DVD-RAM vs. MO for data storage applications
Richard C. Vining
Though prevalent in the consumer world, DVD optical technology is starting to make headway into business markets. DVD-RAM, a rewritable version of DVD, is being positioned for backup and multimedia applications, among others. Additionally, DVD-RAM`s capacity, speed, and price make it attractive for many of the same applications traditionally served by magneto-optical (MO), including document imaging, data archival, and hierarchical storage management.
But like all new technologies, DVD-RAM must overcome several challenges before its full benefit can be realized. For example, since DVD was designed for the consumer electronics and desktop PC markets, and not the industrial data storage market, data protection and data integrity issues deserve examination.
Like MO technology, DVD-RAM drives use sophisticated error correction algorithms to ensure data is readable for the rated life of the media (30 years). This integrity assumes, however, data is written on a defect-free section of the disc. This is where MO and DVD-RAM media differ.
During the manufacturing process, only MO media with fewer than 40 defective sectors on the disk surface are certified and packaged for use. Since there are 1,287,295 sectors on each side of a 5.2GB MO disk (with each sector containing 2,048 bytes of data), defective sectors represent a very small percentage of total writable sectors.
To keep prices as low as possible, most manufacturers do not subject DVD-RAM media to such a certification process. Defective sectors on DVD-RAM discs are discovered and marked as defective only when end users try to write data to those sectors.
A potential problem exists if the disk has more bad sectors than the allotted 256 spare sectors. If this situation occurs on MO media, the disk is marked as full and no new data is written to the disk, resulting in a loss of capacity. This problem is rare with certified MO media, only occurring when a disk is very dirty or has been mishandled.
With DVD-RAM media, the possibility is greater because most media is not be certified and is more susceptible to dust, dirt, and mishandling. If this problem does occur on DVD-RAM media, besides losing all remaining capacity, the disc`s file system may become corrupted, resulting in the loss of access to any of the data already written on the disc.
Fortunately, this is not a significant problem in typical DVD-RAM applications. In most DVD-RAM applications, the user writes a full data set, such as a backup, presentation, or movie, at one time. In this situation, if the operation fails because the spare sector area is full, the user simply discards the disc and rewrites the data to a new piece of media.
In a data storage environment, the possibility of failure is higher. Suppose you have an automated library with 480 DVD-RAM discs for document archival. Each of the discs is assigned to a specific user. It may take the user weeks, months, or even years to fill up the disc. If the disc has more than 256 bad sectors, at some point a write operation will fail and the user may not be able to retrieve the previously recorded information.
With these threats, why would anyone choose DVD-RAM over MO? 1) DVD-RAM is a lower-cost solution that functions very efficiently in most environments. 2) A DVD-RAM library is backward read-compatible with CD, DVD-ROM, and DVD-R media. 3) Some manufacturers do test and certify DVD-RAM media before distribution.
Although the risks of DVD-RAM media are fairly small, end users should consider:
Certified DVD-RAM media. Some companies test each DVD-RAM disc to ensure manufacturing specifications are met. This certification includes writing and reading every data sector on the disc, thereby identifying defective sectors and automatically re-mapping them to spare sectors. If the total number of defective sectors is too high, the disc is rejected.
Pre-packaged magazines. Some vendors provide certified DVD-RAM media in pre-loaded magazines that can be imported into compatible libraries. These magazines eliminate user contact with the media, preventing inadvertent damage from mishandling.
Ensuring that data is securely stored is an important concern for any company. DVD-RAM potentially offers a cost-effective alternative to MO, but with potentially slightly greater risk. On balance, DVD-RAM is a data storage choice worth considering.
Richard C. Vining is an alliance manager at Plasmon IDE (www.plasmon.com), in Eden Prairie, MN.