Commercial storage media and storage management schemes are evolving to keep up with the huge increases in data generated by enterprises. During the next year or two, no dramatically new technologies are likely to replace the three workhorses of data storage: hard disk drives, optical disks and magnetic tape. However, several advances in these fields promise greater capacities and improved performance to better suit users’ needs.

Data management that uses tiered storage provides different access schemes for data depending on the frequency of access. At the real-time-access end of the spectrum, perpendicular recording technologies are increasing the data density of hard disk drives. Data that is less frequently accessed can be put on holographic optical disks, each of which can store as much as a terabyte of data. And virtual tape libraries (VTLs) allow users to shorten access time to what used to be archival data, making it a near-line storage solution. These new wrinkles for magnetic and optical drives are arriving on the market now.

Perpendicular recording
Conventional hard disk drives store data by using tiny read/write heads that magnetize billions of discrete areas as they fly over the surface of thin magnetic platters. An area represents a zero or one, depending on whether it is magnetized clockwise or counterclockwise. But when the magnetized areas are too small and close together, the bits become unstable. Thermal variations can cause the magnetization to flip, thus corrupting data. This super-paramagnetic effect establishes a minimum size for each bit, and thus limits the amount of data that can be stored per unit area in so-called horizontal (or longitudinal) schemes to about 100GB to 200GB per square inch.

Perpendicular recording (sometimes called vertical storage) offers even higher areal densities while leveraging 50 years of hard disk development. Instead of magnetizing bits clockwise or counterclockwise, perpendicular recording magnetizes them up or down. From the manufacturer’s perspective, perpendicular recording requires only a slight modification to the structure of the disk; a soft magnetic layer is placed under the surface of the hard disk. The underlayer works in conjunction with the read/write head to create a stronger magnetic field than can be conveniently generated in conventional hard disk drives, which allows smaller bit sizes. In mid-2005, an areal density of 230GB per square inch for a perpendicular recording hard drive was demonstrated. Perpendicular recording could someday support areal densities as high as 1TB per square inch—almost 10 times the density of horizontal recording.

“The density of capacity has been increasing with enormous speed,” explains Jimmy Zhu, director of the Data Storage Systems Center at Carnegie Mellon University. Using perpendicular recording, Zhu says, “gives you room to push up capacity for a few more generations.” From the user’s view, a device employing perpendicular recording is a normal hard disk drive with the same form factor and interface, but with the potential for improved performance in addition to higher capacities. (The increased number of bits that pass under the read/write head in the same amount of time results in higher data throughput without having to increase the speed of the disk’s rotation.)

Holographic disks
The areal densities of perpendicular recording are quite high, but for data that doesn’t have to be retrieved with the blazing speed of hard disk drives, even higher densities are available. In 2006, one manufacturer squeezed 515GB of data into a single square inch using a holographic optical disk storage technology. This is more than a twofold increase over the data density (200GB per square inch) attained in 2005.

Several new consumer optical technologies are hitting the market now, including Blu-ray and HD DVD. But while those are refinements of optical CD technology, holographic disks are altogether different for two reasons: Holographic methods record and read chunks of data in parallel, and the data is recorded in the volume of the disk, not merely on the surface.

Instead of recording data by changing the phase (and thus the reflectivity) of individual spots on the surface, holographic disks record data into the volume of the disk one page at a time (a page is a two-dimensional array containing as many as 1 million bits). The entire page of bits can be written (or retrieved) in parallel, and more than one page can occupy the same space within the volume.

Holographic media requires two laser beams to record data. A spatial light modulator (such as Texas Instruments’ Digital Micromirror Device) encodes the data onto the signal beam, so that each bit of data is transmitted as a bright or dark spot. The signal beam and the reference beam interfere with one another when they cross in the holographic medium. The light-sensitive medium changes to record the interference of the beams. Later a weaker signal beam can be used to read the data one page at a time.

As with any new medium, holographic storage vendors must educate users on what to expect from the technology and how to use it. The reading and writing technology is more complex than, and not as mature as, other types of optical storage.

Virtual tape libraries
Less revolutionary than either perpendicular or holographic recording, the VTL is a helpful new tool that allows access to what would otherwise be archived data. Magnetic tape remains unbeatable for true archival storage. It’s cheap, portable, well developed and has an infrastructure of support within most organizations. Unfortunately, it can’t provide fast access to data. Managers want the ability to use data that was once destined only for archives. This may be called for only occasionally, but it’s a procedure managers need more often than tape libraries can adequately handle. Virtual tape libraries fill that niche.

A VTL user sees what appears to be a normal backup system: The interface emulates specific tape drives and tape libraries. Both the software and policies implemented for tape libraries remain the same, which can be a huge benefit for organizations that have invested in these areas and aren’t positioned to change at the moment.

But the data is not, in fact, on tape. Instead, hard disk drives emulate tape drives, providing improved access speed. This hardware allows organizations to reap the performance benefits of disk storage designed for storage area networks (SANs). Virtual tape libraries are an intermediate step between networked storage and tape; when the data becomes less useful, it can be transferred from the VTL to real tape.

The advantages and disadvantages of VTLs are fairly straightforward. They offer users the same management schemes already used for tape but provide faster system restores than tape, and the random access of hard disks means that users need not worry about delays inherent in tape streaming. On the other hand, hard disk drives are inevitably more expensive than magnetic tape. (Just about everything is more expensive than tape, which explains tape’s longevity.) Hard disk drives also introduce the possibility of data degradation from file fragmentation, which is not a problem with streamed tape.

More data at a faster rate
In addition to the three technologies discussed in this article, a number of other methods are being researched to improve data storage. For hard disk drives, these include integrating flash memory with the disks, pre-patterning media and using heat-assisted magnetic recording. On the optical front, rewritable Blu-ray DVD technology may eventually be used for serious data storage. As for magnetic tape, a number of innovations may extend the life of that stalwart medium. The only certainty is that while customers continue to generate more amounts of data, developers will continue seeking new ways to satisfy the need for larger and more cost-effective storage media. T

Yvonne Carts-Powell is a science and technology writer based in Boston.

Illustration by Ron Wiemann.

Teradata Magazine-December 2006