Solving The Need To Forage For Sto rage
By Marshall Breeding Optical media can meet an organization's need for data storage on an extremely large scale at an attractive price. The current market offers several alternatives for large-scale data storage. This Buyer's Guide focuses on two of those alternatives: optical disc changers and CD-ROM network servers. Many organizations have discovered
the benefits of optical discs--low cost, high capacity and long media life. Basic optical drives read and write optical media, but require users to change discs manually. Optical disc systems that use robotic changers to manage many discs eliminate the manual labor. These devices provide very large storage capacity through the management of dozens or even hundreds of discs. Although only a few discs can be mounted simultaneously, users can select discs from a much larger storehouse. Additionally, an organization can take the benefits to yet another level by connecting optical disc changers to its network.
But optical disc changers are a good fit for only a few specialized data environments, such as those that have extremely high volumes of locally generated data required by relatively few users at a time. A typical data management strategy will rely on magnetic disk storage for fast access to frequently used data sets, and it will use optical disc changers for less frequently used data and for archiving older information.
Hierarchical storage management (HSM) systems, meanwhile, let organizations take advantage of the most cost-effective data storage media. HSM automatically migrates data to lower-cost platforms as it ages and as the frequency of access to it decreases. Frequently used data, for example, would reside on high-performance magnetic disc arrays. After a specified time, the HSM process might transfer data to rewritable magneto optica l (MO) drives and later to Write Once Read Many (WORM) discs mounted in an optical disc changer. As the data ages and the likelihood of its need diminishes, the discs can be removed from the changer and stored as an archive.
Optical Media Options Optical storage technology comes in several flavors. With some technologies, such as CD-Recordable (CD-R) and WORM, you get one opportunity to write data--a feature often required for guaranteeing the integrity of archived data. Other drives, such as those using MO and phase-change technologies, let data be rewritten multiple times. CD-ROM--also an optical technology--is a read-only media. Optical disc changers are available for discs produced under all of these optical technologies.
One of the most popular optical technologies revolves around the MO process. MO drives use both magnets and lasers to read and write data. An MO drive requires the laser to make a double pass for the writing of data. Another multiple-write process, called phase change (PC ), writes data with only a single pass. WORM drives permit data to be written to a disk only once. CD-R, one of several WORM implementations, employs a purely optical process for writing and reading data.
Under the Hood The internal structure of an optical disc changer includes one or more optical drives, a set of slots for storing discs and a robotic mechanism that selects and loads discs into a drive. One defining characteristic of optical disc changers is the technology's ability to store and manage more discs than the unit has drives.
A critical component of the optical disc changer is the optical drive. Higher-capacity units have multiple drives. The number of drives installed defines the number of discs that can be read simultaneously. Optical disc changers use various methods for storing discs not in active use. Many use cartridges with multiple slots, holding as many as 50 discs per cartridge. Some lower-capacity units use a rotating disc tray.
A robotic mechanism moves a disc from its standby location to the optical drive. The mechanism must quickly locate the desired disc and place it into an available d rive. If there is a disc in the drive, the unit must unload and store it. Well-designed units are less susceptible to jams or mechanical malfunctions.
Updated March 25, 1997