A Buyer's Guide to Understanding and Evaluating Optical Storage

Storage 101: A Beginning
In the Information Age, information is like any other product.

It's bought and bartered every day. It differentiates successful companies from marginal ones. It forms the foundation of every successful enterprise. And, when it's transformed from its raw state into intellectual property, its value soars beyond calculation.

Like any other strategic asset, information must be protected from loss, corruption and theft. But how?

This primer is an important first step in evaluating and understanding your options in storage, especially optical storage. It explores the challenges of data proliferation, compares various approaches to data protection, and outlines specific optical technologies that help address the challenges of data storage in the information age.

No single document has all the answers. But, with this primer, you can begin to evaluate your options in optical storage, and understand how today's technology can protect your company's most valuable asset into the future.

Dealing with Data in the Information Age

Information Overload

Remember the one-gigabyte hard disk? A true breakthrough in its day, it's now virtually obsolete. These days, PCs need that much space just for e-mail and attachments.

The fact is, storage requirements among network clients are exploding, and the pace of data accumulation is not slowing. Increased Internet use, data mining, database access, e-mail traffic, and other common applications are contributing to information overload on networks worldwide. The problem is especially acute in the service sector, imaging environments, customer service departments and other environments that maintain account information online. And, while the demand for storage space is increasing, so too is the need for instant access to that data. In other words, it's not enough to find a place to put it; you have to put it where you can get it back fast.

Where to Put It
There are four broad categories of data storage. Depending on the sensitivity of data, number of data users, frequency of access and your storage budget, you might use one or all of these strategies in your company.

  • Online Storage
    Highest Performance, Highest Cost
    An image or document is considered online when it can be accessed instantly and automatically, without human intervention. Online storage consists primarily of hard disks, which is where most of today's corporate information is stored. Much of the information stored online is reference data, which means it has significant value to an enterprise but is viewed only occasionally. Some estimates suggest that about 20 percent of the all data kept on today's hard disks is actively used, while about 80 percent is only occasionally used. Although much of this information could be moved to lower cost (and lower performance) storage alternatives, it is left on hard disks due to the difficulty of predicting what will be needed, and when. Online storage can be expensive, because although disk mechanism prices are dropping, the cost of back-up and disk management gets higher each year.
  • Near-Online Storage
    High Performance, Mid-Range Cost
    One step removed from online storage, near-online storage is typically accessed quickly and automatically, in seconds versus milliseconds, and without human intervention. Jukeboxes and autoloaders using optical or magnetic tape media are considered near-online storage, and are commonly used as a complement to online storage. Near-online storage is often found in enterprises that have massive amounts of reference data that must be stored for long periods of time, yet must be easily accessible. For example, companies requiring infrequent and unpredictable access to CAD drawings, invoices, title documents, insurance records, customer service records and transaction histories might use near-online storage as a cost-effective extension of online storage. The information is stored permanently and safely; it's easy to retrieve; it's easily managed and controlled; and it does not consume expensive hard disk space.
  • Offline Storage
    Mid to Low Performance, Low Cost
    As the name implies, offline storage refers to data that is maintained offline, requiring human intervention whenever files are requested. Offline storage generally refers to removable disks or tapes that can be shelved for long-term storage or disaster recovery. This strategy offers a partial solution to information overload: data that is not actively used, or data that is so sensitive it requires duplicate backup in a remote location, can be removed from the operating environment and stored offline, sometimes even off-site. Documents that must meet regulatory, legal or audit requirements are a good fit for offline storage, especially when there is little value in having the data online. While offline storage is extremely safe, retrieving or sharing information involves a service request and a restore operation, which might take hours or days. With proper indexing, the data can be reliably located; once the disks or tapes are reinserted in the storage system, files can be restored fairly quickly.
  • Human-Readable Document Storage
    Lowest Performance, Lowest Cost
    Primarily paper and microfilm, human-readable documents are still the most common ways to store and share corporate information. The reasons are clear: data interchange is not an issue, they require little or no hardware, and they can be accessed well into the future. Unfortunately, retrieval is painfully slow, filing is not always reliable, files are easily lost or misplaced, and data sharing gets more and more difficult as the world becomes increasingly digital.

Three Basics of Storage
No matter what combination of storage strategies you choose, three basics must be met.

Industry Standards
The data you store today must be readable in the future, and that's what industry standards are all about. If you store data on optical disks or magnetic tape, make sure you use formats that are supported by ECMA, ISO, IEC and OSTA. These industry consortiums define standards for hardware, data interchange, media and write procedures. They help protect your data by making sure that the data you store today is readable in the future. They also help protect your budgets by allowing you to choose systems and media from a variety of vendors instead of being locked into one vendor with proprietary technology.

Few people calculate the cost of downtime, but minutes or hours of downtime add up to tens or hundreds of thousands of dollars per year in lost productivity, lost revenue and increased expenses. Plus, access is a reasonable expectation among data users: they expect to be able to store and retrieve data quickly, and share and distribute it freely. Any storage solution — especially a storage solution for an increasingly digital, increasingly interconnected world — must store data within easy reach, and keep it online virtually around the clock.

