Hello,

There seems to be quite an overlap in two offerings from MS - AvailabilitySet and Virtual Machine Scale Set. We currently support both of these but it was due to a historic reason that certain regions in azure only consists of a single availability zone. For these regions the only choice is AvailabilitySet to ensure that we get hardware fault tolerance and update fault tolerance. For regions that support more than one availability zones, VMSS becomes the default choice.

Questions:

1. Is VMSS a replacement for AvailabilitySets? Both of them provide fault tolerance (via fault domains)
2. Is the above understand still correct? If not then please clarify on when someone will use AvailabilitySet and not VMSS and vice versa.
3. https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-placement-groups#placement-groups - suggests that placement groups serve the same purpose as AvailabilitySets - does this suggest that one should now not use AvailabilitySets?
4. In the link: https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-faq#do-scale-sets-work-with-azure-availability-sets- it is suggested A common configuration is to put control node VMs (which often require unique configuration) in an availability set, and put data nodes in the scale set. What is the rationale behind this recommendation?

I have already gone through the following links:

• https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/overview
• https://learn.microsoft.com/en-us/azure/virtual-machines/availability-set-overview
• https://learn.microsoft.com/en-us/answers/questions/984077/difference-between-vm-scaleset-and-availability-se

However it is not clear on why one would choose an AvailabilitySet over VMSS. I hope someone gives a detailed answer here.

After going through https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-orchestration-modes#a-comparison-of-flexible-uniform-and-availability-sets it seems that there is now quite an overlap between AvailabilitySet and VMSS.

Hi, Availability set is an object which has the config to place the VMs which you add to the availability set into different fault domain and update domain. VMSS is a product as such which has many functionality that you can provide to your VMs like auto scaling, load balancing etc. The only place where I see an overlap between these 2 is, a VMSS can be deployed in an availability set or in an availability zone to provide high availability.

Using VMSS you could also set the fault domains. So i am still unclear why would i use AvailabilitySet when what VMSS offers is a superset of what AvailabilitySet has to offer.
Link: https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-orchestration-modes#a-comparison-of-flexible-uniform-and-availability-sets offers a comparison and it seems to me that VMSS offers everything that availability set has to offer making it a superset.

Availability set is a feature introduced long back before VMSS. If someone what to deploy they application in an high availability, and doesn't need a VMSS, users can still build their infrastructure using VM and availability set.

Hello Bhargava, Madhav

Welcome to Microsoft Q&A Platform, thanks for posting your query here.

You are correct that there is some overlap between Availability Sets and Virtual Machine Scale Sets, but they serve different purposes and can be used together to provide a highly available and scalable solution.

Availability Sets are used to ensure that virtual machines are distributed across multiple physical servers, racks, and network switches to minimize the risk of a single point of failure. In an Availability Set, virtual machines are placed in different fault domains and update domains to ensure that they are not affected by hardware or software maintenance events at the same time. Availability Sets are a good choice for workloads that require high availability and fault tolerance, but do not require automatic scaling.

Virtual Machine Scale Sets are used to automatically scale out virtual machines based on demand. VMSS can be used to create and manage a group of identical, load-balanced virtual machines that can automatically increase or decrease in response to changes in demand. VMSS is a good choice for workloads that require automatic scaling, but do not require the same level of fault tolerance as Availability Sets.

Placement groups are used to control the placement of virtual machines in a Virtual Machine Scale Set to optimize network performance and minimize the risk of a single point of failure. Placement groups are not a replacement for Availability Sets, but they can be used together to provide a highly available and scalable solution.

Regarding the recommendation to put control node VMs in an Availability Set and data nodes in a Virtual Machine Scale Set, the rationale behind this is that control nodes often require unique configuration and are critical to the operation of the system, so they should be placed in an Availability Set to ensure high availability and fault tolerance. Data nodes, on the other hand, are often identical and can be easily scaled out using a Virtual Machine Scale Set.

