VMMS Availability-Zone behavior

Question

Hi,

when you are creating a Virtual Machine Scale Set, you have the opportunity to optionally specify availability zones:

![192587-image.png][1]

I understand when you specify a single or multiple AZs, the VMs will be created only in those very zones and Azure will try to evenly balance them across them.

What I don't understand is how the VMSS behaves when checking none of the availability zones? Does that mean Azure has complete freedom of choosing which datacenter the VM is created in? Are then multiple VMs restricted only to the datacenter of Azure's choosing or can the placement be fully arbitrary?

Thanks in advance!

Answer 1

Hello @metalheart ,

Thank you for reaching out to the Microsoft Q&A platform. Happy to answer your question.

VMMS and Availability zones are 2 different concepts, VMMs are set of similar VMs where the number of VM instances can automatically increase or decrease in response to demand or a defined schedule
Let's go over availability zones and VMs without availability zones,

An availability zone is a physically separate location (like a datacenter) within an Azure Region that provides redundancy and reliability that protects you from a disaster like a flood that might impact the whole datacenter.

Azure availability zones are physically separate locations within each Azure region that are tolerant to local failures. Failures can range from software and hardware failures to events such as earthquakes, floods, and fires. Tolerance to failures is achieved because of redundancy and logical isolation of Azure services. To ensure resiliency, a minimum of three separate availability zones are present in all availability zone-enabled regions.

Azure availability zones are connected by a high-performance network with a round-trip latency of less than 2ms.

Specfic Questions:

1. I understand when you specify a single or multiple AZs, the VMs will be created only in those very zones and Azure will try to evenly balance them across them.
When you create a VM with an AZ, it means that you have chose an additional redundant option for your VMs availability in another physical datacenter within the same region. It also means that the They can be configured in two ways. They can be either zone redundant, with automatic replication across zones, or zonal, with instances pinned to a specific zone.
Please read through this document to understand the availabiliy zones well az-overview

2. What I don't understand is how the VMSS behaves when checking none of the availability zones? Does that mean Azure has complete freedom of choosing which datacenter the VM is created in? Are then multiple VMs restricted only to the datacenter of Azure's choosing or can the placement be fully arbitrary?
When you create a VM or VMMS in azure say without availability zones, you will specifiy the location of your VMs only not the datacenters. With Availability zone, you would still to the same thing, which is specify the location which is refferred to as region, plus here in AZs you would additional choose which datacenters within the same region you would like to use for redundancy for your VMs as explained in definition above.

Hope this helps!

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