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metalheart-8020 asked Prrudram-MSFT commented

VMMS Availability-Zone behavior

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!

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1 Answer

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Prrudram-MSFT answered Prrudram-MSFT commented

Hello @metalheart-8020,

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.

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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!

Please "Accept as Answer" and Upvote if the answer provided is useful, so that you can help others in the community looking for remediation for similar issues.





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I understand the concept of AZs. Now when I create a VMSS in Azure Portal, I don't understand the difference in behavior (where the individual VMs are created) based on the options selected for Availability Zone for below cases:

192935-image.png

a) Availability Zone = None
b) Availability Zone = all three AZs checked (in this given region)

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@metalheart-8020

When you select none, the VMSS doesn't have any redundancy with availability zones. When you select the 3 zones it means the VM or VMSS VMs will be made available when the actual DC is down due to any failure or disaster. It doesn't show the AZ VMSS VMs explicitly in the portal.


201924-image.png

From the above screenshot, it is my actual VMSS whose VMs are created primarily in one of the DCs in the region i selected. As i chose to all the 3 zones, it means the VMSS VMs will be
Example :

If you’re deploying a web tier consisting of 2 VMs in Ireland, you can now make sure that VM1 is placed in Availability Zone 1 and VM2 is placed in Availability Zone 2. If zone 1 was to fail, you (and your customers) would still be able to access VM2 in AZ2.

This means your service won’t have to run from a separate Azure region and will be faster as a result. This is especially useful if your customers are concentrated in a single region.


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