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Collocation and stacking
The growth in tape cartridge size has created a glut of underutilized tape capacity in IT environments. Backup apps typically write data to tape serially, which leaves significant cartridge capacity unused. For example, Job 1 may be written to Cartridge A and use only 10% of that cartridge's capacity. Job 2 would then be written to Cartridge B, leaving 90% of Cartridge A unused.

By embedding additional layers of intelligence in the tape data-layout process, virtual tape vendors can improve tape utilization. Collocation, or stacking, places unrelated volumes/jobs from a virtual tape drive onto a physical one in a manner that maximizes the utilization of the cartridge. In addition to significant reductions in cartridge count (some users have seen a 10 times reduction, which isn't surprising given that utilization rates tend to be around 15% to 20%), collocation also speeds up restores. With collocation, a volume restore no longer requires the restoration of the entire tape. Products supporting collocation or stacking include ADIC's Pathlight VX, Diligent's VTF Mainframe, EMC's Clariion Disk Library and Neartek's VSE2.

Virtual tape holds tremendous promise. It's a powerful technology that effectively bridges the gap between traditional tape and disk-based data protection architectures as non-disruptively as possible because

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virtual tape preserves your operational processes. But the wrong virtual tape product can be disastrous for your storage environment, so a thorough evaluation of available products is essential.


This was first published in November 2005

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