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Active data stays online and old stuff goes offline, but what about data you need only occasionally?

Everyone thinks about online data in the same way: You write it, read it, rewrite it and keep it forever. But this type of "active" data is actually a minority in most environments. Many organizations have far more data that's written once, read a few times and kept alive forever. You might say this bulk data is "write once, read several times" (WORST), and it can bloat your storage environment.

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Online, offline and nearline

Online, nearline and WORM
Companies have traditionally lumped all of their active data together. For open systems, the content of a disk was online and the content of a tape was offline (see "Online, offline and nearline"). This was more a symptom of the limitations of open systems than any real requirement; Unix and Windows hosts couldn't access tape without relying on a third-party backup application. Storage was online on an active disk or off-line on tape simply because there was no other choice.

But mainframe systems have long been more creative, even using tape to temporarily store data for later recall by apps. This was called hierarchical storage management (HSM) for many years, but the concept has emerged in open systems as so-called nearline storage. When we implement this type of storage, we're saying data might be needed, but we can afford to wait a bit for it.

This was first published in February 2007

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