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Can you help me with my software licensing question?
We're planning on having an active stack and a warm standby stack, where the sql server databases are mirrored.
The servers in the warm standby stack aren't used at all (apart from health checking, and updates), until we fail over.
Do we need to power off these machines until we fail over?
I've been told we need to, based on Microsoft's licencing terms and conditions.
It depends on if you bought SA on your servers and whether or not they are on all the time or not whether or not you need additional licenses. Cold DR servers are not required to be licensed providing you have SA, warm servers are required to be licensed. As indicated previously, contact your preferred MS licensing specialist for details
If it's installed you need to pay for the license if you don't have SA on both the licenses for the servers and the CALs (see link provided by vinny in the comments).
On or off really doesn't matter I don't think. The only place you get a break with SQL server is the secondary node in an A/P cluster.