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Deduped VTL Greener Than Tape?

HDS CTO and blogger Hu Yoshida started quite the little blog flame war with a post here that suggested a real world customer found their tape library was using more power than a VTL. Responses included IBM blogger Tony Pearson, The Backup Blogger, and SearchStorage's Beth Pariseau, with comments by other noted storage pundits. It didn't make any sense to me, so I decided to do the math myself.

Figuring a midsize LTO-4 tape library with 10 drives and 300 slots for a capacity of around 400 TB (assuming around 1.6:1 compression) vs. a deduping VTL with 48 1-TB Western Digital Green Power drives for 40 TB of usable space after RAID overhead (RAID-6 or course). With deduplication, it will hold 700 TB to 1.5 PB of backup data. I could use fewer drives to better match the tape libraries capacity, but even with 48 drives I don't think this will be as fast as 10 LTO-4 drives.

The 10 tape drives will draw 340 W when active and 130 W when idle. Fibre Channel bridge, control electronics, and idle draw of the robotics should be around 300 W to 500 W. The robotics are actually moving so little I'll ignore the power it uses. Assuming a 40% duty cycle, the library should use about 12 Kwh a day.

The WD drives draw 7.5 W when active and 4 W when idle, the Xeon server that runs the whole thing (RAID controllers, etc.) about 500 W. Assuming the same 40% duty cycle, that's about 18 Kwh a day.

OK, tape's still greener, but it's close.

Someone want to fight over the math?

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