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Most HSM applications use tape, but a few use optical disks. Similar to CDs or DVDs, these optical "platters" use light rather than (or in addition to) magnetic charges to store data. Historically, this media has had the advantage of being less expensive than magnetic disk while retaining the ability to access any piece of data at random. This stands in contrast to tape drives, which must reel through much of their length to locate a piece of data and often read much more data than is needed to get at a single, valuable piece of information.

Although rewritable optical disks have long been available, the majority are of the write-once type commonly called write once, read many (WORM). So, in a classic case of the cart driving the horse, businesses have attempted to identify applications requiring immutable media with long load times. Predictably, these applications are rare, limiting the usefulness of WORM in the real world.

One would think that archiving would be an ideal use of WORM media because data integrity is an important part of compliance. But, ironically, another aspect of WORM media actually interferes with its usefulness. Because data can't be deleted, and a single "platter" can store many gigabytes, immutable media can interfere with compliance policies that call for deleting a single piece of data.

With optical WORM effectively marginalized, many vendors began offering a new type of nearline storage with both random access

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and atomic control. This nearline storage was developed primarily by disk vendors and the apple didn't fall far from the tree; most offerings are simply repurposed disk storage arrays.

That's not to say there isn't innovation in nearline storage. To the contrary, developing certifiable WORM storage based on conventional disk has proved challenging. Some vendors even went in an entirely different direction, creating so-called content-addressable storage (CAS) that deals with objects rather than files or blocks.

This was first published in February 2007

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