Tiering 5 PB of Cold Data Under a Power Cap
A media company has 5 PB of archive content. 60% has not been accessed in 12 months. The current disk archive draws 120 kW of IT power at a PUE of 1.6, occupies 20 racks, and is due for a five-year refresh. A vendor proposes an LTO-9 tape library with 8 drives, 6,000 slots, 15 kW of IT power, and 4 racks. Electricity costs $0.12/kWh. The company has a 15-minute recall SLA for 10% of requests and a 4-hour SLA for the rest. The board has capped five-year archive spend at $2.5 million and wants a sustainability story as well as lower cost. The tape proposal assumes media migration every seven years, but the vendor has not included migration labor, offsite vaulting, or software licenses. The archive team must show whether tape tiering meets the SLAs and where the proposal is financially incomplete.
1. Calculate the five-year facility power cost for the disk and tape tiers. Show the PUE adjustment.
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Disk: 120 kW IT x 1.6 PUE = 192 kW facility. 192 x 8,760 = 1,681,920 kWh/yr. At $0.12 = $201,830/yr; five years = $1,009,152. Tape: 15 kW IT x 1.6 = 24 kW facility. 24 x 8,760 = 210,240 kWh/yr; $25,229/yr; five years = $126,144. Power savings are about $883,000 over five years. That is material but not the whole TCO. Verify whether the 15 kW is IT load or facility load; if it is already facility load, do not apply PUE twice. Also separate power from cooling, floor space, and refresh costs.
2. Design a tiering and recall policy that meets both SLAs without keeping all 5 PB on disk.
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Keep a disk cache sized to the 15-minute SLA, not all 5 PB. Use request history to cache the active 10% of projects or a 200-500 TB working set. Put cold content on tape with a catalog that supports prefetch. For 4-hour requests, eight LTO-9 drives at 400 MB/s can stream about 11.5 TB/hour theoretical; real recall is lower with mounts and seeks. Use duplicate tapes for popular content or a second library. Set a policy: files untouched for 12 months go to tape; any project in active production stays on disk. Test recall against both SLAs before committing.
3. What TCO and sustainability items are missing from the tape proposal, and how would they change the decision?
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Missing: media migration labor every 5-7 years, spare drives, library maintenance, software licenses, offsite vault, tape disposal, floor-space cost, staff training, and test restores. Tape media is cheap but migration is not free; LTO generations arrive faster than seven years, so the archive may need migration sooner. Sustainability: tape saves power but creates media waste and still needs cooling; disk refresh creates e-waste. Compare lifecycle emissions, not just kWh. Add a 10-15% contingency and require a migration plan before board approval. The tape tier still likely wins on power and floor space, but the proposal is incomplete without these costs.