A group of discrete virtual machines spread across fault domains.
A group of identically configured virtual machines spread across fault domains.
Use Availability Set for predictable workloads.
Use Scale Set for unpredictable workloads (autoscale).
Has 3 fault domains and 5 update domains by default
Has 5 fault domains and 5 update domains by default
Virtual machines are created from different images and configurations.
Virtual machines are created from the same image and configuration.
Virtual machines are automatically distributed within a data center.
Virtual machine scale sets can be distributed within a single datacenter or across multiple data centers.
Number of VMs
You can only add a virtual machine to the availability set when it is created.
Scale sets can increase the number of virtual machines based on demand.
Availability set has no additional charge. You only pay for the computing resources.
Scale sets have no additional charge. You only pay for the computing resources.