Kernel Upgrade Downtime Calculator

Work out kernel upgrade downtime instantly with clear inputs, formula shown and shareable results.

About Kernel Upgrade Downtime Calculator

A rolling kernel upgrade trades window length against capacity loss: larger batches finish sooner but take more of the fleet offline simultaneously. The batch size must leave enough capacity to carry production traffic, so it follows from headroom rather than from convenience — and verification time per batch is what catches a bad kernel before it reaches the whole estate.

Formula

Rolling upgrade window

batches = ceil(hosts / batch size); window = batches x (reboot time + verification); capacity offline = batch size / hosts

Worked example

With the default values below, Kernel Upgrade Downtime Calculator returns Total maintenance window of 160. Change any input above and every figure here is recalculated in your browser.

Inputs used

Hosts to upgrademinimum 1: 120
Reboot and service start timeminimum 0.1 min: 5 min
Hosts rebooted simultaneouslyminimum 1: 6
Verification per batchminimum 0 min: 3 min

Results

Total maintenance window: 160 min
Reboot batches: 20
Window in hours: 2.67 h
Capacity offline at any moment: 5.0%

Step by step

1Values used
2Rolling upgrade window
3Total maintenance window= 160 min
4Reboot batches= 20
5Window in hours= 2.67 h
6Capacity offline at any moment= 5.0

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges 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 large should the batch be?

No larger than your redundancy allows. If the service is sized for N-1, reboot one at a time; with 30 percent headroom you can take a quarter of the fleet.

Does live patching remove the need for this?

It covers many critical fixes without reboot, but not full kernel version upgrades. Plan for rolling reboots regardless, just less frequently.