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DLT (Digital Linear Tape) is a tape backup drive used primarily on Unix machines, which can store up to 40GB of (uncompressed) data per DLT cartridge. The cartridges are about half the size of a VHS tape. The units typically transfer data at about 5 megabytes/sec.

DLTs are not just for Unix, they are common in most large NT / Windows 2000 high end servers. For example, Compaq offer DLTs on all their servers, as well as DLT arrays under their StorageWorks banner.

The Digital Linear Tape (DLT) is an old and venerable tape format dating back many years. Back in the day, a cartridge held about 2.6GB native. Stretching my mind back, it seems like the first time I saw one of these was on a DecStation, but they called it a TK50, which doesn't seem right, except I'm almost positive the form factor was the same. Who knows.

Basically these are 1/2 inch multi-track metal particle tapes in a square plastic cartridge of about the thickness of a VHS casette and half the width. Of many available types, there's DLT III (10 GB), DLT IIIXT (15 GB), and DLT IV (20, 35, 40 GB) - all sizes native uncompressed. Tracks are stacked densely in a configuration called Linear Serpentine Recording. The modern tapes (IIIXT and above) are certified for up to 1 million passes, and rated for a 30 year shelf life. Not bad. The format was invented and is still owned and licenced by Quantum, although there are a number of open standards describing the technologies used.

DLT has tended to be more expensive than the alternatives, with drives often costing hundreds or thousands more than comparable DAT models, a more recent arrival on the backup drive market, and media going for over $70 per tape. However, DLTs are in wide and common use, especially at sites which have been operating since before there was a decent alternative in the size and price range served by these systems (read: unix/other mainframe ops). It's also considered to be more reliable and durable a backup solution than the prevailing alternatives (which in addition to DAT/AIT, include Exabyte and Travan); perhaps deservedly. It's what admins like to call "mature."

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