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Stefano d'Antonio
Stefano d'Antonio

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Virtual machine scale sets flexible orchestration mode (and benefits over regular VMs)

Introduction and VMSS benefits

When we close our eyes and we try and picture "the cloud", two quintessential IaaS services come to mind: Virtual machines and Virtual machine scale sets (in Azure).

Historically, VMs and VMSSs are the first iteration of our cloud migration journey; a VM facilitates the lift-and-shift pattern and a VMSS our first step towards leveraging cloud scaling for our workload.

A traditional VMSS sacrifices control in favour of simpler deployment, simpler management, faster recovery and faster horizontal scaling.

You define an image and the service will stamp instances of that blueprint on demand, it can auto-scale based on the usage, on failure, on updates; all on your behalf.

Downsides of VMSS

Those glorious benefits have some drawbacks:

1) VMSS API diverges from the standard VM API for individual instances
2) Lack of RBAC granular permissions per VM
3) Lack of Azure Site Recovery and Azure Backup support

In addition, to handle VMs availability, there are two different patterns making the experience inconsistent between VMSSs and VMs; for datacenter redundancy for VMs, availability sets must be used, VMSSs have native support for distribution across fault and update domains.

Enter VMSS flexible

A new option for VMSS has been created, the "orchestration mode".

The classic experience, has been re-branded as uniform orchestration, but is no different from the traditional VMSS experience.

The new option, still in preview at the time of writing, is called flexible orchestration and it promises to:

  • Unify the experience, no more availability sets, the availability experience will be handled in the same way between individual VMs and VMSSs
  • Individual instances have full control with the same VM API as regular VMs
  • VMs can be added to VMSS flex after creation
  • Custom instance naming
  • Target individual VMs with extensions
  • Assign machines to specific fault domains
  • In guest OS security patching (without re-imaging)
  • Mix Windows and Linux in the same set

Why should I use that instead of regular VMs?

VMSS flex shines when managing a substantial number of VMs (30+).

It provides a single control plane for distribution of the machines across a datacenter with automatic and optimized spread.

When dealing with many machines, it can be a significant overhead to manage availability.

It does also support the template-based scaling like a uniform scale set should you need that, but, I believe, its main purpose is to unify the experience between VMs and VMSSs.

It supports up to 1000 instances spread across fault domains, whereas uniform supports only up to 100. Fault domains are also treated the same as update domains.

There are obvious advantages (and some cons) over a uniform mode, including having different machine sizes and other features listed above in the article; in addition, there is a comprehensive table in the official Microsoft documentation that would be pointless to copy and paste here.

Current limitations

VMSS flex does not currently support availability zones.

Does not support single placement groups.

How to get started

To try VMSS flex you need to register the feature in your subscription, see the full guide here: https://docs.microsoft.com/en-us/azure/virtual-machine-scale-sets/virtual-machine-scale-sets-orchestration-modes#register-for-flexible-orchestration-mode

Quickstart ARM template to deploy VMSS flex: https://github.com/Azure/azure-quickstart-templates/tree/master/quickstarts/microsoft.compute/vm-vmss-flexible-orchestration-mode

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