Latency between two Azure VMs

Question

I have an ASP.NET 4.5, chatty, web application which is hosted on one Large (4 cores 7GB) Azure VM. The WEB application is loosely coupled to the data tier via a dedicated WCF Service. The application database is hosted by a dedicated SQL Server instance on another Large (4 cores 7GB) Azure VM. The WCF endpoint communicates with the DB VM via an ASP.NET Connection String that employs the DB VM public DNS name - e.g. xyz.cloudapp.net.

Both the WEB and DB VMs appear to be operating in a different subnet but both are in the same Azure location; differing second and onwards octet values.

When running the exact same solution on one Medium (2 cores 3.5GB) Azure VM, the latency issues are much lower.

I am looking for suggestions on how to reduce the WEB to DB latency as much as possible.

Answer 1

If you have two VMs in the same data center that need to communicate with each other, don't use their public DNS. Create an Affinity Group, create a virtual network in that affinity group, and then place both VMs in the virtual network (you might need to shut them down, delete them without deleting their VHDs, and then create them from the data disks in the new vnet). Accessing VMs through the DNS (thus through the Azure LB) adds about 0.5ms latency to each request - Not recommended for a chatty app.

Answer 2

It almost sounds like you have the two VM's running in two separate cloud services. Might I suggest placing both machines in the same cloud service? This should allow you to access the database server from the web tier via the short DNS name (aka the server name). This should not only help secure the database server by allowing you to remove any input endpoints you have declared on it, but also reduce latency since calls will be made directly from one VM to another and not pass through the Azure Fabric load balancer (which is what fronts all calls coming to the cloud service URL).

Answer 3

in this blog post I have measured latency in various network configurations.

[https://nicolgit.github.io/azure-measuring-latency-across-availability-zones-in-we/]

I think it is useful to understand the impact on latency of different "typical" network architectures.