the yield, the number of viable chips produced in a unit time. Chips are produced in batches printed on the surface of a single large silicon wafer, which is cut up and non-working samples are discarded. Fabrication has improved yields over time by using larger wafers, and producing wafers with fewer failures. The lower limit on this process is about $1 per completed chip due to packaging and other costs.[16]
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 1kbit part that cost about $50 in large batches, or about 5 cents per bit. 64Mbit parts were common in 1999, which cost about 0.00002 cents per bit (20 microcents/bit).[16]
Bit cell– the length, area or volume required to store a single bit
Mark Kryder, who projected in 2009 that if hard drives were to continue to progress at their then-current pace of about 40% per year, then in 2020 a two-platter, 2.5-inch disk drive would store approximately 40 terabytes (TB) and cost about $40.
Computer data storage or digital data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers.
Disk storage is a data storage mechanism based on a rotating disk. The recording employs various electronic, magnetic, optical, or mechanical changes to the disk's surface layer. A disk drive is a device implementing such a storage mechanism. Notable types are hard disk drives (HDD), containing one or more non-removable rigid platters; the floppy disk drive (FDD) and its removable floppy disk; and various optical disc drives (ODD) and associated optical disc media.
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnetic material. The platters are paired with magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces. Data is accessed in a random-access manner, meaning that individual blocks of data can be stored and retrieved in any order. HDDs are a type of non-volatile storage, retaining stored data when powered off. Modern HDDs are typically in the form of a small rectangular box.
A hard disk drive platter or hard disk is the circular magnetic disk on which digital data is stored in a hard disk drive. The rigid nature of the platters is what gives them their name. Hard drives typically have several platters which are mounted on the same spindle. A platter can store information on both sides, typically requiring two recording heads per platter, one per surface.
Non-volatile random-access memory (NVRAM) is random-access memory that retains data without applied power. This is in contrast to dynamic random-access memory (DRAM) and static random-access memory (SRAM), which both maintain data only for as long as power is applied, or forms of sequential-access memory such as magnetic tape, which cannot be randomly accessed but which retains data indefinitely without electric power.
IBM manufactured magnetic disk storage devices from 1956 to 2003, when it sold its hard disk drive business to Hitachi. Both the hard disk drive (HDD) and floppy disk drive (FDD) were invented by IBM and as such IBM's employees were responsible for many of the innovations in these products and their technologies. The basic mechanical arrangement of hard disk drives has not changed since the IBM 1301. Disk drive performance and characteristics are measured by the same standards now as they were in the 1950s. Few products in history have enjoyed such spectacular declines in cost and physical size along with equally dramatic improvements in capacity and performance.
Magnetic storage or magnetic recording is the storage of data on a magnetized medium. Magnetic storage uses different patterns of magnetisation in a magnetizable material to store data and is a form of non-volatile memory. The information is accessed using one or more read/write heads.
Millipede memory is a form of non-volatile computer memory. It promised a data density of more than 1 terabit per square inch, which is about the limit of the perpendicular recording hard drives. Millipede storage technology was pursued as a potential replacement for magnetic recording in hard drives and a means of reducing the physical size of the technology to that of flash media.
The Microdrive is a type of miniature, 1-inch hard disk produced by IBM and Hitachi. These rotational media storage devices were designed to fit in CompactFlash (CF) Type II slots.
Perpendicular recording, also known as conventional magnetic recording (CMR), is a technology for data recording on magnetic media, particularly 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. The first industry-standard demonstration showing unprecedented advantage of PMR over longitudinal magnetic recording (LMR) at nanoscale dimensions was made in 1998 at IBM Almaden Research Center in collaboration with researchers of Data Storage Systems Center (DSSC) – a National Science Foundation (NSF) Engineering Research Center (ERCs) at Carnegie Mellon University (CMU).
Heat-assisted magnetic recording (HAMR) is a magnetic storage technology for greatly increasing the amount of data that can be stored on a magnetic device such as a hard disk drive by temporarily heating the disk material during writing, which makes it much more receptive to magnetic effects and allows writing to much smaller regions.
Travelstar was a brand of 2.5-inch hard disk drive (HDD) that was introduced by IBM in 1994 with the announcement of the Travelstar LP. At 12.5 mm high with two platters, they were available in 360, 540 and 720 MB capacities. Initial models were industry-leading for small form factor HDDs in terms of areal density, data transfer rates and shock tolerance (500g).
Mark Howard Kryder was Seagate Corp.'s senior vice president of research and chief technology officer. Kryder holds a Bachelor of Science degree in electrical engineering from Stanford University and a Ph.D. in electrical engineering and physics from the California Institute of Technology.
In 1953, IBM recognized the immediate application for what it termed a "Random Access File" having high capacity and rapid random access at a relatively low cost. After considering technologies such as wire matrices, rod arrays, drums, drum arrays, etc., the engineers at IBM's San Jose California laboratory invented the hard disk drive. The disk drive created a new level in the computer data hierarchy, then termed Random Access Storage but today known as secondary storage, less expensive and slower than main memory but faster and more expensive than tape drives.
A solid-state drive (SSD) is a solid-state storage device. It provides persistent data storage using no moving parts. It is sometimes called semiconductor storage device or solid-state device. It is also called solid-state disk because it is frequently interfaced to a host system in the same manner as a hard disk drive (HDD).
Magnetic-tape data storage is a system for storing digital information on magnetic tape using digital recording.
Patterned media is a potential future hard disk drive technology to record data in magnetic islands, as opposed to current hard disk drive technology where each bit is stored in 20–30 magnetic grains within a continuous magnetic film. The islands would be patterned from a precursor magnetic film using nanolithography. It is one of the proposed technologies to succeed perpendicular recording due to the greater storage densities it would enable. BPM was introduced by Toshiba in 2010.
Random-access memory is a form of electronic computer memory that can be read and changed in any order, typically used to store working data and machine code. A random-access memory device allows data items to be read or written in almost the same amount of time irrespective of the physical location of data inside the memory, in contrast with other direct-access data storage media, where the time required to read and write data items varies significantly depending on their physical locations on the recording medium, due to mechanical limitations such as media rotation speeds and arm movement.
Higher performance in hard disk drives comes from devices which have better performance characteristics. These performance characteristics can be grouped into two categories: access time and data transfer time .
Two-dimensional magnetic recording (TDMR) is a technology introduced in 2017 in hard disk drives (HDD) used for computer data storage. Most of the world's data is recorded on HDDs, and there is continuous pressure on manufacturers to create greater data storage capacity in a given HDD form-factor and for a given cost. In an HDD, data is stored using magnetic recording on a rotating magnetic disk and is accessed through a write-head and read-head. TDMR allows greater storage capacity by advantageously combining signals simultaneously from multiple read-back heads to enhance the recovery of one or more data-tracks. In this manner, data can be stored with higher areal-density on the disks thus providing higher capacity in each HDD. TDMR is a read-back technology and thus applies equally well to future recording (writing) technologies such as heat-assisted magnetic recording (HAMR) and microwave-assisted magnetic recording (MAMR).
References
↑ "ExaDrive®". Nimbus Data. 22 July 2016. Retrieved 2020-11-16.
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