SLA for Azure Shared Disks and Availability Set

Question

Hello,

I would to know if the usage of the feature "Azure shared disks" (https://learn.microsoft.com/it-it/azure/virtual-machines/windows/disks-shared) negatively impacts the SLA in case of two VMs in an availability set.

Two VMs in an availability set gets you an SLA that is at least 99.95%. Yet Storage Accounts only get an SLA of 99.9%. As managed disks would be different storage accounts (in the background) you can see why 99.95% is offered despite individual disks only getting 99.9%. However, if a shared disk is used, will this bump the SLA down to 99.9% for the VMs? Or will there be a higher SLA for the shared disks?

Thank you in advance,
Leonardo.

Answer 1

@Leonardo Queirolo
As noted on SLA for Virtual Machines page, "This SLA does not apply to Availability Sets leveraging Azure shared disks". So you will have the lower SLA when using Shared Disks.

Hope this helps! Let us know if you have further questions.

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Answer 2

In fact, since "When using Availability sets and virtual machine scale sets with Azure shared disks, storage fault domain alignment with virtual machine fault domain is not enforced for the shared data disk", and because any of the components can independently fail (VM AS and disk), shouldn't the correct SLA be the compound of both SLAs? 99.95% x 99.9% = 99,85%

Answer 3

The SLA of your cluster is based on the weakest link. The weakest link in this case is your shared storage at 99.9%.