User:Nren4237/Areal density
Areal density is a measure of the quantity of information
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
By far the densest type of memory storage experimentally to date is
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
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
Although the effect on performance is most obvious on rotating media, similar effects come into play even for solid-state media like
One defining electrical property is the resistance of the wires inside the chips. As the cell size decreases, through the improvements in
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.[4] 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
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.[6]
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).[6]
See also
- Bit cell – the length, area or volume required to store a single bit
- Patterned media
- Shingled magnetic recording (SMR)
References
- ^ "Archive HDD Data Sheet" (PDF). Seagate Technology. November 2014.
- ^ M. Mallary; et al. (July 2002). IEEE Transactions on Magnetics. 38 (4): 1719–1724 http://ieeexplore.ieee.org/xpl/articleDetails.jsp?tp=&arnumber=1017762&contentType=Journals+%26+Magazines&queryText%3DOne+terabit+per+square+inch+perpendicular+recording+conceptual+design. Retrieved 2 April 2013.
{{cite journal}}: Explicit use of et al. in:|author=(help); Missing or empty|title=(help) - ^ "Reading the fine print takes on a new meaning". stanford.edu.
- ^ Shilov, Anton (2013-10-29). "WD Continues to Widen Gap with Seagate as Average Selling Prices of Hard Disk Drives Continue to Fall". xbitlabs. xbitlabs.com. Retrieved 2014-08-11.
Average selling prices of hard disk drives in $USD
- ^ Cost of Hard Drive Storage Space
- ^ a b "DRAM 3". iiasa.ac.at.