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The Cost Ledger

How the saving is computed so that it survives a finance review: against a counterfactual, net of every charge, attributed to decisions, and reconciled.

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Inputs Placed bytes catalogue replica · daily Movement outcomes bytes · requests Recall charges retrieval · mounts Platform cost hardware · power share Price Ladder version price at placement Compute Counterfactual bill all hot · $0.020 Realised bill by tier Tiering charges retrieval · early delete Attribute Per decision tenant · class · tier Publish Net saving target ≥ 45% Platform cost ratio target ≤ 8% of net Chargeback check ± 2% of storage ledger The Cost Ledger — Net Saving Against an All-Hot Counterfactual Data store Decision point Application we own Opportunity Security / platform A saving with no counterfactual is an assertion, and a gross saving is not a saving. Both numbers are published side by side every month. v 1.0 · owner FinOps · date 2026-09

Decisions

  • Every byte is priced at the ladder version under which it was placed. A price change in the ladder does not silently rewrite last quarter's saving.
  • Charges are attributed to the decision that caused them: retrieval to the demotion that put the object out of reach, early deletion to the demotion that preceded the delete, movement to the policy version.
  • The ledger is reconciled daily against the storage team's chargeback, which is built from Ceph usage and tape inventory, within ±2%. A gap bigger than that is a finding about one of the two ledgers.

Numbers

  • All-hot counterfactual at 41 PB: about $820,000 a month. A 45% net reduction is about $369,000 a month, or $4.4 M a year.
  • The 8% platform-cost ceiling is therefore about $350,000 a year.

Risk carried openly

  • The platform's own hardware (catalogue fleet, Kafka, ClickHouse, movers) is estimated at $450,000 to $600,000 a year amortised, before staff. The 8% target is not met in year one. It is met in year two or three as the corpus grows 34% a year against a mostly fixed platform, and that is stated rather than hidden.
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Solution Architecture v1.0 · Storage Platform Architecture · 2026-09 · 31 views · open source, fully on-premises26-cost-ledger