Difference in Azure Availability Sets and Scale Sets

Question

Can someone define the difference between the two in the ARM portal. Also what would be the PowerShell if i need to add a new Azure RM VM to a existing AS(Availability Set)? Thanks Prab

Answer 1

An availability set consists of a set of discrete VMs which have their own names and individual properties, but are spread across fault domains, which means when you have more than one VM in a set it reduces the chances of losing all your VMs in event of a hardware failure in the host or rack.

A scale set consists of a set of identically configured VMs, also spread across fault domains (in fact a scale set is an implicit availability set with 5 fault domains). The main difference is scale sets, being identical, make it very easy to add or remove VMs from the set while preserving high availability, which in turn makes it easy to implement autoscale, and to perform operations on the whole set or a subset of VMs. There are also API calls that support re-imaging and upgrading VMs, allowing you to roll out an update while keeping the service running. They are useful for cloud architectures which require deploying large numbers of similar VMs, or need to be elastic. A typical architecture might use a scale set for agent or worker nodes, and an availability set for master or control nodes. See https://azure.microsoft.com/en-us/services/virtual-machine-scale-sets/ for more detail.

For your question about adding a new VM to an existing AS, see Azure Resource Manager: move VM to availability group and https://gallery.technet.microsoft.com/Set-Azure-Resource-Manager-f7509ec4

Answer 2

1. The main difference is that Scale Sets have Identical VMs where in Availability Sets does not require them to be identical.

2. Availability set, in concept, are for enhancing application availability in case one primary VM fails/needs update another VM from Fault/Update domain can be provisioned

   Scale sets on another hand, in concept, are designed for automatic scaling (horizontal) in application where load can vary extensively to fulfill more compute needs.

3. Provisioning new VM in Azure when needed is easier for Scale sets as all other VMs are same in all aspects & replica of one golden copy.

For more details refer:

https://docs.microsoft.com/en-us/azure/virtual-machine-scale-sets/

https://docs.microsoft.com/en-us/cli/azure/vm/availability-set?view=azure-cli-latest