Does Deallocating/Reallocating Azure VMs Affect AG Synchronization Latency?

Question

Hi,

I'm considering deallocating and reallocating Azure VMs that are part of a SQL Server Always On Availability Group with a three-node cluster, to potentially improve performance by moving to a cleaner, possibly less loaded physical server.

My concern is the impact this could have on the synchronization speed between the AG replicas, especially since these VMs must maintain a consistent and fast sync to ensure high availability.

If I deallocate and reallocate each node one by one, is there a risk that the VMs might be placed on physical hosts that are farther apart, leading to increased latency? And if so, could this impact the AG synchronization adversely?

Would appreciate insights into whether Azure tries to optimize the placement of VMs post-reallocation to minimize latency within the same region or availability set, especially in high-availability configurations like SQL Server AGs.

Thank you for your assistance.

Answer 1

Hello Jacob James

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

If you deallocate and reallocate each node one by one, there is a risk that the VMs might be placed on physical hosts that are farther apart, leading to increased latency. This could impact the AG synchronization adversely. However, Azure offers a feature called proximity placement groups that can help you optimize network latency.

Proximity placement groups improve the overall application performance by reducing the network latency among virtual machines.

When you deallocate and reallocate each node one by one, you can specify the proximity placement group for each VM to ensure that they are placed on physical hosts that are close to each other, minimizing latency within the same region or availability set. This can help ensure that the AG synchronization is not adversely impacted.

In summary, you can use proximity placement groups to optimize network latency between Azure VMs and minimize the risk of increased latency when deallocating and reallocating VMs that are part of a SQL Server Always On Availability Group.

Ref:

https://learn.microsoft.com/en-us/azure/virtual-machines/co-location

https://learn.microsoft.com/en-us/azure/virtual-machines/windows/proximity-placement-groups-portal

https://azure.microsoft.com/en-us/blog/introducing-proximity-placement-groups/

Hope this helps.