Virtual tape libraries in depth


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When an IDT deletes a file, it automatically reclaims the space. But a VTL has no idea that the tape it's holding has expired. A workaround is to manually re-label tapes when they expire. When the VTL sees a new label being written to the tape, it knows it can throw away the rest of the data on that tape.

File system devices support simultaneous read and write, but VTLs don't. If a backup is writing to one virtual tape, another process can't read that tape to do a restore or copy. But this only happens if you're backing up and restoring/copying at the same time -- probably a rare occurrence that can be made even less likely by using smaller virtual tapes.

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Why dedupe and FC disk don't mix

Fibre Channel (FC) is essential to the enterprise and data deduplication is important as well. But the only way to get both in a backup appliance is to buy a virtual tape library (VTL). Why don't they just make a deduplicated logical unit number (LUN) that's accessible via FC? The short answer is that it's a lot harder than it sounds. Giving you a LUN allows you to pick your own file system, which the appliance would then need to support. Windows, Linux, Solaris, HP-UX, AIX, MacOS, etc., all have their own completely incompatible file systems. The IDT vendor would have to test deciphering all of the various backup formats on all of the various file systems as well. Think of that test matrix.

But GreenBytes Inc. has gotten close. It's about to offer an iSCSI deduplicated LUN with its GB-X Series of storage appliances.

This was first published in December 2009

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