Data Protection
If information is important enough to be backed up, then it's important enough to be backed up accurately. Data users expect complete accuracy, protection from loss, and when the inevitable disaster happens — facility damage, virus infection, system crash or operator error — they expect data to be restored quickly. In fact, restoring data accurately is the only reason for backing it up in the first place. That means any storage solution you choose should include built-in utilities for data protection, overwrite protection, error-checking and disaster recovery.

Your Options in Optical Storage
How It Works
Optical drives read data by bouncing a laser beam off the recording layer of an optical disk, and registering differences in how the light is reflected.

The read-write head contains a laser, mirrors and lenses to send and receive the reflected light. As the head moves over the disk, it focuses its laser on the disk's data layer, and translates the reflections into the 1s and 0s of digital data. While all optical drives read data in a similar way, different technologies use different methods to record data.

Recording Methods
CD-ROM, CD-R (compact disc-recordable) and CD-RW (compact disc-rewritable) drives record data by putting marks on a disk's recording layer. A finished disk contains marks and spaces (i.e., non-marked areas). Each mark or space represents one bit of data. Marks are non-reflective, spaces are reflective. Since they reflect light differently, the laser is able to read the recording surface and translate recorded data into 1s and 0s.

Magneto-optical (MO) technology takes a different approach. Instead of reading the intensity of reflected light, an MO drive reads the direction of reflected light. The recording surface of an MO disk is magnetic, much like a common hard disk. To record data, a laser heats a tiny spot on the disk directly above the recording layer. Once it reaches a certain temperature, the spot on the disk's recording layer can be manipulated by a magnet, which moves the recording material in one of two directions. Each direction reflects light differently, allowing a low-power laser to read the reflections and translate what it sees into 1s and 0s. Unlike other technologies, the MO recording method and MO media support a virtually unlimited number of rewrites, with a media shelf life of at least 100 years.

The Technologies
There are currently over 800,000 5.25-inch optical storage drives in operation worldwide. Though not as ubiquitous as hard disks, microfilm readers or file cabinets, optical drives and jukeboxes are quickly being implemented in computing environments around the world, especially as the importance and quantity of corporate information grows. Optical storage is uniquely designed for an increasingly digital world, where information must be stored safely, shared globally, and accessed instantly, sometimes over vastly distributed networks.

There are three basic categories of optical storage: CD-ROM, rewritable and WORM (write once, read many). All are viable, all have strengths and weaknesses. Here's a quick look at the technologies and most common recording formats in use today.

  1. CD-ROM
    CD-ROM (compact-disc read-only-memory) technology is the grandfather of optical storage, found in every consumer audio CD player in the world. CD-ROM drives use an optical laser to read marks that are recorded in a continuous spiral track on the disk's recording layer. The drives don't write data, they only read it — a virtually unlimited number of times, with no degradation of disk or data. Handled correctly, a CD-ROM will easily outlive its owner. This combination of durability and high capacity makes CD-ROM a popular choice for distributing high volumes of stable data, such as computer software, catalogs, technical manuals, reference works, and of course, music.
  2. Rewritable Optical
    Rewritable optical drives not only read but also write data to optical disks. Unlike WORM disks, a rewritable optical disk can be erased and rewritten many times, just like a hard disk. Two common technologies comprise the rewritable optical arena:

    • CD-RW (compact disc-rewritable) products represent an increasingly popular application of rewritable optical technology. A CD-RW disk is like a giant floppy disk, allowing you to repeatedly write and rewrite large volumes of data. It has most of the benefits of a typical CD-ROM, allowing you to store and distribute large volumes of information that can be read by any CD-ROM drive. When the data needs to be changed, it can be erased and re-saved on the same CD-ROM. The only drawbacks: the write process takes some time using current technology, and CD-RW discs have a maximum capacity of 650 MB.
    • Magneto-Optical (MO) technology brings four advantages to the table. 1) Since its write method changes the magnetic flux directions on a disk's recording layer, and does not physically deform the disk like other optical technologies, MO disks are subject to less wear and degradation, and therefore last longer than disks that use other recording methods. 2) Magneto-optical technology supports both WORM and rewritable functionality, so an MO drive or optical jukebox can provide both the permanence of WORM and the flexibility of rewritable. 3) Industry standards are solidly in place and well-defined to assure that the MO disks of today will be readable in the MO drives of tomorrow. 4) Magneto-optical disks offer capacities of up to 5.2 GB per cartridge.
  3. WORM (Write Once, Read Many)
    Commonly applied in CD-R (compact disc-recordable) and MO storage systems, WORM drives can write data to an optical disk, then read the data a virtually unlimited number of times. Each sector on a WORM disk can be written just once, and cannot be erased, overwritten or altered. For that reason, WORM technology is perfectly suited to long-term storage of information such as financial records, accounts and medical records that require an unalterable audit trail. WORM drives use one of four proven recording methods:
    • Continuous Composite Write-Once (CCW) combines the magneto-optical recording process with overwrite protection on the disks and drives.
    • Bubble-forming/Dye-Polymer Write-Once uses a laser to form small bubbles on the recording surface of a disk.
    • Phase-Change Write-Once uses a laser to alter the molecular structure of a disk's recording surface.
    • Ablative uses a laser to create holes, or pits, in a disk's recording surface.

