Plan availability set fault and update domains so a rack failure or maintenance wave still leaves enough capacity.
An availability set spreads VMs across two or three fault domains and up to twenty update domains, so the worst-case simultaneous loss is one full fault domain — roughly a third of the set with three domains. Sizing for that means provisioning enough VMs that the survivors still carry peak load, which is where the fault-domain fraction in the formula comes from. The 99.95% availability set target only covers connectivity to at least one instance, not your application's capacity, so a set sized exactly for peak becomes an outage the moment a fault domain drops; update domains additionally cap how fast Azure can patch your fleet.
Azure Availability Set
VMs per fault domain = ceil(VMs ÷ fault domains); to keep peak capacity through a fault-domain loss you need ceil(required × fault domains ÷ (fault domains − 1)) VMs; downtime = (100 − SLA%) × 438 minutes per month.
VMs per fault domain = ceil(VMs ÷ fault domains); to keep peak capacity through a fault-domain loss you need ceil(required × fault domains ÷ (fault domains − 1)) VMs; downtime = (100 − SLA%) × 438 minutes per month. An availability set spreads VMs across two or three fault domains and up to twenty update domains, so the worst-case simultaneous loss is one full fault domain — roughly a third of the set with three domains. Sizing for that means provisioning enough VMs that the survivors still carry peak load, which is where the fault-domain fraction in the formula comes from.
The 99.95% availability set target only covers connectivity to at least one instance, not your application's capacity, so a set sized exactly for peak becomes an outage the moment a fault domain drops; update domains additionally cap how fast Azure can patch your fleet.
This calculator takes 5 inputs: VMs in the availability set, Fault domains, Update domains, VMs needed to carry peak load, Availability target. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.