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 storage generation replaces moving parts with solid-state electronics.
Through all this evolution runs a constant thread: Storage got faster and it got smaller, packing more data into less space. We measure this storage density (also called areal density) 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 was just announced by Seagate Technology LLC. For perspective, researchers believe that 1Tbit/in.2 represents the theoretical limit for current magnetic storage, and we may approach that limit in just a few years. What happens when we hit the wall? Where do we turn for more storage? A number of technologies that could help are under development.
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.
Magnetic Media: Denser and Cheaper
| Date | Disk type | Drive capacity | Areal density | Cost/MB | Density improvement | Relative cost improvement |
|---|---|---|---|---|---|---|
| 1956 | IBM RAMAC | 5MB | 2Kbit/in.2 | $10,000 | Not applicable | Not applicable |
| 1989 | Typical Western Digital | 4MB | 12Mbit/in.2 | $25 | 6,000 times | 400 times |
| 2009 | Typical high-capacity hard drive | 1TB | 200Gbit/in.2 | $0.00015 | 100 million times | 75 million times |