In summary, Availability Sets and Virtual Machine Scale Sets serve different purposes and can be used together to provide a highly available and scalable solution. Availability Sets are a good choice for workloads that require high availability and fault tolerance, while Virtual Machine Scale Sets are a good choice for workloads that require automatic scaling. Placement groups can be used to optimize network performance and minimize the risk of a single point of failure in a Virtual Machine Scale Set.

Hope this clarifies your queries.

Hello @vipullag-MSFT

Thanks for your answer. I have already read all of what you have stated in the provided links by MS. However that still does not answer my question. Look at the comparison table:

https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-orchestration-modes#a-comparison-of-flexible-uniform-and-availability-sets and it seems that VMSS is a super set of availability set.

Hello Bhargava, Madhav

Yes, you are correct that Virtual Machine Scale Sets (VMSS) provide a superset of the functionality provided by Availability Sets. VMSS provides all the features of Availability Sets, such as fault domains and update domains, and also provides additional features such as automatic scaling, rolling upgrades, and support for managed disks.

However, it's important to note that Availability Sets are still a valid choice for workloads that require high availability and need to be spread across multiple fault domains and update domains. Availability Sets are a simpler option compared to VMSS and may be more suitable for certain workloads that don't require automatic scaling or other advanced features provided by VMSS.

In summary, while VMSS provides a superset of the functionality provided by Availability Sets, Availability Sets are still a valid choice for certain workloads that require high availability and don't require the advanced features provided by VMSS.

Hello @vipullag-MSFT

Thanks for a quick response.

However, it's important to note that Availability Sets are still a valid choice for workloads that require high availability and need to be spread across multiple fault domains and update domains.

This can also be done by VMSS.

Availability Sets are a simpler option compared to VMSS and may be more suitable for certain workloads that don't require automatic scaling or other advanced features provided by VMSS.

You can select manual scaling option and choose not to scale a machine-set. This would then become equal to having an AvailabilitySet. From costing perspective also there seems to be no difference. The software stack evolves and so does the requirements of higher availability and scalability. One can still opt to start with VMSS without using any of its scaling features and when that is required the configure the scale-set rules for automatic-scaling. I still do not see the need for an AvailabilitySet.

However, it's important to note that Availability Sets are still a valid choice for workloads that require high availability and need to be spread across multiple fault domains and update domains.

This can also be done using VMSS.

Availability Sets are a simpler option compared to VMSS and may be more suitable for certain workloads that don't require automatic scaling or other advanced features provided by VMSS.

One can switch off automatic scaling in VMSS and make the scaling manual. Still trying to understand why would one choose to use AvailabilitySet. Requirements evolve, if you do not require automatic scaling initially but then later on wish to have that feature (even if you completely wish to control it and therefore set it to manual), then instead of then switching from AvailabilitySet to VMSS you could just continue to use VMSS right from start.

This can also be done using VMSS.

Yes, that's correct. Virtual Machine Scale Sets (VMSS) are a newer Azure service that provides similar functionality to Availability Sets.

I still do not see the need for an AvailabilitySet.

You are correct that VMSS can be manually scaled and that it is possible to use VMSS without using its scaling features. In that case, VMSS can provide similar functionality to Availability Sets. However, it's important to note that VMSS is a more complex service than Availability Sets, and it may not be the best choice for all workloads. For example, if you have a simple application that doesn't require automatic scaling or other advanced features provided by VMSS, then Availability Sets may be a simpler and more suitable option. Additionally, Availability Sets have been around longer than VMSS and are a well-established feature of Azure, so some users may prefer to use them for that reason. Ultimately, the choice between Availability Sets and VMSS depends on the specific requirements of your workload and your preferences as a user.

We noticed that you rated an answer as not helpful. Thank you for taking time to share your feedback here in Microsoft Q&A platform.

Requesting you to check recent responses provided and requesting you to take a resurvey.