Redundancy Calculator (N+1, N+2)
Size an N+1 or N+2 cluster: units required, spare capacity, per-unit load after a failure and the cost of redundancy.
Inputs
Units to Deploy
5
Units Needed to Carry the Load
4
Per-Unit Load After One Failure
90.0%
Cost of the Redundant Units
$15,000
Simultaneous Failures Survivable
1
Step by step
Values used
Total load to carry = 3,600 units of load; Capacity per unit = 1,000 units of load; Redundancy level = N+1; Cost per unit = 15,000 currency
Redundancy Calculator (N+1, N+2)
N = ceil(load ÷ unit capacity); deployed = N + spares (or 2N for full duplication); post-failure load per unit = load ÷ (deployed − 1).
Units to Deploy
= 5
Units Needed to Carry the Load
= 4
Per-Unit Load After One Failure
= 90.0
Cost of the Redundant Units
= 15,000
Simultaneous Failures Survivable
= 1
How it works
N is the number of units the load actually demands, and the redundancy level adds units on top so that failures do not reduce served capacity. The post-failure figure is the check that matters: if it exceeds 100%, the cluster is redundant on paper but overloads the survivors. N+1 that pushes the remaining units to 115% of rated capacity is not redundancy, it is a delayed cascading failure.
Formula
Redundancy Calculator (N+1, N+2)
N = ceil(load ÷ unit capacity); deployed = N + spares (or 2N for full duplication); post-failure load per unit = load ÷ (deployed − 1).
- N
- Units needed to carry the load with nothing spare
- spares
- Units that may fail without breaching capacity
Frequently Asked Questions
How is Redundancy Calculator (N+1, N+2) calculated?
N = ceil(load ÷ unit capacity); deployed = N + spares (or 2N for full duplication); post-failure load per unit = load ÷ (deployed − 1). N is the number of units the load actually demands, and the redundancy level adds units on top so that failures do not reduce served capacity. The post-failure figure is the check that matters: if it exceeds 100%, the cluster is redundant on paper but overloads the survivors.
Why does Redundancy Calculator (N+1, N+2) matter?
N+1 that pushes the remaining units to 115% of rated capacity is not redundancy, it is a delayed cascading failure.
What values do I need to enter?
This calculator takes 4 inputs: Total load to carry, Capacity per unit, Redundancy level, Cost per unit. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
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