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Host Failure Impact Calculator

Quantify a host failure: VMs affected, capacity lost, post-failure utilisation, restart window and the availability cost.

About Host Failure Impact Calculator

Two independent things happen when a host dies. Capacity redistributes onto the survivors, and because the denominator shrinks the utilisation rise is non-linear — losing one host of eight at 65% pushes survivors to 74%, but losing two pushes them to 87%. Separately, HA has to detect the failure, elect a master and restart the VMs in batches, which is what the users actually experience as downtime. HA guarantees a restart, not a fast one: a hundred VMs restarting 32 at a time with a 45-second application start is over two and a half minutes of outage before detection is even counted. Substitute your own cost-of-downtime figure; the default is a placeholder.

Formula

Host Failure Impact

VMs affected = ceil(VMs ÷ hosts) × hosts lost; post-failure utilisation = current utilisation × hosts ÷ surviving hosts; restart window = detection time + ceil(VMs affected ÷ concurrent restarts) × restart time.

post-failure utilisation
Load redistributed across the survivors, which rises non-linearly
concurrent restarts
HA restart batch size, not the total VM count
detection time
Heartbeat timeout plus master election before restarts begin

Worked example

With the default values below, Host Failure Impact Calculator returns VMs Requiring Restart of 28. Change any input above and every figure here is recalculated in your browser.

Inputs used

Hosts in clusteraccepts 2–2,000 hosts
8 hosts
VMs in clusteraccepts 1–500,000 VMs
220 VMs
Hosts lostaccepts 1–100 hosts
1 hosts
Cluster utilisation before the failureaccepts 1–100 %
65 %
Restart time per VMaccepts 1–3,600 seconds
45 seconds
VMs restarted concurrentlyaccepts 1–512 VMs
32 VMs
Failure detection and election timeaccepts 1–600 seconds
30 seconds
Cost of downtime per houraccepts 0–10,000,000 USD
4,000 USD

Results

VMs Requiring Restart
28 VMs
Cluster Capacity Lost
12.5%
Utilisation After the Failure
74.3%
Restart Window
1m
Cost of the Outage
$10.61
Survivability Verdict
Survivors absorb the load comfortably

Step by step

  1. 1Values used
  2. 2Host Failure Impact
  3. 3VMs Requiring Restart= 28 VMs
  4. 4Cluster Capacity Lost= 12.5
  5. 5Utilisation After the Failure= 74.3
  6. 6Restart Window= 1.25
  7. 7Cost of the Outage= 10.61
  8. 8Survivability Verdict= Survivors absorb the load comfortably

Assumptions and limits

  • The result is derived only from the 8 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Host Failure Impact calculated?

VMs affected = ceil(VMs ÷ hosts) × hosts lost; post-failure utilisation = current utilisation × hosts ÷ surviving hosts; restart window = detection time + ceil(VMs affected ÷ concurrent restarts) × restart time. Two independent things happen when a host dies. Capacity redistributes onto the survivors, and because the denominator shrinks the utilisation rise is non-linear — losing one host of eight at 65% pushes survivors to 74%, but losing two pushes them to 87%. Separately, HA has to detect the failure, elect a master and restart the VMs in batches, which is what the users actually experience as downtime.

Why does Host Failure Impact matter?

HA guarantees a restart, not a fast one: a hundred VMs restarting 32 at a time with a 45-second application start is over two and a half minutes of outage before detection is even counted. Substitute your own cost-of-downtime figure; the default is a placeholder.

What values do I need to enter?

This calculator takes 8 inputs: Hosts in cluster, VMs in cluster, Hosts lost, Cluster utilisation before the failure, Restart time per VM, VMs restarted concurrently, Failure detection and election time, Cost of downtime per hour. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.

Why does utilisation rise faster than the capacity lost?

Losing one host of eight removes 12.5% of capacity, but the same workload now divides across seven hosts instead of eight — a 14.3% increase in per-host load. The effect compounds with each additional host lost, which is why small clusters need proportionally more headroom.

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Host Failure Impact Calculator | Calcrivo