The first storage media — paper tape and punched cards — were inefficient, slow and bulky. These gave way to magnetic storage: core memory, drums and, finally, hard drives. For backup, there were removable media: magnetic tape reels and cartridges, floppy disks and removable hard drives. Then optics (CD-ROM and DVD drives) supplanted magnetism for archival uses. Today's computers need to store more data than ever. The most recent generation of storage replaces moving parts with solid-state electronics.
Through all this evolution runs a constant thread: Storage has become faster and smaller, packing more data into less space. This storage density (also called areal density) is measured in units of bits per square inch (or bit/in. 2 ). The increase in density over time, particularly with magnetic media, has been remarkable; the cost-effectiveness is astronomical.
A hard drive with a density of 329Gbit/in. 2 has recently been announced by Seagate. For perspective, researchers believe that 1Tbit/in. 2 represents the theoretical limit for current magnetic storage, and that limit may be approached in just a few years. What happens when we hit the wall? Where do we turn for more storage? Several technologies that could help are under development.
1. Longitudinal vs Perpendicular Magnetic Recording
In longitudinal recording, magnetic data bits are aligned parallel to the disk surface, following concentric tracks. This limits storage density to 100 to 200Gbit/in. 2 or so. Perpendicular recording, introduced commercially in 2005, puts data bits in a vertical magnetic alignment that is perpendicular to the disk surface; it's as if the data bits are standing up rather than lying down.
Normally, the amount of magnetic material used to record a bit must be sufficiently large to retain its polarity so that it can't be accidentally reversed. Perpendicular recording allows the use of finer-grained material in which it's more difficult to reverse the magnetic orientation. Thus, perpendicular recording permits physically smaller bits; theoretically a density of 1Tbit/in. 2 would be possible.
2. Heat-Assisted Magnetic Recording
Still in the research stages, HAMR uses a laser to heat the storage medium while writing to it. It uses a different type of recording medium than conventional magnetic technology. That new medium is often an iron-platinum alloy. This allows much higher storage densities (potentially up to 50Tbit/in. 2 ), but requires the application of heat to change the magnetic polarity in the area that delineates each bit.
3. Patterned Media
Regular magnetic disks store each bit across several hundred grains of magnetic material. With patterned media, photolithography lays down a uniform grid of small magnetic cells, each storing one bit in less space, permitting higher storage density.
In 2007, Fujitsu America achieved a storage density of 1Tbit/in. 2 using this method.
4. Holographic Storage
Three-dimensional holographic images store more information in less space by recording not only on the surface of the storage medium, as do other technologies, but also within an entire volume of space in the medium. (Technically speaking, holographic storage density should be measured in units of bits per cubic inch, but that's not yet a common usage).
Holograms are created by splitting a single laser light into two beams: One is a reference and another carries the signal. Where the reference beam and the data-carrying signal intersect, the interference patterns are recorded in a light-sensitive storage medium. (Initially, this physical storage device will be a DVD-size disk). Because multiple beams can be used at different angles, hundreds of unique holograms can be recorded in the same volume of material. In one sense, this is similar to a dual-layer DVD, except that it contains hundreds of layers, each recorded at a different angle so that they are not parallel to one another. The stored information is reconstructed by deflecting the reference beam off the hologram and projecting it onto a detector that reads an entire data page (more than 1 million bits) at once.
An early offerer of holographic storage is InPhase Technologies, whose Tapestry product line incoroporates holographic storage. A real advantage of holographic storage is that its transfer speed (160MB/s) is far higher than the speeds other optical media can deliver.
5. Solid-State Drives
These use on-chip RAM or flash memory that emulates a hard drive. With no moving parts, solid-state drives are silent and sturdy. With no mechanical delays, they usually provide fast access time and low latency.
StorageTek developed the first modern SSD in 1978. M-Systems (now owned by SanDisk) introduced keychain-size, flash-based solid-state drives in 1995; these are now used successfully as replacements for hard disk drives, and as convenient backup and data-transfer devices (often called thumb drives).
These days, smaller SSDs are commonly found in mainstream consumer netbooks and subnotebooks, while SSDs capable of holding 100GB or more are available at high prices.
Additional reporting by David Watson