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ADR-013: Weighted Average DRR (not Simple Mean)

Status: Accepted Date: 2026-02-19

Context

The summary report must show an overall DRR for the entire VM fleet. Two options exist: arithmetic mean of individual DRRs, or capacity-weighted harmonic mean.

Decision

Compute overall DRR as total_provisioned / total_required, not as the arithmetic mean of individual VM DRRs.

Rationale

  • A 1 GiB VM and a 100 TiB VM should not contribute equally to the fleet average
  • The formula matches how storage arrays actually behave across a mixed workload
  • Consistent with how the required capacity total is computed row-by-row

Alternatives Considered

  • Simple arithmetic mean: Mathematically incorrect for capacity sizing; a single tiny VM with DRR=1 would drag down the average unfairly

Consequences

  • Headline DRR can differ significantly from the arithmetic mean in skewed fleets
  • Pre-sales customers must understand the metric is a weighted result
  • Calculation is deterministic and reproducible from the per-VM numbers