Recording Formats
There are many flavors of optical storage. Here's a quick comparison of the most common recording formats in use today.

Magneto-Optical (MO)
MO drives use a laser and magnet to record data on optical disks. The write procedure causes no measurable wear on the media, so write cycles are virtually unlimited. Read/write speeds are extremely fast — just slightly slower than hard disk speeds — and data reliability is extremely high, with a hard error rate that is 10 times better than half-inch tape.

MO disks and drives are governed by industry standards set by ANSI, ISO and ECMA, which define physical form factors, logical sector formatting, registration of bad sectors and more. Industry standards assure backward compatibility among multiple generations of MO disks. Realize, however, that media interchange relies on compatible file systems; MO disks stored under Windows 95® are accessible only on another Windows 95 or compatible file system.

Most CD-RW drives employ the phase-change recording format, which uses a plastic disk with a metal recording layer to store data. Heat generated by the drive's laser focuses on the disk's recording layer, and changes spots on the disk from an amorphous to crystalline state. Whenever a spot is reheated, it changes back to an amorphous state. Writing data consists of heating the desired spots to change their state. When reading, the laser detects differences between the amorphous and highly reflective crystalline spots. Because a high-intensity laser is required to heat the recording layer, the disks are very stable and their data will remain unchanged for up to 100 years. Phase-change recording has one primary limitation: the high-intensity laser is expensive compared to most other optical read/write heads.

Bubble-forming devices use a laser to vaporize a disk's dielectric recording surface, causing small bubbles to form. The bubbles change the reflectivity of the disk's recording surface, a difference that is detected during reads.

Other Technologies

Most rewritable MO devices use a two-pass write procedure: the first pass erases the disk's surface area, the second pass records information. An emerging technology known as LIMDOW (light intensity modulation/direct overwrite) recording allows one-pass writing on special MO disks. LIMDOW devices will offer higher write performance than today's MO technology since the device writes directly to the media substrate with one head pass. However, the media and hardware are more complex and more expensive. For that reason, LIMDOW is not currently a widely accepted format.

Digital Video Disk (DVD) closely resembles a standard CD in size, color and physical format, but holds approximately seven times more data. A typical CD holds about 650 MB of data, whereas today's DVD-ROM disks hold 4.7 GB, with a target capacity of about 17 GB in the future. Recordable and rewritable versions of DVD exist today as well. A two-hour feature-length movie can fit on a DVD, making it an attractive medium for the entertainment industry as well as for PC makers. In addition to DVD-ROM, DVD-recordable technologies are also becoming available, with multiple technologies now competing for dominance in the market.

How to Choose
By Data Requirement

The key to building a long-term storage strategy rests in knowing the role of data within your company, and understanding the expectations of those who use it. Here are few checkpoints to consider.

  • Value/Sensitivity
If your company's data is irreplaceable, or if its loss would threaten the viability of your enterprise, then a combination of online and offline (or even off-site) storage is highly recommended.
  • Volume
If you generate medium to large amounts of data, an online storage solution may suffice. For high to extremely high volumes of data, a combination of online and near-online storage is recommended.
  • Accessibility
If information is important enough to be backed up, then it should be protected from corruption and loss, and should allow a quick restore when the inevitable happens, such as facility damage, virus infection, system crash or operator error. Write-once optical solutions, which include built-in utilities for data protection, overwrite protection and error-checking, are recommended.
  • Longevity
If data must be readable and accessible for decades, an optical disk-based solution is desirable since the disks can be stored for up to 100 years without maintenance or data loss.
  • Security
Removable media provides the ultimate security. Optical disks, magnetic tapes, paper files and microfiche can all be removed from a storage system and stored safely offline under lock and key.
  • Legal Requirements
Information that might be used in audits or legal proceedings should be stored on media that cannot be easily altered, such as write-protected optical disks, magnetic tapes or film. Since this type of data is often stored for future reference (and herefore is not actively used), removability is desirable.
  • Price/Performance
It always come down to these two issues. If price is no object and performance is paramount, go with online storage for all of your company's data. If price is an issue but performance is still paramount, consider a combination of online and near-online storage. If all you care about is low price, and if accessibility and performance truly don't matter, store everything on paper or microfiche. In short, the right solution depends entirely on your budget and expectations, which can be defined only by you.


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© Hewlett-Packard Company 1999

Francis Bacon