Write-Once-Read-Many (WORM) technology is becoming increasingly common in today's storage environments because of its ability to help companies meet regulatory and compliance requirements for preserving data in a provably unaltered format.
There are three approaches to WORM: physical WORM, embedded WORM and software WORM.
The earliest form of WORM technology, and still the most widely available, was physical WORM (P-WORM), or hardware WORM. In P-WORM, the medium is inherently write-once. The most familiar examples are CD-R and DVD-R. More sophisticated versions from vendors like Pioneer use magneto-optical (MO) disks.
The biggest drawback to P-WORM technology is its limited media capacity. The most common write-once optical drives, DVDs, store less than 10 GB per disk. Even the highest-capacity, commercially available technologies store less than 40 GB per disk. You can increase storage limits by using jukeboxes and other multiple disk storage devices, but when large amounts of information have to be stored, this approach is a bit clumsy.
Another drawback to P-WORM is cost. Even write-once DVDs become expensive when a lot of data has to be written to them regularly, and disk handling has to be automated.
Because of these limitations, P-WORM is traditionally used only on a subset of the enterprise's information. Many companies store only the data covered by regulatory requirements on P-WORM (although some financial
Embedded WORM (E-WORM) uses higher-capacity media such as tape and makes it write-once by hardware, firmware, media design or a combination of the three. This approach has become increasingly popular as major storage vendors such as EMC Corp. and Network Appliance Inc. add WORM capabilities to their products (in EMC's case, with Centera; in NetApp's case, its NetStore and SnapLock appliances).
E-WORM designs typically employ a combination of device drivers to designate WORM drives and firmware in the drive itself to limit the drive to write-once. The systems also often use a special tape or another medium that cannot be written in a conventional drive.
Since E-WORM storage capacities run into the terabytes and beyond, they are suited for enterprise-scale storage. And because the underlying technologies, such as magnetic tape, are thoroughly familiar to storage administrators, they are usually easy to integrate into existing storage operations. However, unlike P-WORM media, which are highly standardized, E-WORM implementations are almost all proprietary, meaning that the problem of vendor lock-in is always rearing its ugly head.
Software WORM (S-WORM) relies on features built into the operating system. The least common WORM family, it is mostly found in Unix and Linux systems.
Rick Cook has been writing about mass storage since the days when the term meant an 80 K floppy disk. The computers he learned on used ferrite cores and magnetic drums. For the last 20 years he has been a freelance writer specializing in issues related to storage and storage management.
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This was first published in February 2006