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Azure VM Availability SLA

This post is focusing on the availability SLA of Azure Virtual Machines. You might think that there is not so much to say about it, even so, the reality is different.
Let me ask you a simple question when you talk about availability and Azure Virtual Machines:
  1. When do you have 99.9% availability?
  2. When do you have 99.95% availability?

Case 1: Single VM
When there is only one instance of VM, the SLA guaranty us minimum 99.9% availability. There are no actions that you can take to improve this value. Remember that this is the minimum value that is guaranteed. This value means that the real value can be higher than 99.9%.

Case 2: Two or more VMs in the same Availability Set
When there are two or more VMs in the same Availability Set, you have the availability SLA to 99.95%. This value means that:
  • The availability of the two or more VMs combined is at least 99.95%. 
  • On the other hand, this does not mean that for each VM the availability is 99.95%. Per node the availability can be lower but combined, you get 99.95%.

Case 3: Single VM inside a Scale Set
When you have only one VM in a Scale Set, the same SLA is offered as for a Single VM – 99.9%. Having a single, VM inside a VM does not offer additional VM availability.

Case 4: Two or more VMs on a Scale Set with at least Two Fault Domains
Having at least two VMs inside at least two fault domains guaranty us an availability SLA at 99.95%.

Things to consider
There is no increased availability offered if you have only one VM as a single VM or inside a Scale Set (99.9%). Inside a Scale Set, if you have two VMs inside the same Fault Domain the SLA for availability remains the same as for one VM (99.9%). It is required to have multiple VMs inside different Fault Domain or inside an Availability Set to get 99.95% availability.


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