Server licenses

The diagram below shows a server farm consisting of 3 physical servers with 2 virtual machines running SQL Server 2025 on the first physical server. 2 Server licenses have been assigned to that server to allow the running of those two virtual machines, and this is all correctly licensed.

One of the technical benefits of virtualization is that virtual servers can move between physical servers and this is desirable for all sorts of reasons. It might be load balancing – so perhaps the 2 SQL Server virtual machines move to the second physical server because it has more capacity available at a given time than the first server that they’re currently running on. Or it could be that the first server fails, and the virtual machines fail over to the second server. The dynamic moving of virtual machines between physical servers is a technical feature that can be managed by the IT team within parameters that they specify, but what happens from a licensing perspective?

Let’s assume that the 2 SQL Server virtual machines have moved to the second physical server – perhaps for either of the two reasons mentioned above. Are the licenses allowed to move too? Well, licenses CAN be moved between servers as long as they obey the “90-day rule”. This rule states that licenses can be moved between machines no more frequently than every 90 days. In this scenario, we could move the licenses to the second server once, but would then have to wait 90 days before we moved them again. This isn’t really an agile enough solution for most server farms that use virtualization.

So, what options are there? Well, you could assign 2 licenses to each of the physical servers, as shown below, so that all eventualities would be covered: both virtual machines could run on a single server, or each virtual machine could run on a separate server.

You can probably imagine that this is potentially a pretty expensive option; if we were licensing these 2 SQL Server machines in a physical environment we would just need 2 licenses, but when we deploy them in a virtual infrastructure we need 6 licenses. Any time we want to add a SQL Server virtual machine we need to purchase another 3 licenses – one for each server in this example.

The alternative is to make sure that you have License Mobility across Server Farms rights. This benefit is available when you buy Software Assurance with a license, and it essentially allows you to ignore the 90-day rule. So, in the diagram below, if we buy 2 licenses with Software Assurance then they are temporarily assigned to whichever physical servers are hosting the virtual machines and as the virtual machines move, the licenses can (in effect) move too. Licenses purchased as Software Subscriptions are also eligible for this benefit.