hyper-v failover clustering: licensing costs

ndidomenico used Ask the Experts™
We are looking at implementing hyper-v failover clustering between 2 phsycal hyper-v server for redundancy. What Microsoft licensing costs are involved / software required to purchase ?

Must the 2 servers be exactly identical in terms of hardware, or could we have the failover server with less expensive  (ex: Sata drives instead of sas, slower raid controller, etc). this is for a small installation (40 users) looking at implementing failover in case of failure of the main server, while the defective server is fixed.

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Here's some notes on a recent chat with a Microsoft License specialist.

Me: So for short outages, migrating the 3 VMs to a failure node is perfectly acceptable?
Rachel: I apologize for the delay. I'll be right with you..
Rachel: Not quite. You could reassign the license to another machine, but you wouldn't be able to move it back (reassign it yet again) for 90 days. The specific language from the product use rights is:
Rachel: Generally, you may not reassign:
Rachel: • licenses on a short-term basis (i.e., not within 90 days of the last assignment)
Rachel: • licenses for the Windows desktop operating system
Rachel: • licenses for Rental Rights for Office or Windows
Rachel: • Software Assurance coverage separately from the underlying license.
Rachel: In all cases, when you reassign a license from one device or user to another, you must remove the software from the former device or from the former user’s device or block further access by or to the former device or by or to the former user’s device.

Rachel: All right. If you had Datacenter on the working server, you wouldn't need to reassign the license at all, since Datacenter supports an unlimited number of virtual instances. So you should be able to move the VMs from the failed machine to the working server without exceeding the allowed number of instances, whereas with Enterprise, there's a limit of 4 VMs per license.
In a nutshell, if you were to put Windows server 2008 R2 Datacenter on both of your servers you wouldn't have any licensning restrictions.  If that were the case you could have VMs running on both servers, and if one of them failed the VMs could come up on the running server.

There are some requirements:
You must have a SAN where the VM files are stored. That SAN must be connected to both servers.
The servers must be in a cluster.
You must have at least 3 network connections for each server
Both servers must have the same family of Proc, but the raid controller and drive types won't matter.
Also, if the primary server goes down, all of your VMs will be turned off, then brought up on the backup server - it's basically like they are rebooted.  Hyper-v supports Live migration which doesn't have any down-time, but both servers need to be running to facilitate that.

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