Azure SQL database read only replica compute unit architecture

Question

I have one quick architecture question. Azure business-critical tier database has a feature called "Read scale-out". I want to make use of it, but there is one question regarding computing architecture when I enable the "Read scale-out".

If I select 40 cores business-critical tier, does that mean I will have 80 cores total compute power for both primary and secondary replicas or compute unit is shared for both replicas?

Thanks, Alex.

Answer 1

As per this Microsoft official document

The secondary replicas are provisioned with the same compute size as the primary replica. The read scale-out feature allows you to offload read-only workloads using the compute capacity of one of the read-only replicas, instead of running them on the read-write replica. This way, some read-only workloads can be isolated from the read-write workloads and will not affect their performance.

So, by default, primary and secondary replicas have the same compute size but they do not share it. It means if primary replica has 40 vCores then secondary will also have the same and they do not share.

But if you enable the read scale-out feature, the read-only workloads will be offloaded from read-write replica by using the compute capacity of one of the read-only replica. This feature improves the performance.

Please go through the shared document for better understanding and more details.